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Johnson Matthey Technol. Rev., 2020, 64, (4), 507

doi:/10.1595/205651320x15953337767424

从爱琴海水和土耳其居吕克湾沉积物分离出来的抗药性和重金属耐受细菌

量化具有环境修复应用潜力的已识别细菌种类的抗药性

重金属和耐抗生素细菌具有环境生物修复应用潜力。在 2011 年至 2013 年,我们对从土耳其爱琴海采集的沉积物和海水样本中的耐性菌进行了研究。我们采用膜过滤技术对海水样品中的生物指示菌进行了检测,并针对样品中的异养需氧菌采用了涂布平板技术和 VITEK®2 Compact 30 微鉴定系统。采用最小抑菌浓度法对耐重金属菌执行检测,用纸片扩散试验来检测耐抗生素细菌。从沉积物样品中分离出的所有细菌对利福平、磺胺、四环素和氨苄青霉素的耐药性均为 100%。有 98% 的分离菌对呋喃妥因和土霉素有抗药性。与海水样品相比,从沉积物中分离出来的细菌耐药性和重金属耐受性更高。这些细菌对铜的金属耐受性最高(58.3%),然后依次是锌(33.8%)、铅(32.1%)、铬(31%)和铁(25.2%)。结果表明,我们可以将来自沉积物和海水的细菌对抗生素和重金属的抗性视为细菌对(包括海洋污染在内的)环境影响的反应。

Antibiotic and Heavy Metal Resistant Bacteria Isolated from Aegean Sea Water and Sediment in Güllük Bay, Turkey

Quantifying the resistance of identified bacteria species with potential for environmental remediation applications

  • Gülşen Altuğ*
  • Department of Marine Biology, Faculty of Aquatic Sciences, Istanbul University, Balabanağa Mahallesi Ordu Caddesi No 8, Laleli, Fatih, Istanbul, 34134, Turkey
  • Mine Çardak
  • Department of Fisheries Technology, Faculty of Çanakkale Applied Sciences, Çanakkale Onsekiz Mart University, Terzioğlu Campus, Çanakkale, 17020, Turkey
  • Pelin Saliha Çiftçi Türetken
  • Department of Marine Biology, Faculty of Aquatic Sciences, Istanbul University, Balabanağa Mahallesi Ordu Caddesi No 8, Laleli, Fatih, Istanbul, 34134, Turkey
  • Samet Kalkan
  • Department of Marine Biology, Faculty of Fisheries and Aquatic Sciences, Recep Tayyip Erdoğan University, Zihni Derin Campus, Rize, 53100, Turkey
  • Sevan Gürün
  • Department of Marine Biology, Faculty of Aquatic Sciences, Istanbul University, Balabanağa Mahallesi Ordu Caddesi No 8, Laleli, Fatih, Istanbul, 34134, Turkey
  • *Email: galtug@istanbul.edu.tr
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Article Synopsis

Heavy metal and antibiotic-resistant bacteria have potential for environmental bioremediation applications. Resistant bacteria were investigated in sediment and seawater samples taken from the Aegean Sea, Turkey, between 2011 and 2013. Bioindicator bacteria in seawater samples were tested using the membrane filtration technique. The spread plate technique and VITEK® 2 Compact 30 micro identification system were used for heterotrophic aerobic bacteria in the samples. The minimum inhibition concentration method was used for heavy metal-resistant bacteria. Antibiotic-resistant bacteria were tested using the disk diffusion method. All bacteria isolated from sediment samples showed 100% resistance to rifampicin, sulfonamide, tetracycline and ampicillin. 98% of isolates were resistant against nitrofurantoin and oxytetracycline. Higher antibiotic and heavy metal resistance was recorded in bacteria isolated from sediment than seawater samples. The highest levels of bacterial metal resistance were recorded against copper (58.3%), zinc (33.8%), lead (32.1%), chromium (31%) and iron (25.2%). The results show that antibiotic and heavy metal resistance in bacteria from sediment and seawater can be observed as responses to environmental influences including pollution in marine areas.

1. Introduction

In the era of Industry 4.0, with global climate change, increasing population and developing technology, the spread of heavy metal pollutants in aquatic areas is increasing. Bacterial resistance and metal accumulation capability are common phenomena that can be exploited for the bioremediation of the environment, hence these resistant bacteria may be potential candidates for biotechnological applications. Despite the risks caused by antibiotic-resistant bacteria, heavy metal-resistant bacteria can be used in detoxification processes to convert a toxic form to a harmless form of a substance by developing biotransformation mechanisms. Bioremediation studies have been carried out to identify candidate species (14).

In recent years, the increase in pollution by toxic compounds and heavy metals in marine areas makes it increasingly important to study the relationships between bacteria and toxic compounds. Studies related to the transformation of compounds into different forms via bacterial metabolic processes for the removal of toxic substances from the environment have gained importance. Detection of bacteria that are resistant to heavy metals in natural environments constitutes the first step to provide data for remediation studies.

Bacteria are some of the most important components in marine ecosystems. Since bacteria adapt to new conditions created by environmental variables around them, knowledge of bacteria provides data in terms of defining environmental factors, public health status and ecosystem function. Marine areas are exposed to domestic and industrial wastes depending on local technology levels and population. Many xenobiotic micro pollutants, antibiotic derivatives and metabolites reach the sea from human activity. This concerning issue is considered an important factor for global health with respect to the evolution and detection of antibiotic resistance in bacterial pathogens (5). Since the spread of antimicrobial resistance is not restricted by phylogenetic, geographic or ecological borders, studies describing regional status of bacterial resistance in natural areas are important.

Antibiotic resistance can spread rapidly among bacterial species (6). It is known that the occurrence of antibiotic-resistant bacteria in natural environments reduces the effectiveness of antibiotics in the treatment of infectious diseases. Due to the increasing global resistance of bacteria against antibiotics, humanity is constantly being forced to develop new antibiotic derivatives. This vicious circle is one of the most important problems of our age and poses a threat for the future. Thus, it is important to know the resistance levels of bacteria and to produce regional antibiotic resistance profiles in natural areas. Aquatic environments constitute a way to disseminate not only antibiotic-resistant bacteria but also the resistant genes in natural bacterial habitats (7). It has been well documented that the aquatic environment is a potential reservoir of antibiotic-resistant bacteria, furthermore the prevalence and persistence of antibiotic resistance in bacterial pathogens is a threat and a source of considerable concern to public health (815). It is known that environmental factors such as overpopulation, livestock farming, insufficient drainage and sanitation infrastructure may provide hotspots for environmental antibiotic-resistant bacteria transmission (16).

In aquatic environments, antibiotic-resistant bacteria can be accompanied by heavy metal-resistant bacteria that are often induced by the presence of metal caused by anthropogenic activities and environmental factors (16, 17). Heavy metals are introduced into the marine environment in different ways. Accumulation in sediment can affect aquatic life negatively for a long time. Bacteria that will take part in the transformation of heavy metal salts into harmless forms must be resistant to the heavy metals. Bacteria that cannot adapt to the changes metabolically will be eliminated and therefore various pollution inputs accumulated in the sediment will affect the composition of microbial diversity. Sediments containing harmful, inorganic or organic particles are relatively heterogeneous in terms of physical, chemical and biological properties and are an important source of heavy metal contamination (11). It has been reported that microplastics mediate the spread of metal- and antibiotic-resistant pathogens due to their ability to adsorb various pollutants (18, 19). Bacteria resistant to heavy metals in marine areas have developed various resistance mechanisms to counteract heavy metal stress. Only bacteria that can withstand the current heavy metal concentration can survive in these areas.

Heavy metals accumulate in biota via food chains and are transferred between organisms in marine environments. This cumulative process, named biomagnification, is higher in the sea than in terrestrial environments (15, 20) and this implies significant effects of heavy metal pollution in marine areas. On the other hand, heavy metal-resistant bacteria can play a role in detoxification by converting a toxic form into a harmless form through biotransformation mechanisms that develop in natural environments. These mechanisms include the formation and sequestration of heavy metals in complexes and the reduction of a metal to a less toxic species (21, 22).

Metal-resistant bacteria have developed very efficient and varying mechanisms for tolerating high levels of toxic metals and thus they carry an important potential for controlling heavy metal pollution (23). In many prokaryotes, it has been shown that the mechanism for resisting heavy metals develops over time. This process has been studied in species such as Escherichia coli and Staphylococcus aureus. It is reported that many different species of Pseudomonas, Bacillus, Enterobacter, Providencia and Chryseobacterium are efficient for reducing heavy metals (14).

It is known that the occurrence of bacteria resistant to antibiotics and heavy metal salts in the sea is related to the pollutants present in the environment. For the reasons highlighted above, it is important to determine the profile of antibiotic and heavy metal-resistant bacteria in marine environments. Marine areas which have different environmental inputs present novel media for bacterial studies.

For the present study, the Güllük Bay of the Aegean Sea, Turkey, was chosen since it is a dynamic area due to marine transportation, seasonal population growth depending on tourism, aquaculture, recreational and agricultural activities and terrestrial pollution inputs transported from rivers.

Probable faecal source analysis conducted in Güllük Bay showed that the primary source of the detected bacteriological pollution is anthropogenic (24). A significant part of domestic wastewater in the region collects in sealed septic tanks. It is possible for the wastewater to reach the sea by mixing the sedimentary septic tanks with groundwater. Chemical and biological studies (2433) confirm that regional pollutants have reached Güllük Bay.

It is well known that sewage transported via domestic wastewater carries antibiotics to marine environments. This has an effect on metabolic capabilities of bacteria in marine environments. For example, β-lactam antibiotic derivatives used for human infection treatment may enter marine environments via domestic wastewater. Bacteria may obtain resistance via intercellular contact mostly using a conjugation mechanism (34). The existence of antibiotic-resistant bacteria is an indicator of domestic pollution. Furthermore, antibiotic-resistant bacteria may cause a vicious cycle. This problem has grown in recent years due to systematic use of antibiotics in animal husbandry and overuse of antibiotics (35, 36).

The frequencies of heavy metal-resistant bacteria and antibiotic-resistant bacteria were investigated in seawater and sediment samples collected from Güllük Bay in the period between May 2011 and February 2013.

2. Material and Methods

2.1 Sampling Area

Güllük Bay is an important location due to its natural resources. The region is open to different environmental influences and inputs due to tourism, port activities, marine transportation, domestic and industrial wastes and fish farms. The bay is also affected by the presence of Sarıçay Creek, Kazıklı Port, Güllük Port and Akbük Port (2426). Fish farms were operated in Güllük Bay until 2008. Although they have been relocated away from the coastal regions to an offshore area, the indirect effects of this long-time pollution may have contributed to the sediment.

The export of feldspar and bauxite from the region has been conducted from ports within the borders of Güllük Bay. The port is mainly used by dry cargo and other cargo-type ships. It is reported that an annual average of 800,000 tonnes of ballast water is transported to the bay from 157 different ports. The amount of ballast water carried is reported as: 68% from the Mediterranean, 21% from the Aegean Sea, 7% from the Sea of Marmara, 2% from the Atlantic Ocean and 1% from the Black Sea and Red Sea, respectively (37). The operation of many tourism-oriented boats in Güllük Bay is also among the possible polluters of the bay due to bilge water and wastewater. More than half of Turkey’s sea bream and sea bass production was in farms operating in the coastal areas of the Güllük Bay for many years. These farms have been operating in the offshore areas of the region for the past 10 years. The domestic wastewater of the human population, reaching approximately 50,000 around the region in the summer months, and the wastes of small industrial establishments such as yogurt, yeast and olive oil producers that directly reach streams are the other main sources of pollution in Güllük Bay. The population of the Bodrum peninsula, which is 25,000 in winter, can reach 1,500,000 in summer (27). The change in the population between the seasons was among the biggest pollution sources according to the terrestrial bioindicator bacteria distribution in coastal areas in the region (24, 26).

Sampling stations were selected to represent tourist areas (G1, G5, G7, G8); harbours (G4, G6); fresh water entry-exit points of the Sarıçay Creek (G9); fish farms (G11, G12, G13); and the deepest point in the bay as a reference station (G14). Figure 1 shows the location of Güllük Bay and the sampling stations.

Fig. 1

Location of Güllük Bay and seawater (0–30 cm surface, mid-point and bottom-point) and sediment sampling stations

Location of Güllük Bay and seawater (0–30 cm surface, mid-point and bottom-point) and sediment sampling stations

2.2 Sampling

Seawater and sediment samples were collected from 12 different sampling stations in Güllük Bay between May 2011 and February 2013. Three units of seawater samples were taken from each station at surface (0–30 cm), mid-point and bottom-point water (Figure 1). In each sampling process covering 12 stations, 36 seawater samples were collected. In the spring and summer months monthly, at other times seasonally, a total of 432 seawater samples were collected in the period between May 2011 and February 2013. The seawater samples were collected using a Nansen bottle that was cleaned with acid (10% HCl in distilled water), sterilised with alcohol (50:50, v/v) and rinsed with sterile water. The seawater samples were then transferred into brown sterile glass bottles and transported to the laboratory as a cold chain.

Surface sediment samples were collected using Ekman grab (HYDRO-BIOS Apparatebau GmbH, Germany, 15 × 15) from the sampling stations which have various depths from 8 m to 66 m (Figure 1). A total of 144 units of sediment samples were collected during the two-year study from 12 stations (one from each station). The sediment samples were transferred into sterile zip seal bags from Ekman grab and transferred in the cold chain to the laboratory.

2.3 Bacterial Isolation and Identification

Bacterial heavy metal and antibiotic resistance were tested in heterotrophic aerobic bacteria isolated from seawater and sediment samples. Heavy metal and antibiotic resistance of indicator bacteria (faecal coliform, coliform and faecal Streptococcus) isolated from the seawater samples were also tested.

2.3.1 Seawater Samples

Indicator bacteria and heterotrophic aerobic bacteria analyses were performed on the seawater samples. The membrane filtration technique was used to detect indicator bacteria. A sample containing 300 ml seawater was diluted serially (10−5 dilution) and filtered through membrane filters (0.45 μm, Sartorius AG, Göttingen, Germany). The filters were placed on m-Endo, m-FC and m-Azide media (Sartorius AG). The plates were incubated for 24 h (at 37 ± 0.1°C; at 44 ± 0.1°C for m-FC). Brown‐red colonies growing on the azide medium were considered as suspicious faecal Streptococcus, blue colonies growing on the m-FC medium as suspicious faecal coliform and yellow-green colonies with yellow-metallic gloss on the m-Endo medium as suspicious coliform. Cytochrome oxidase test (API® 20 Strep, bioMérieux, France) was performed on suspicious coliform colonies and oxidase negative colonies were evaluated numerically. Cytochrome oxidase (API® 20 Strep, bioMérieux) and indole tests were performed on the suspicious colonies of faecal coliform.

Colonies with oxidase negative and indole positive results were evaluated as faecal coliform. Suspicious Streptococcus colonies, to which the catalase test was applied (1 ml, 3% H2O2), were incubated on Bile Esculin Agar (BEA) for 18 h at 37°C for esculin hydrolysis and 40% bile resistance control. Blackening in the medium and the formation of black shadow around the colony, positive of esculin hydrolysis, and the number of colonies showing growth in the medium were evaluated as 40% bile resistant, and catalase negative and breeding colonies in BEA were evaluated as faecal Streptococcus. Counted colonies were multiplied by the 10−5 dilution factor to determine the number of colony forming units (CFU) 100 ml−1 in the original sample (38).

The spread plate technique was used for heterotrophic aerobic bacteria analyses in seawater. Seawater samples 0.1 ml with 10−5 dilution were used for duplicate spreading on the DifcoTM Marine Agar 2216 (Becton, Dickinson and Company, USA) and the plates were incubated for five days at 22 ± 0.1°C. At the end of the incubating period, counted colonies were multiplied by the 10−5 dilution factor to determine the number of CFU ml−1 in the original sample. An average of 10 different colonies were picked and restreaked several times to obtain pure cultures. The pure isolates were Gram-stained. For identification of spore-forming bacilli, the isolates were stained with Indian ink according to the negative staining technique and were evaluated using a light microscope (Nikon E110, Nikon, Japan). The isolates were then tested using Gram‐negative fermenting and non‐fermenting bacilli (GN), Gram-positive cocci and non-spore-forming bacilli (GP) and Gram‐positive spore-forming bacilli (BCL) cards in the automated micro identification system VITEK® 2 Compact 30 (bioMérieux) (39).

2.3.2 Sediment Samples

The spread plate technique was used for heterotrophic aerobic bacteria analyses in sediment samples. Each sediment sample was mixed and homogenised. Then 1 g sample was taken from each and serially diluted with sterile commercial seawater. 0.1 ml samples of 10−5 dilutions were taken and spread on DifcoTM Marine Agar 2216. The plates were incubated for five days at 22 ± 0.1°C. Growing colonies were evaluated as CFU g−1 (40). Further processes related to heterotrophic bacteria identification were continued by using VITEK® 2 Compact 30 similarly to the seawater samples described above.

2.4 Bacterial Resistance Against Antibiotics

The antibiotic resistance of the isolates was examined by the Kirby–Bauer method with slight modifications. Two or three colonies of each isolate were suspended with 5 ml of DifcoTM Marine Broth 2216 and diluted with sterile water against the 0.5 McFarland turbidity standard to approximately 106 cells ml−1 and swabbed as 2 ml on DifcoTM Marine Agar 2216. Antibiotic discs (Oxoid, UK) containing ampicillin (10 μg), nitrofurantoin (300 μg), oxytetracycline (30 μg), sulfonamide (300 μg), rifampicin (2 μg), tetracycline (10 μg) and tetracycline (30 μg) were incubated for two to three days at 37°C. The results were interpreted according to the guidelines of the Clinical Laboratory Standard Institute (CLSI) (41). All isolates that showed resistance were classified as ‘resistant’. Other isolates that did not show resistance were classified as ‘sensitive’ or ‘susceptible’.

2.4.1 Multiple Antibiotic Resistance

The multiple antibiotic resistance (MAR) index of a given sample was calculated by the equation: a/(bc), where a represents the aggregate antibiotic resistance score of all isolates from a sample; b is the total number of isolates; and c is the number of isolates from a sample (42). Bacterial isolates that displayed resistance to three or more antibiotic agents were designated as multiple antibiotic resistant (ranging from two to 10).

2.5 Bacterial Resistance Against Heavy Metal Salts

Different concentrations (50 μg ml−1, 100 μg ml−1, 150 μg ml−1, 200 μg ml−1 and 250 μg ml−1) of heavy metal salts (FeSO4, ZnSO4, CuSO4, Cr2(SO4)3 and Pb(NO3)2) were used to test the bacteria resistivity against iron, zinc, copper, chromium and lead. The microdilution method was followed with minor modifications to determine the resistance of isolates to heavy metals (43). Stock solutions of metal salts prepared in distilled water were sterilised by filtration (0.20 μm). In U-well microtiter plates, serial dilutions of heavy metals were prepared and then each well was inoculated with bacteria inoculation. The OxoidTM Turbidometer (Thermo Fisher Scientific Inc, USA) provides the inoculum density standardisation for 0.5 McFarland which is necessary to ensure accurate reproducible results. Before the addition of bacterial inoculation, no precipitation was seen. The plates were incubated at 37°C for 24 h and then examined for visual turbidity. The lowest concentration of the metal salt, at which growth was inhibited (indicated by lack of turbidity), was taken as the minimum inhibitory concentration (MIC) (44) Samples of 10 μl were drawn from each well without turbidity and were subcultured on agar plates to determine bactericidal concentration.

Reference strains of Escherichia coli (ATCC® 25922TM), Salmonella enterica (ATCC® 2577TM) and Staphylococcus epidermidis (ATCC® 12228TM) which are susceptible to Cu2+, Zn2+, Pb2+, Cr2+ and Fe3+ and metal-free plates were used in the control tests to evaluate the viability of the strains and culture media. All of the experiments were carried out in triplicate.

3. Results

3.1 Bacterial Resistance Against Antibiotics

Table I shows the antibiotic-resistant, intermediate or susceptible bacteria species isolated from the seawater and sediment samples in this study.

Table I

Antibiotic Resistant, Intermediate or Susceptible Bacteria Species Isolated from Seawater and Sediment

Sample Order/class tested (%) Bacterial isolates tested (n) Antibioticsa
AM (10 μg) TE (30 μg) S (300 μg) TE (10 μg) RD (2 μg) F/M (300 μg) OT (30 μg)
Seawater Proteobacteria/Alpha proteobacteria (27%) Brevundimonas diminuta (3) R: 66.7%I: 33.3%S: 0.0% R: 33.3%I: 0.0%S: 66.7% R: 100%I: 0.0%S: 0.0% R: 33.3%I: 33.3%S: 33.3% R: 100%I: 0.0%S: 0.0% R: 66.7%I: 0.0%S: 33.3% R: 66.7%I: 33.3%S: 0.0%
Brevundimonas vesicularis (4) R: 75%I: 0.0%S: 25% R: 50%I: 0.0%S: 50% R: 100%I: 0.0%S: 0.0% R: 50%I: 0.0%S: 50% R: 100%I: 0.0%S: 0.0% R: 50%I: 0.0%S: 50% R: 25%I: 0.0%S: 75%
Sphingomonas paucimobilis (38) R: 71.05%I: 2.63%S: 26.31% R: 31.57%I: 5.26%S: 63.15% R: 97.38%I: 0.0%S: 2.63% R: 42.10%I: 7.89%S: 50% R: 97.36%I: 0.0%S: 2.63% R: 60.52%I: 0.0%S: 39.47% R: 26.31%I: 34.21%S: 39.47%
Sphingomonas thalpophilum (4) R: 50%I: 0.0%S: 50% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0%I: 25%S: 75% R: 100%I: 0.0%S: 0.0% R: 75%I: 0%S: 25% R: 25%I: 25%S: 50%
Proteobacteria/Beta proteobacteria (%) Burkholderia cepacia (3) R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 0.0%I: 0.0%S: 100%
Burkholderia mallei (3) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Neisseria animaloris (3) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0% S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Proteobacteria/Gamma proteobacteria (53%) Acinetobacter lwoffii (3) R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 100%S: 0.0%
Aeromonas hydrophila (4) R: 100%I: 0.0%S: 0.0% R: 50%I: 0%S: 50% R: 100%I: 0.0%S: 0.0% R: 50%I: 0.0%S: 50% R: 50%I: 0.0%S: 50% R: 50%I: 0.0%S: 50% R: 0.0%I: 50%S: 50%
Aeromonas salmonicida (4) R: 50%I: 0.0%S: 50% R: 50%I: 0.0%S: 50% R: 100%I: 0.0%S: 0.0% R: 50%I: 0.0%S: 50% R: 100%I: 0.0%S: 0.0% R: 50%I: 0.0%S: 50% R: 50%I: 0.0%S: 50%
Aeromonas sobria (3) R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 100%S: 0.0%
Aeromonas veronii (3) R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0% S: 100%
Citrobacter sedlakii (3) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Cronobacter dublinensis subsp. lausannensis (3) R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Enterobacter aerogenes (6) R: 33.3%I: 33.3%S: 33.3% R: 33.3%I: 0.0%S: 66.6% R: 66.6%I: 0.0%S: 33.3% R: 66.6%I: 0.0%S: 33.3% R: 100%I: 0.0%S: 0.0% R: 33.3%I: 0.0%S: 66.6% R: 33.3%I: 66.6%S: 0.0%
Enterobacter cloacae subsp. dissolvens (4) R: 100%I: 0.0%S: 0.0% R: 0.0%I: 100%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100%
Enterobacter cloacae (4) R: 0.0%I: 50%S: 50% R: 0.0%I: 0.0%S: 100% R: 0.0%I: 100%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100%
Enterobacter cloacae complex (4) R: 100%I: 0.0%S: 0.0% R: 50%I: 0.0%S: 50% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 50%I: 50%S: 0%
Escherichia coli (30) R: 78.5%I: 10.7%S: 10.7% R: 71.4%I: 10.7%S: 17.8% R: 92.9%I: 3.5%S: 3.5% R: 89.3%I: 0.0%S: 10.7% R: 100.0%I: 0.0%S: 0.0% R: 100.0%I: 0.0%S: 0.0% R: 75%I: 0.0%S: 25%
Klebsiella pneumoniae subsp. ozaenae (3) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Pasteurella canis (3) R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 0.0%I: 0.0%S: 100%
Proteus vulgaris group Proteus penneri (3) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Pseudomonas aeruginosa (13) R: 100%I: 0.0%S: 0.0% R: 90.91%I: 0.0%S: 9.09% R: 100%I: 0.0%S: 0.0% R: 90.91%I: 0.0%S: 9.09% R: 100%I: 0.0%S: 0.0% R: 90.91%I: 0.0%S: 9.09% R: 90.91%I: 0.0%S: 9.09%
Raoultella ornithinolytica (3) R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 0.0%I: 0.0%S: 100%
Raoultella ytica (3) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Serratia marcescens (5) R: 100%I: 0.0%S: 0.0% R: 66.6%I: 0.0%S: 33.4% R: 100%I: 0.0%S: 0.0% R: 66.6%I: 0.0%S: 33.4% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 66.6%I: 0.0%S: 33.4%
Shewanella putrefaciens (13) R: 81.81%I: 0.0%S: 18.18% R: 45.45%I: 18.18%S: 36.36% R: 100%I: 0.0%S: 0.0% R: 72.72%I: 0.0%S: 27.27% R: 100%I: 0.0%S: 0.0% R: 63.63%I: 0.0%S: 36.36% R: 54.54%I: 36.36%S: 9.06%
Stenotrophomonas maltophilia (7) R: 80%I: 0.0%S: 20% R: 40%I: 0.0%S: 60% R: 100%I: 0.0%S: 0.0% R: 20%I: 50%S: 50% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 20%I: 0.0%S: 80%
Vibrio vulnificus (4) R: 50%I: 0.0%S: 50% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 0.0%I: 50%S: 50%
Enterococcus faecium (3) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Firmicutes/Bacilli (9%) Alicyclobacillus acidocaldarius (3) R: 0.0%I: 100%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 100%S: 0.0%
Bacillus cereus (7) R: 100%I: 0.0%S: 0.0% R: 60%I: 0.0%S: 40% R: 100%I: 0.0%S: 0.0% R: 80%I: 20%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 60%I: 20%S: 20%
Bacillus pumilus (5) R: 66.7%I: 0.0%S: 33.3% R: 66.7%I: 0.0%S: 33.3% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 66.7%I: 0.0%S: 33.3% R: 66.7%I: 0.0%S: 33.3%
Staphylococcus xylosus (3) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Staphylococcus aureus (3) R: 0.0%I: 0.0%S: 100% R: 0.0%I: 0.0%S: 100% R: 0.0%I: 100%S: 100% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 0.0%
Staphylococcus warneri (5) R: 33.3%I: 0.0%S: 66.7% R: 33.4%I: 0.0%S: 66.7% R: 66.7%I: 0.0%S: 33.3% R: 66.7%I: 0.0%S: 33.3% R: 33.3%I: 66.7%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 33.3%I: 0.0%S: 66.7%
Bacteroidetes/Flavobacteriia (8%) Chryseobacterium indologenes (13) R: 54.54%I: 18.18%S: 27.27% R: 36.36%I: 0.0%S: 63.63% R: 100%I: 0.0%S: 0.0% R: 45.45%I: 0.0%S: 54.54% R: 100%I: 0.0%S: 0.0% R: 54.54%I: 0.0%S: 45.45% R: 45.45%I: 18.18%S: 36.36%
Myroides spp. (5) R: 100%I: 0.0%S: 0.0% R: 66.6%I: 0.0%S: 33.3% R: 100%I: 0.0%S: 0.0% R: 66.6%I: 0.0%S: 33.3% R: 100%I: 0.0%S: 0.0% R: 66.6%I: 0.0%S: 33.3% R: 66.6%I: 0.0%S: 33.3%
Actinobacteria/Actinomycetales (3%) Dermacoccus nishinomiyaensis (3) R: 0.0%I: 0.0%S: 100% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 0.0%I: 0.0%S: 100%
Kocuria kristinae (4) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 50%I: 0.0%S: 50% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 50%I: 0.0%S: 50%
Kocuria varians (3) R: 0.0%I: 100%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 0.0%I: 0.0%S: 100% R: 0.0%I: 1000%S: 0.0%
Micrococcus luteus (4) R: 50%I: 0.0%S: 50% R: 50%I: 0.0%S: 50% R: 100%I: 0.0%S: 0.0% R: 50%I: 0.0%S: 50% R: 100%I: 0.0%S: 0.0% R: 50%I: 0.0%S: 50% R: 0.0%I: 100%S: 0.0%
Sediment Proteobacteria/Alpha proteobacteria (7%) Brevundimonas diminuta (1) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Sphingomonas paucimobilis (1) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Sphingomonas thalpophilum (1) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Proteobacteria/Beta proteobacteria (7%) Neisseria animaloris (3) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Chromobacterium violaceum (1) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Proteobacteria/Gamma proteobacteria (43%) Aeromonas caviae (1) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Aeromonas sobria (1) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Pseudomonas aeruginosa (1) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Serratia marcescens (5) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Shewanella algae (15) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Shewanella putrefaciens (11) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Vibrio alginolyticus (14) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 66.7%I: 0.0%S: 33.3% R: 66.7%I: 0.0%S: 33.3%
Vibrio fluvialis (11) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Vibrio parahaemolyticus (13) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Vibrio vulnificus (11) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Firmicutes/Bacilli (34%) Alicyclobacillus acidoterrestris (11) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Bacillus cereus (23) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Bacillus pumilus (11) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Lactococcus garvieae (13) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Bacteroidetes/Flavobacteriia (7%) Chryseobacterium indologenes (12) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Myroides spp. (12) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%
Actinobacteria/Actinomycetales (2%) Micrococcus lylae (11) R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0% R: 100%I: 0.0%S: 0.0%

aAmpicillin (AM, 10 μg), nitrofurantoin (F/M, 300 μg), oxytetracycline (OT, 30 μg), sulfonamide (S, 300 μg), rifampicin (RD, 2 μg), tetracycline (TE, 10 μg) and tetracycline (TE, 30 μg). Resistant (R), intermediate (I) or susceptible (S)

Bacterial species isolated from the seawater samples showed considerable resistance to rifampicin (98%), sulfonamide (98%) and ampicillin (76%) and considerable sensitivity to tetracycline-30 μg (52%), tetracycline-10 μg (39%) and oxytetracycline (33%). Almost all the bacterial species isolated from sediment samples showed resistance to rifampicin (100%), sulfonamide (100%), ampicillin (100%), nitrofurantoin (98%), tetracycline-30 μg (100%), tetracycline-10 μg (100%) and oxytetracycline (98%) while they showed almost no sensitivity to antibiotics except nitrofurantoin (2%) and oxytetracycline (2%). Pseudomonas aeruginosa (24%) and Sphingomonas paucimobilis (20%), isolated from seawater samples, showed higher resistance to antibiotics than did Raoultella oxytica, Staphylococcus xylosus, Kocuria kristinae, Aeromonas salmonicida and Proteus vulgaris strains. On the contrary, Aeromonas caviae, Alicyclobacillusacidoterrestris, Brevundimonas diminuta, Chryseobacterium indologenes, Lactococcus garvieae, Neisseria animaloris, Pseudomonas aeruginosa, Serratia marcescens, Shewanella algae and Vibrio parahaemolyticus isolates from the sediment samples showed resistance to all antibiotics (Table I).

The highest number of antibiotic-resistant bacteria were detected from the sediment samples. The frequency of resistant bacteria (%) to oxytetracycline (30 μg), nitrofurantoin (300 μg), rifampicin (2 μg), tetracycline (10 μg), tetracycline (30 μg), sulfonamide (300 μg) and ampicillin (10 μg) from the seawater and sediment samples are shown in Figure 2. The frequencies of antibiotic resistance in bacteria species from seawater and sediment samples are shown in Figure 3.

Fig. 2

The frequency of bacteria resistant to specific antibiotics (%) in the seawater and sediment samples

The frequency of bacteria resistant to specific antibiotics (%) in the seawater and sediment samples

Fig 3

The frequencies of antibiotic resistance in bacteria species from (a) seawater samples and (b) sediment samples

The frequencies of antibiotic resistance in bacteria species from (a) seawater samples and (b) sediment samples

A total of 258 and 158 isolates were tested against antibiotics from seawater and sediment samples, respectively. The frequencies of resistance against seven antibiotics in bacteria species isolated from the seawater samples were recorded as 49% in Gammaproteobacteria, 22% in Alphaproteobacteria, 3% in Betaproteobacteria, 14% in Bacilli, 8% in Flavobacteriia and 4% in Actinomycetales. The resistance frequencies against seven antibiotics in bacteria isolated from the sediment samples were recorded as 43% in Gammaproteobacteria, 34% in Bacilli, 7% in Alphaproteobacteria, 7% in Betaproteobacteria, 7% in Flavobacteriia and 2% in Actinomycetales.

3.2 Multiple Antibiotic Resistance Indexes

The MAR index was calculated for each of the antibiotic-resistant bacteria. If the MAR index is lower than 0.2, it shows a non-point based source of pollution and if it is higher than 0.2 it shows point‐based pollution and a high risk of contamination by excessive antibiotic presence (23). Table II shows the MAR indexes.

Table II

Multiple Antibiotic Resistance Indexes and Resistance Ratios

Number of antibiotics to which bacteria show resistance MAR Index Resistance, % p-value
1 0.0052 0.75187 0.3635
2 0.0104 18.0451 0.6513
3 0.0022 12.7819 0.1323
4 0.0294 12.7819 0.1325
5 0.048 9.7744 0.3635
6 0.0576 14.2857 0.4084
7 0.0208 31.57894 0.1234

The MAR indexes of the study showed possible exposure of these bacterial isolates to the tested antibiotics. The MAR index of bacteria isolated from all stations around fish farm areas (0.0576) was 2.6 times greater than the MAR index for the combined non-fish farm areas (0.022).

3.3 Bacterial Resistance Against Heavy Metals

The frequencies of heavy metal resistance in the bacteria species isolated from the seawater samples were recorded as 76.72% in Gammaproteobacteria, 71.82% in Alphaproteobacteria, 80.01% in Bacilli, 56.92% in Flavobacteriia and 75% in Actinomycetales. The frequencies of resistance to Cu2+, Zn2+, Pb2+, Cr2+ and Fe2+ were detected as an average of 58.3%, 33.8%, 32.1%, 31.0% and 25.2% respectively in 258 bacterial strains isolated from seawater samples.

The frequencies of heavy metal resistance in bacteria species isolated from the sediment samples were recorded as 100% in Alphaproteobacteria, 100% in Betaproteobacteria, 97.5% in Flavobacteriia, 95% in Gammaproteobacteria, 72.5% in Bacilli and 66.6% in Actinomycetales. The frequencies of resistance to Cu2+, Zn2+, Pb2+, Cr2+ and Fe2+ were detected as an average of 33.3%, 30.3%, 25.5%, 35.3% and 28.4% respectively in 158 strains isolated from the sediment samples.

The frequencies of heavy metal-resistant bacteria isolated from sediment samples were higher than the frequencies of heavy metal-resistant bacteria isolated from the seawater samples. Table III shows the heavy metal resistance in bacteria isolates from seawater and sediment in Güllük Bay.

Table III

Heavy Metal Resistance in Bacteria Species from Seawater and Sediment in Güllük Bay, Turkey

Heavy metals Sampling sides Metal concentrations, μg ml−1 Isolates Resistant isolates
0.8 1.6 3.1 6.5 12.5 25 50 100 200 >200 n n %
Cu2+ Seawater 3 3 3 7 8 9 10 11 6 258 149 58.3
Sediment 7 4 9 3 6 17 17 29 11 158 53 33.3
Zn2+ Seawater 3 6 4 9 8 7 11 8 2 258 86 33.8
Sediment 4 2 2 8 4 19 36 22 6 158 48 30.3
Pb2+ Seawater 1 8 2 7 7 10 11 12 2 258 82 32.1
Sediment 1 9 4 4 3 18 13 23 17 158 40 25.5
Cr2+ Seawater 3 2 7 4 6 7 12 6 4 16 258 79 31.0
Sediment 3 4 5 8 4 9 11 10 27 32 158 56 35.3
Fe2+ Seawater 1 2 9 24 15 6 258 67 25.2
Sediment 3 13 3 5 9 15 24 29 158 45 28.4
Total number of tested isolates 416

The MICs of the isolates ranged from 0.004 mM to 2.5 mM. The isolates from sediment samples obtained from stations close to fish farms showed higher frequency of resistance against chromium, copper and zinc than other stations. The highest resistance (MIC value: 2.5 mM) was displayed against Cr+ by all isolates. Bacillus isolates showed a higher resistance to chromium, lead and copper than Pseudomonas isolates, and Vibrio isolates showed higher resistance to zinc, copper and chromium than Escherichia coli. Tolerance to the maximum MIC (>2.5 mM) for chromium was 10.1% for Bacillus and 0.8% for Pseudomonas isolates. Bacillus isolates from sediment samples showed higher resistance to chromium, lead, iron and copper than Klebsiella spp. and Escherichia coli strains from seawater samples. Similarly, Shewanella spp. and Serratia spp. strains from the sediment samples also showed higher resistance than the species mentioned above.

4. Discussion

Indicator bacteria levels reported in Güllük Bay and the presence of pathogenic bacteria (25, 26) support the relationship between the resistance data detected in the current study with bacteriological pollution levels. In the present study, bioindicator bacteria showing human-induced pollution input isolated from seawater had the highest frequency of resistance against nitrofurantoin (100%) and sulfonamide (95%). Sulfonamides were the first antibiotics developed for clinical use. Sulfonamides have been widely used to treat bacterial and protozoan infections in humans, domestic animals and fish since their introduction to clinical practice in 1935 (4547). The results of higher resistance against sulfonamide in the present study were similar to the findings of sulfonamide resistance in another study (48). For example, there were significant increases in numbers of bacteria resistant to oxytetracycline, oxolinic acid and florfenicol in sediments from an aquaculture site compared with those from a non-aquaculture control site. Interestingly, in another study a similar number of antibiotic-resistant bacteria were isolated from aquaculture and non-aquaculture sites (49). Gram-negative bacteria (predominantly Plesiomonas shigelloides and Aeromonas hydrophila) were isolated from aquaculture ponds in the south-eastern USA and it was reported that antibiotic resistance to tetracycline, oxytetracycline, chloramphenicol, ampicillin and nitrofurantoin were higher in antibiotic-treated ponds compared to non-treated rivers (50). It was determined that bacteria isolated from Sopot Beach, Poland, were resistant to ampicillin (51). A high percentage of bacteria were reported as resistant to streptomycin (100%), cefazolin (89.8%), ampicillin (83.7%) and trimethoprim-sulfamethoxazole (69.4%), whereas a low percentage of bacteria were resistant to cefepime (12.3%) and meropenem (14.3%) in the aquaculture region of İskenderun Bay, Turkey (52).

In the current study, higher numbers of sulfonamide, rifampicin and ampicillin-resistant bacteria were recorded in the stations around aquaculture areas than other stations. Sphingomonas paucimobilis, Escherichia coli and Enterobacter cloacae isolated from both seawater and sediment at the stations around aquaculture areas had the highest levels of antibiotic resistance. The development of resistant pathogens in aquaculture environments is well documented (53, 54) and evidence of transfer of resistance encoding plasmids between aquaculture environments and humans has been presented recently (55). It has been reported that antibiotic‐resistant bacteria are present in a seafood ecosystem where antibiotics have never been used (56). This is interesting in terms of showing that aquaculture areas may be adversely affected by the presence of environmental antibiotic-resistant bacteria.

In the present study, a high percentage of the bacteria Sphingomonas paucimobilis were isolated, which was especially prevalent in Güllük Bay. The natural habitat of Sphingomonas has not been defined, but it is widely distributed in the natural environment especially in water and soil (57). The second most prevalent species were Escherichia coli and Enterobacter cloacae. Escherichia coli is an indicator of faecal contamination in aquatic environments. Enterobacter cloacae is the most frequent species associated with nosocomial infections along with Klebsiella pneumoniae that is a growing problem in human healthcare. The highest number of Bacillus cereus was isolated from the sediment underneath fish farms. A few Bacilli of marine origin have been reported to produce unusual metabolites different from those isolated from terrestrial bacteria (58). Due to the ubiquity and ability of the Bacillus species to survive under difficult circumstances, Bacillus strains are considered to be species of certain habitats (59, 60). In the current study, Bacillus pumilus, B. thuringiensis, B. mycoides and B. cereus were isolated from the sediment samples of the stations around fish farms.

The high frequency of resistance among bacterial isolates in the present study confirms the earlier reports regarding the role of antimicrobial use that plays a role in selecting antibiotic-resistant bacteria in water and aquatic sediments (4652). Many previous studies have shown that the increases in antibiotic resistance in human medicine, agriculture and aquaculture are directly related to the amounts of antimicrobials used (6165).

Infections caused by antibiotic-resistant bacteria are one of the most important public health concerns worldwide. Currently, MARs have been reported in a wide range of human pathogenic or opportunistic bacteria such as Vibrio sp. (66), Klebsiella pneumoniae (67), Salmonella sp. (68), Pseudomonas aeruginosa and also in pathogens (69, 70). Reservoirs of antibiotic resistance can interact between different ecological systems and potential transfer of resistant bacteria or resistant genes from animals to humans may occur through the food chain (70). In the current study, the MAR index of multiple antibiotic-resistant bacteria was found to be 2.6 times greater in the stations around fish farm areas (0.057) than the other stations (0.022).

Marine sediments offer more informative results than seawater about environmental pollution due to the accumulation of various pollutants at the bottom of the sea, therefore analysis of sediments is widely used in tests. The association of microorganisms with sediment particles is one of the primary factors in assessing microbial fate in aquatic systems. In this study, the bacteria isolated from sediment in all samples showed a higher resistance rate than bacteria isolated from seawater. Detection of higher antibiotic resistance in sediment bacteria than bacteria isolated from seawater showed that sediment bacteria were exposed to more antibiotics. Natural ecosystems containing high concentrations of heavy metals are also frequent. Heavy metal resistance genes are commonly found in environmental bacteria (71). The resistance to seven heavy metals has been reported in the order Cu > Mn > Ni > Zn > Pb > Cd > Fe for seawater bacteria isolated from the Golden Horn, Istanbul, Turkey (17). Heavy metal resistance in bacteria found in seawater from the Mediterranean has been reported as Cd > Cu > Cr = Pb > Mn; in Karataş, Turkey Cd > Cu > Cr = Mn > Pb; and İskenderun Bay, Cu > Cd > Mn > Cr > Pb (72).

In the present study, resistance to five different heavy metals (Zn2+, Pb2+, Cu2+, Cr3+ and Fe3+) were investigated for all isolates. Trends in heavy metal resistance vary depending on the sample sites: Güllük Bay, fish farm water column: Cu > Zn > Pb > Cr > Fe; sediment: Cr > Cu> Zn > Fe > Pb. Frequency of bacteria resistance to heavy metals shows the direct effects of metal pollution. Neisseria animaloris, Aeromonas caviae and Bacillus cereus isolated from sediment samples were the most tolerant of all the heavy metal salts. Chryseobacterium indologenes displayed the highest degree of sensitivity to all metal salts while Lactococcus garvieae showed the highest degree of sensitivity to Zn2+, Pb2+, Cu2+ and Fe3+. Kocuria kristinae, Escherichia coli and Acinetobacter lwoffii, which were isolated from the seawater underneath the fish farm, displayed similar sensitivities to all tested heavy metal salts. Resistances to heavy metals for Aeromonas and Pseudomonas isolates were similar to those from İskenderun Bay, with cadmium, 35.0% and 56.5%; copper, 98.3% and 75.4%; chromium, 38.3% and 31.9%; lead, 1.7% and 7.2%; manganese, 43.3% and 44.9%; and zinc 35.0% and 41.3%, respectively (72).

Both Gram-positive and Gram-negative bacteria can resist heavy metals (73). Resistance to toxic metals in bacteria probably reflects the level of environmental contamination with these substances and it may be related to the concentration of bacteria (74). The present project found heavy metal pollution in Güllük Bay sediment samples at all stations. In the sediment samples, the heavy metal contents were reported at varying rates: between 1 μg g−1 and 209 μg g−1 for lead; 10 μg g−1 and 259 μg g−1 for zinc; 1 μg g−1 and 59 μg g−1 for copper; 0.1 μg g−1 and 46 μg g−1 for chromium; <0.01 μg g−1 and 2.8 μg g−1 for cadmium; <0.01 μg g−1 and 0.4 μg g−1 for arsenic; and 0.6% and 5.9% for aluminium, respectively. The region was defined according to cadmium, lead and zinc levels as moderately polluted. Recorded high metal values were evaluated as an indicator of domestic and industrial inputs, carried via Sarıçay Creek, port operations and tourism activities within Güllük Bay (75). In the current study, the high frequencies of heavy metal-resistant bacteria detected in the sediment samples support this data. Bacterial heavy metal resistance detected in the study may depend on many factors. A possible explanation for differences in heavy metal resistance is the proximity of Güllük Bay to iron-steel factories. Additionally, Güllük Harbour is a serious pollution source. It was reported that 2862‐unit ships carried 4.8 million tonnes of ballast water to Güllük Harbour during 2007–2012 (37). Another potential source of increased resistance may be the discharge of thermal power plants located 107 km, 46 km and 39 km away from Güllük Bay. The effects of thermal power plant discharge on the accumulation of heavy metals have been reported in other studies (29, 75).

The association between antibiotic resistance and resistance to heavy metals is quite common in the same organism. The increasing numbers of antibiotic and heavy metal-resistant bacteria could be a result of gene transfer activities demonstrating that industrial pollution most likely selects for antibiotic resistance and vice versa (58). In this study, similarly, the most antibiotic-resistant bacteria such as Sphingomonas paucimobilis, Escherichia coli and Enterobacter cloacae were also resistant to heavy metals. Metal‐resistant isolates from Güllük Bay also showed high resistance to sulfonamide, rifampicin and ampicillin. Bacteria from different sources such as humans, animals and soil can transfer or exchange their resistance genes. At the same time, water contaminated with antibiotics, disinfectants, pesticides and heavy metals might encourage selection and result in antibiotic and heavy metal resistance. Marine environmental conditions are extremely dynamic compared to the terrestrial environment, allowing bacteria to bring resistance mechanisms they have developed together while being adapted to the varying conditions. This makes the isolation of various bacteria useful to assess environmental pollution and provides a pathway to possible solutions to remove pollution from marine environments. For bacteria to take part in the transformation of any heavy metal salt into a harmless form, those bacteria must firstly be resistant to the heavy metal; thus the data related to frequency of metal resistant bacteria can provide knowledge on the continual accumulation or transformation of heavy metals in the marine environment.

The findings of the current study provide data regarding the distribution of heavy metal- and antibiotic-resistant bacteria in seawater and sediment samples of Güllük Bay, Aegean Sea, Turkey. As a result, preliminary data on candidate bacteria will offer opportunities for further studies on the elimination of heavy metal contamination by the detection of heavy metal-resistant bacteria.

5. Conclusions

Analyses of the presence of antibiotic resistance in bacteria provide knowledge on pollution sources such as septic systems on regional ecosystems. Since antibiotic-resistant bacteria can affect pathogen virulence, these pollution sources can induce pathogens and can create health risks for both humans and the ecosystem. In the present study, bacteria resistant to antibiotics and heavy metals in seawater and sediment were investigated. The bacterial information obtained provides essential data for identifying the regional distribution of resistant bacteria. Levels of resistance against heavy metals and antibiotics in bacteria isolated from seawater and sediments of the Aegean Sea were quantified. Bacteria isolated from Güllük Bay sediment were resistant to all antibiotics tested and exhibited higher resistance than those isolated from seawater. The frequency of antibiotic-resistant bacteria was higher around fish farms and near the exit of Sarıçay Creek. The widespread resistances of indicator bacteria to antibiotics suggest the presence of anthropogenic influences due to domestic waste and maritime transport.

In order for bacteria to take part in the transformation of heavy metal salts into harmless forms, they must initially be resistant to heavy metals. The frequency of resistance thus provides information regarding the continual accumulation or transformation of heavy metal salts in the marine environment. The findings of the present research have shown the existing contamination status of Güllük Bay via heavy metal and antibiotic resistance tests. The study region is under pressure of pollution as stated in previous research (25, 26, 75) and the bacterial resistance data of the current study showed that there is a prevalence of resistant bacteria in the region that may be due to indirect effects of environmental dynamics and pollution.

In this study, the presence of higher levels of resistant bacteria in sediment compared to seawater may indicate the presence of microplastics in the sediment as well as the probability that the sediment is a suitable medium for accumulation of metals and antibiotics. Further studies on this subject will provide detailed data on the spread of antibiotic- and metal-resistant bacteria in marine sediments.

The present study showed bacterial responses to environmental stress and influences in terms of antibiotic and heavy metal resistance both in sediment and seawater samples at Güllük Bay, Turkey. These findings highlight the necessity of holistic assessments with a ‘one health’ approach and the need to control bacteria entering marine areas due to human activities, considering the contributions of resistant bacteria to global distribution. The data may also provide a useful resource to help identify strains of bacteria for environmental remediation applications.

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Acknowledgments

The authors wish to thank the Scientific and Technical Research Council of Turkey (TÜBITAK, project number: 110Y243, 2011) and Istanbul University Scientific Research Project Unit (İÜ BAP Project/19347) for their financial support.

The Authors


Gülşen Altuğ is a professor in the Department of Marine Biology of the Faculty of Aquatic Science at Istanbul University, Turkey. Her research focuses on marine bacteriology, including bacterial diversity and micro-geographical variations, clinical, industrial and ecological uses of marine isolates, bacterial pollution, epibiotic bacterial communities and anti-bacterial characteristics, bacterial remediation (oil degrading capacity of marine isolates) and resistant bacterial isolates against heavy metals and antibiotics. She is also the inventing founder of the biotechnology company named BIYOTEK15 R&D Training and Consulting Industry and Trade Ltd Company in Entertech of Istanbul University Technocity.


Mine Çardak is an associate professor at Çanakkale Onsekiz Mart University, School of Çanakkale Applied Sciences, Department of Fisheries Technology, Turkey. Her researches focus on marine bacteriology, bacterial resistance against heavy metals and antibiotics, bacterial pollution and biotechnology. She has worked as a scientist since 2000.


Pelin Saliha Çiftçi Türetken is a researcher at İstanbul University, Faculty of Aquatic Sciences, Department of Marine Biology. Her research focus on marine bacteriology, bacterial remediation, bacterial resistance and biotechnology. She has a PhD degree in marine biology. She has worked as an academic at university since 2005.


Samet Kalkan has a PhD degree from Istanbul University, Institute of Graduate Studies in Science and Engineering, Department of Marine Biology. He currently works as a doctor scientist at Recep Tayyip Erdogan University- Faculty of Fisheries, Department of Marine Biology, Turkey. He has worked as academic at university since 2010. His main researches focus on marine bacteria, bacterial diversity, bacterial pollution, resistant bacteria against heavy metals-antibiotics, also marine biotechnology. He has scientific abroad experiences in Italy and Portugal.


Sevan Gürün graduated with a degree in Biology from Istanbul University. He has a PhD degree from Istanbul University, Institute of Graduate Studies in Science and Engineering, Department of Marine Biology. He worked as a researcher in various scientific projects. He has been working as a researcher in a private company since 2016. His expertise focuses on bacterial diversity, marine bacteria, bacterial pollution, bacterial biotechnology, resistant bacteria against heavy metals and antibiotics.

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