Antibiotic resistance of bacteria was observed in various aquatic environments including seas, rivers, lakes, coastal areas, surface water and sediments. The increased implementation of antibiotics into these environments through medical therapy, agriculture and animal husbandry has resulted in new selective pressures on natural bacterial aquatic populations. Antibiotic resistance of heterotrophic bacteria isolated from the surface microlayer and subsurface water of freshwater coastal polymictic and low-productive lake was studied. Antibiotic resistance was determined by the single disc diffusion method. The resistance level of bacteria to various antibiotics differed considerably. Bacteria were most resistant to ampicillin, ciprofloxacin, clindamycin, erythromycin and penicillin. Majority of bacterial strains showed resistance to 4–6 out of 18 antibiotics tested. As a rule, neustonic bacteria (antibiotic resistance index, ARI 0.44) were more resistant to the studied antibiotics than planktonic bacteria (ARI 0.32). 70–90% of neustonic bacteria were resistant to ampicillin, clindamycin and erythromycin, 60– 70% of planktonic bacteria were resistant to ampicillin, ciprofloxacin and penicillin. Differences between pigmented and non-pigmented bacteria in their resistance to the tested antibiotics were observed. Above 40% of achromogenic bacterial strains were resistant to ampicillin, clindamycin and penicillin. Among bacterial strains characterised by their ability to synthesize carotenoids, more than 30% was resistant to ampicillin and ciprofloxacin. Bacterial resistance level to antibiotics depended on their chemical structure. Bacteria isolated from study lake were most resistant to quinolones and lincosamides while they were most susceptible to tetracyclines and aminoglycosides. Results presented in this paper indicate that antibiotics are a significant selection factor and probably play an important role in regulating the composition of bacterial communities in aquatic ecosystems. Adaptive responses of bacterial communities to several antibiotics observed in the present study may have possible implications for the public health.
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