Occurrence of antibiotic-resistant bacteria in the sea surface microlayer of coastal waters in the southern North Sea

The emergence of antibiotic-resistant bacteria in coastal waters is a global health problem posing potential risks to the health of humans who depend on coastal resources. Monitoring and increased efforts are needed to maintain the health of marine ecosystems. The sea surface microlayer (SML) is poorly studied for antibiotic resistance of the inhabiting bacteria. Therefore, we examined the antibiotic resistance patterns of 41 bacterial strains isolated from the SML in a harbor in the southern North Sea. The strains are affiliated with 17 genera typically found in the marine environment. Using the disc diffusion assay, we found extensive resistance, particularly to gentamycin, kanamycin, nalidixic acid, penicillin, sulfadimidine, and streptomycin. A broth microdilution assay showed high minimum inhibitory concentrations (MICs) for most isolates for amikacin, aztreonam, ceftazidime, cefepime, minocycline, and tobramycin. Genome analysis of three strains affiliated with the genera Pseudoseohaeicola, Nereida, and Vibrio, all showing a highly resistant phenotype, revealed the presence of 57, 42, and 90 genes, respectively, associated with antibiotic resistance. Over 50 % of these genes are multidrug efflux pumps. Our study shows that the SML in anthropogenic-influenced coastal regions harbors a wide diversity of antibiotic-resistant bacteria equipped with a broad range of multidrug efflux pumps.