Genetic diversity of oxytetracycline-resistant bacteria and tet(M) genes in two major coastal areas of South Korea

•The genetic diversity of tet(M) was examined in two coastal areas of South Korea.•Analysis of tet(M) sequences revealed high diversity and site specificity.•Genetic identification from culturable oxytetracycline (OTC)-resistant bacteria was examined.•Vibrio, Acinetobacter, Photobacterium, Aeromonas and Klebsiella act as reservoirs.•The presence of OTC-resistant pathogenic species encourages further risk assessments.

This study aimed to analyse the prevalence and genetic diversity of the tet(M) resistance gene as well as the species composition of oxytetracycline-resistant bacteria in coastal areas of South Korea. Both culturable and non-culturable bacterial communities were sampled in 2010 and 2011 within two coastal areas (Wando and Geoje). tet(M) and 16S rRNA gene sequences were obtained by PCR and sequencing and were used for phylogenetic analyses. Quantitative PCR (qPCR) was performed to determine the prevalence of tet(M) in sampled areas. Phylogenetic analyses revealed high heterogeneity of tet(M) sequences between Wando and Geoje areas. Sequences found in Wando were highly similar to each other and were similar to sequences found in Japanese aquaculture sites. A larger diversity of tet(M) sequences was obtained from natural assemblage in Geoje. qPCR showed a high prevalence of tet(M) in Wando's aquaculture sites (up to 10−2 gene copies per 16S rRNA copy) and for industrial sites in Geoje (up to 10−3 gene copies per 16S rRNA copy). 16 rRNA-based identification showed that Vibrio sp. and Photobacterium sp. were the most representative species in Wando, whereas in Geoje a broader range of species was found, with several isolates identified as pathogenic Acinetobacter, Aeromonas and Klebsiella. Altogether, these results showed a high prevalence of tet(M) in both sites, and different origins of contamination were identified. Because several pathogenic strains of oxytetracycline-resistant bacteria were reported for the first time, these results strongly warrant further analyses and actions to prevent contamination events by antibiotics in South Korea.