Marine bacteria harbor the sulfonamide resistance gene sul4 without mobile genetic elements.

Marine bacteria are possible reservoirs of antibiotic-resistance genes (ARGs) originating not only from clinical and terrestrial hot spots but also from the marine environment. We report here for the first time a higher rate of the sulfonamide-resistance gene in marine bacterial isolates compared with other genes. Among four sulfonamide-resistance genes (, , , and ), was most abundant (45%) in 74 sulfonamide-resistant marine isolates by PCR screening. The order of abundance was (33 isolates) (6 isolates) (5 isolates) (1 isolate). Whole-genome sequencing of 23 isolates of -expressing α- and γ-proteobacteria and bacilli revealed that was not accompanied by known mobile genetic elements. This suggests that in these marine isolates is clonally transferred and not horizontally transferable. Folate metabolism genes formed a cluster with , suggesting that the cluster area plays a role in folate metabolism, at which functions as a dihydropteroate synthase. Thus, might be expressed in marine species and function in folate synthesis, but it is not a transferable ARG.