Origins and environmental mobility of antibiotic resistance genes, virulence factors and bacteria in a tidal creek's watershed

Aims To compare bacterial compositions of watershed run‐offs released by a human settlement and a forested area, and to evaluate their role as carriers of antibiotic resistance and virulence genes. Methods and Results Run‐offs from a forested area and a small settlement in a tidal creek' s watershed were compared for bacterial composition and profiles of 16 tetracycline resistance (TRG), eight virulence (VG) and integrase1 and 2 genes. Integrase 1 gene was detected only once. No integrase 2 gene was observed. VGs were detected only in settlement's run‐offs, and TRG incidence frequency there was twice as high as in the forest's run‐offs. Gene incidences revealed a positive correspondence to the rainfall, and weak correlations to water parameters. Metagenomic, Principle Coordinates and Shannon analyses together revealed distinctive bacterial compositions of the forest's and settlement's run‐offs. Passage of the latter through a salt marsh resulted in the elimination of TRGs and three‐fold decrease in VG incidence, and their bacterial composition was shifted towards that of the tidal creek. Conclusions The settlement was a major source of TRGs and VGs in the watershed, but these contaminants were mitigated by a salt marsh system. Significance and Impact of the Study Our data revealed the role of small settlements in biological contamination of the coastal waters. They also indicated that salt marshes are capable of reducing not only chemical but also biological contamination of run‐offs.