Monitoring sewage and effluent water is an effective approach for the detection of the mobile colistin resistance genes (mcr) and associated bacterial hosts in the human population and environment in the USA

Colistin, a last-resort antibiotic, is reserved for treating recalcitrant infections. Colistin has not been approved for agricultural use in the USA. Therefore, it has been suggested that the occurrence and spread of the mobile colistin resistance genes (mcr) were relatively limited in the USA. To evaluate the latter suggestion, we investigated the occurrence of mcr-positive Gram-negative bacterial isolates in sewage (a representative of the fecal microbial community of a human population) and environment (effluent and surface water) in Georgia, USA. Raw sewage and water samples were collected from major wastewater treatment plants and an associated water reservoir. The samples were screened on the selective chromogenic Rapid'E.coli2 agar supplemented with colistin. Colonies were investigated for mcr using gene-specific PCR analyses. The Kirby-Bauer disk diffusion and broth micro-dilution assays were used to determine susceptibility against different antibiotics. Whole Genome Sequencing (WGS) was used to investigate the resistome, virulome, and plasmidome of the mcr-positive isolates. Biofilm assays were used to evaluate the persistence of mcr. mcr-9-positive Serratia nevei strains were isolated from sewage, while mcr-3-positive Aeromonas jandaei strains were detected in effluent and surface water samples. The isolates were multidrug-resistant, and WGS analyses highlighted the detection of additional antibiotic resistance genes. Furthermore, mcr-9 was found to be located on an IncHI2 plasmid, while mcr-3 was on the chromosome. Also, mcr-9 persisted in 12-day old biofilms. Sewage analyses suggested that the plasmid-borne mcr-9 was potentially circulating in the human population, while the detection of mcr in effluent and surface waters highlighted a potential for environmental dissemination.