Emergence of Mobile Colistin Resistance Genes within Los Angeles County Wastewater

Abstract

Wastewater surveillance of antibiotic resistance genes (ARGs) provides insight into the circulating antibiotic-resistant profile of its serviced population. Continual surveillance and characterization of wastewater ARGs could help identify emerging ARGs of clinical importance and their dissemination across bacterial species and locations. In this study, we detected four mobile colistin-resistant (mcr) gene variants (mcr-3.12, mcr-4.3, mcr-5.1, and mcr-9.1) in Los Angeles County wastewater between September 2020 to February 2022. Through sequencing of cultured colistin-resistant bacterial isolates and wastewater plasmids, we characterized the genetic environments of the detected mcr gene variants and revealed conserved mcr gene cassettes between our data set and published data. Moreover, we discovered two novel small plasmids (approximately 8.29 and 8.67 kbp) with near identical mcr-5 gene cassettes flanked by a nonhomologous region. Within cultured colistin-resistant bacterial isolates, we detected the presence of mcr-3.12 in Aeromonas hydrophila and mcr-9.1 in Hafnia alvei. Our findings underscore the utility of wastewater surveillance to detect and characterize circulating ARGs and help uncover intermediate carriers along their dissemination routes.