Prevalence of Plasmid-mediated Colistin Resistance Gene mcr-1 in Domestic Wastewater

Abstract

Colistin is one of the last-resort antibiotics. However, the transmissible plasmid-mediated colistin resistance gene encoded by mcr-1 was first detected in China in 2015. The dissemination of mcr-1 is of great concern. This study investigates the prevalence of mcr-1 in urban sewage in Vietnam and Japan because domestic sewage reflects the health status of urban citizens. Water samples were collected from To Lich River and Nhue River in Hanoi, Vietnam, in September 2019. These rivers are severely polluted by untreated domestic wastewater; thus, these samples can be regarded as urban sewage. We also collected wastewater samples from three different municipal wastewater treatment plants (A, B, and C) in Japan in October 2019. DNA was extracted from the samples, and mcr-1 abundance was analyzed via quantitative PCR. Quantitative PCR demonstrated that mcr-1 was detected in all samples, indicating that healthy citizens carry some bacteria harboring mcr-1. The abundance of mcr-1 in the influent of wastewater treatment plant A (1.9 × 106 gene copies/L) was lower than that in the other samples in Japan and Vietnam (8.1 × 106–1.3 × 107 gene copies/L). The log reduction values of mcr-1 in wastewater treatment ranged 1.5–4.2 log10, resulting in abundance of 7.0 × 102–7.3 × 104 copies/L in the final effluents. The comparable levels of mcr-1 in urban sewage in Vietnam and Japan indicate the global spread of transmissible colistin resistance. Wastewater is considered as an important monitoring target for mcr-1.