High prevalence of Escherichia coli co-harboring conjugative plasmids with colistin- and carbapenem resistance genes in a wastewater treatment plant in China

Emergence and dissemination of resistance to last-resort antibiotics such as carbapenem and colistin is a growing, global health concern. Wastewater treatment plants (WWTPs) link human activities and the environment, can act as reservoirs and sources for emerging antibiotic resistance, and likely play a large role in antibiotic resistance transmission. The aim of this study was to investigate occurrence and characteristics of colistin-and carbapenem-resistant Escherichia coli (CCREC) in wastewater and sludge samples collected over a one-year period from different functional areas of an urban WWTP in Jinan city, Shandong, China. A total of 8 CCREC were isolated from 168 samples with selective agar and PCR, corresponding to high prevalence of 4.8%, co-harboring car-bapenem resistance genes (blaNDM) and colistin resistance gene (mcr-1) and subsequently whole-genome sequenced. Additionally, all isolates were multidrug-resistant by antimicrobial susceptibility testing and car-ried a variety of antibiotic resistance genes. Two isolates carrying virulence genes associated with avian path-ogenic E. coli were identified, one belonging to the high-risk clone O101:H9-ST167. Southern blotting was used to characterize CCREC isolates and plasmids carrying blaNDM-genes or mcr-1 could be transferred to a recipient strain E. coli J53 by in vitro conjugation assays. Resistance to other antibiotic classes were sporadically co-transferred to the transconjugant. Transposition of and mcr-1-carrying element from a transferable IncHI2-plasmid was observed among two CCREC clones isolated within 4 days of each other. The occurrence of multidrug-resistant CCREC capable of transferring their antibiotic resistance genotypes via conjugative plasmids is alarming. WWTPs bring bacteria from different sources together, providing opportunities for horizontal ex-change of DNA among compatible hosts. Further dissemination of the colistin-, carbapenem-, or both colistin-and carbapenem resistant E. coli could lead to a serious threat to public health.

Automated summary

This study investigated the occurrence and characteristics of carbapenem and colistin-resistant Escherichia coli (CCREC) in wastewater and sludge samples from a urban wastewater treatment plant in Jinan, China. The study found that 8 out of 168 samples tested positive for CCREC, with high prevalence of multidrug resistance and the presence of virulence genes. The study also observed that the resistance genes could be transferred through conjugation, indicating the potential for further dissemination of resistant E. coli.