First surveillance and draft genome sequences of circulating mcr-1-carrying, multidrug-resistant Escherichia coli in...

The global rise of colistin resistance, primarily mediated by the mcr-1 gene, threatens the efficacy of one of the last-resort treatments for multidrug-resistant Gram-negative infections. While mcr genes have been reported in clinical, veterinary and limited environmental samples in Qatar, there is a notable gap in data regarding their presence in wastewater.

Fifteen composite wastewater samples were collected across multiple rounds in Qatar. Escherichia coli isolates were screened for colistin resistance, and mcr-1-positive strains were identified. Antimicrobial susceptibility testing was performed to determine resistance profiles. Whole-genome sequencing was conducted to characterize plasmid scaffolds harbouring mcr-1. Transformation assays were used to verify the transferability of resistance, and biofilm assays were conducted to evaluate persistence of resistance under biofilm conditions.

Out of the 15 composite wastewater samples, only 3 yielded E. coli - one isolate from each sample - and all were positive for the mcr-1 gene. The mcr-1-positive E. coli isolates (DSR2, DNR3 and DSR3) were all further screened and exhibited multidrug resistance while remaining susceptible to selected carbapenems and cephalosporins. Whole-genome sequencing indicated diverse mcr-1-carrying plasmid types: IncI2Δ, IncX4 and IncHI2A. Transformation assays confirmed that these plasmids conferred colistin resistance to recipient strains. In biofilm assays, the IncHI2A-carrying isolate retained colistin-resistant colonies up to day 6, in contrast to the other two plasmid types.

This study detected transferable mcr-1-bearing plasmids in wastewater E. coli in Qatar, stressing the value of wastewater surveillance for tracking environmental spread and horizontal transfer of last-resort antibiotic resistance.