Prevalence of mcr-positive Enterobacterales and characterization of mcr-9-carrying plasmids from pig farm ecosystems in Fujian, China.

OBJECTIVE

Polymyxins are last-resort antibiotics for multidrug-resistant Gram-negative bacterial infections, but their efficacy is threatened by plasmid-mediated colistin resistance genes (mcr). We investigated the prevalence and plasmid characteristics of mcr-positive Enterobacterales at the pig-worker-environment interface across four pig farms in Fujian Province, China.

METHODS

A total of 930 samples were collected from pigs, farm workers, and farm-associated environmental matrices. mcr variants were screened by PCR. Antimicrobial susceptibility testing was performed, and representative isolates were further characterized by short-read sequencing and long-read sequencing, conjugation, fitness cost and persistence assays.

RESULTS

Only mcr-1 and mcr-9 were detected. Among K. pneumoniae isolates, 39/263 (14.8%) carried mcr-1 and 21/263 (8.0%) carried mcr-9. In E. coli, both genes were each detected in 7/521 (1.3%). Environmental samples yielded 14 mcr-1-positive K. pneumoniae and 16 mcr-9-positive isolates, including 2 E. coli, 10 K. pneumoniae, 3 Raoultella ornithinolytica and 1 Enterobacter roggenkampii. Genomic analysis showed that mcr-1 was primarily located on IncHI2 (n = 5), IncX4 (n = 14), and IncI2 (n = 15) plasmids, while mcr-9 was predominantly carried by IncHI2 plasmids (n = 4) and IncF(n = 2) plasmids. Compared with mcr-1 IncHI2 plasmids, mcr-9-positive plasmids showed higher conjugation efficiency, lower fitness cost, and greater within-host persistence.

CONCLUSIONS

mcr-positive Enterobacterales were detected in pigs, farm workers, and farm-associated environments. These findings suggest that mcr-9-carrying plasmids may contribute to the persistence of colistin resistance in swine-farm ecosystems, supporting continued One Health surveillance and improved farm hygiene and management.