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Purpose: Several subtypes of plasmid-mediated fosfomycin resistance gene fosA in Enterobacteriaceae have been reported worldwide and have caused concern. The present study characterized a novel member of fosA gene located on a plasmid from Escherichia coli. Materials and Methods: A fosfomycin-resistant E. coli isolate PK9 was recovered from a chicken meat sample in 2018. The presence of fosA genes was detected by PCR and sequencing. Whole-genome sequencing (WGS), conjugation, and cloning were performed to identify the mechanism responsible for fosfomycin resistance. Oxford Nanopore MinION sequencing was carried out to characterize the plasmid carrying fosfomycin resistance gene and the genetic context of the novel fosA variant. Results: A novel fosA gene with significant homology (>98%) with fosA6 and fosA5 genes was identified by WGS and was named fosA10. FosA10 shared 56.1% to 98.6% amino acid sequence identity with other reported plasmid-mediated FosA enzymes. Fosfomycin resistance and fosA10 gene were successfully transferred to E. coli C600 by conjugation. Cloning confirmed that FosA10 could confer fosfomycin resistance (MIC > 128 μg/mL). The fosA10 gene was localized on a 53kb IncFII (F35:A-:B-) plasmid. The ΔlysR-fosA10-Δhp fragment (4328 bp), located between two copies of IS10R, showed 100% identity with the chromosomal sequences of 17 Klebsiella pneumoniae strains of ST664 and one of ST3821 in GenBank. Conclusion: Our findings indicated that the fosA10 gene of E. coli might be captured from the chromosome of K. pneumoniae by IS10, which further demonstrated that K. pneumoniae might act as a reservoir of fosA-like genes acquired by E. coli.

The rise of the plasmid-encoded colistin resistance gene mcr-1 is a major concern globally. Here, during a routine surveillance, an unexpectedly high prevalence of Escherichia coli with reduced susceptibility to colistin (69.9%) was observed in a Chinese broiler farm. Fifty-three (63.9%) E. coli isolates were positive for mcr-1 . All identified mcr-1 -positive E. coli (MCREC) were multidrug resistant and carried other clinically significant resistance genes. Furthermore, the mcr-1 genes were mainly located on the IncI2 and IncHI2 plasmids. Conjugation experiments unraveled the co-transfer of mcr-1 with other antibiotic resistance genes ( bla CTX-M-55 , bla CTX-M-14 , floR , and fosA3 ) via the IncI2 ( n =3) and IncHI2 ( n =4) plasmids. The stable genetic context mcr-1 - pap2 was common in the IncI2 plasmids, whereas IS Apl1 - mcr-1 - pap2 -IS Apl1 was mainly found in the IncHI2 plasmids. The dominance of mcr-1 -bearing IncI2 and IncHI2 plasmids and co-selection of mcr-1 with other antimicrobial resistance genes might contribute to the exceptionally high prevalence of mcr-1 in this broiler farm. Our results emphasized the importance of appropriate antibiotic use in animal production.

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