Prevalence of Plasmid-Mediated Fosfomycin Resistance Gene fosA3 Among CTX-M-Producing Escherichia coli Isolates from Chickens in China

The aim of this study was to investigate the prevalence of fosfomycin resistance gene fosA3 and characterize plasmids harboring fosA3 among CTX-M-producing Escherichia coli from chickens in China. A total of 234 CTX-M-producing E. coli isolates collected from chickens from 2014 to 2016 were screened for the presence of plasmid-mediated fosfomycin resistance genes (fosA, fosA3, and fosC2). Clonal relatedness of fosA3-positive isolates was determined by pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST). The genetic environment of fosA3 was analyzed by polymerase chain reaction (PCR) mapping. Plasmids were studied by using conjugation experiments, PCR-based replicon typing and plasmid MLST. Sixty-four (27.4%) fosA3-positive E. coli isolates were identified in this study. The gene blaCTX₋M₋₅₅ (31/64) was predominant among these strains, followed by blaCTX₋M₋₁₄ (18/64) and blaCTX₋M₋₆₅ (14/64). Various PFGE patterns and sequence types (STs) indicated that these isolates were clonally unrelated. Seven different genetic environments of fosA3 were identified and two new combinations (ISEcp1-blaCTX₋M₋₆₅-ΔIS903D-IS26-fosA3-orf1-orf2-Δorf3-IS26 and IS26-ISEcp1-blaCTX₋M₋₃-orf477-blaTEM₋₁-IS26-fosA3-orf1-orf2-Δorf3-IS26) were discovered for the first time. Conjugation experiments were successful for 47 isolates and 33 transconjugants harbored a single plasmid. Plasmids carrying fosA3 belonged to incompatibility group IncFII (17/33), IncI1 (2/33), IncHI2 (3/33), and IncB/O (1/33). F33:A-:B- plasmids carrying blaCTX₋M₋₅₅, IncHI2/ST3 plasmids carrying blaCTX₋M₋₆₅, and F2:A-:B-plasmids carrying blaCTX₋M₋₅₅ were found in E. coli isolates from different provinces. Our results revealed a considerable prevalence of fosA3 gene among CTX-M-producing E. coli with clonal diversity from chickens in China. The transmission of different kinds of plasmids is responsible for the dissemination of fosA3 in chicken farms in China.