Characterization of an IncF2:A-:B- plasmid harboring fosA3 gene in a Shiga toxin-producing Escherichia coli O26:H11 isolated from a child with diarrhea in Japan.

The efficacy of antimicrobial therapy against Shiga toxin-producing Escherichia coli (STEC) infections has been a matter of global debate. In Japan, fosfomycin is often prescribed for pediatric patients to prevent life-threatening sequelae. Recently, E. coli strains harboring plasmid-mediated fosfomycin-resistant genes (fos genes) have been disseminated worldwide, yet the prevalence of these genes among STEC remains unclear. This study aimed to determine the prevalence of fosfomycin resistance among STEC strains isolated from humans, as well as to analyze the molecular characteristics of a conjugative plasmid carrying a fos gene in a STEC strain. Two fosfomycin-resistant STEC O26:H11 strains were identified among 206 STECs by the disk diffusion method. The primary fos gene, fosA3, was identified on an IncF2:A-:B- plasmid in one strain. This plasmid could be transferred to recipients, and the frequency of transfer was higher for STEC O26 recipients compared to STEC O157 recipients. Sequence-based comparative analysis of the plasmid suggested horizontal transfer of fosA3 gene among diverse bacterial strains, with insertion sequences playing a pivotal role in its mobilization. These findings suggest the possibility of fosA3 gene disseminating among diverse STEC strains and emphasize the importance of monitoring fosfomycin resistance and characterizing resistance plasmids in STEC strains.