First detection of conjugative plasmid-borne fosfomycin resistance gene fosA3 in Salmonella isolates of food origin.

Fosfomycin is a naturally occurring antibacterial agent with a broad spectrum of antimicrobial activity against both Grampositive and Gram-negative bacteria (1). Recently, a fosfomycin resistance gene, fosA3, was detected in Escherichia coli and Klebsiella pneumoniae isolates (2–6). This gene is normally plasmid mediated, surrounded by IS26 transposase genes, and often detectable in CTX-M-producing and multidrugresistant E. coli isolates (2–6). It has been suggested that the increasing prevalence of fosA3 is due to dissemination of the IncI and IncN plasmids rather than clonal expansion of specific strains (4). This study aimed to determine whether fosA3-borne plasmids can disseminate among other closely related Enterobacteriaceae species such as Salmonella and to identify the underlying mechanisms regulating its dissemination potential. Two Salmonella isolates, S76 and S79, obtained from meat samples purchased from supermarkets and wet markets in Hong Kong in 2013, were shown to be resistant to quinolones and thirdgeneration cephalosporins (Table 1). Surprisingly, these two isolates were also resistant to fosfomycin. Salmonella strain S76 was determined to be Salmonella enterica serovar Derby; this strain was isolated from chicken product purchased in supermarket on February 22, 2013, whereas strain S79, which was found to be S. enterica serovar Enteritidis, was isolated on the same day from a different chicken product purchased from a different supermarket in Hong Kong. Multilocus sequence typing (MLST) analysis of these two isolates showed that S76 belonged to ST11, whereas S79 belonged to ST460. Previous studies had shown that ST11 was associated with invasive S. Enteritidis and ceftriaxone-resistant S. Enteritidis strains carrying CTX-M-14 and CTX-M-15 on IncI1 and IncFII plasmids, respectively (7–9). In contrast, ST460 had not been reported to be associated with CTX-M-producing S. Derby previously. The gene fosA3 was detectable in both strains by PCR screening of fosfomycin resistance genes and transferrable to E. coli J53 (4, 10). Conjugative plasmids in E coli J53 that harbored the fosA3 gene were shown to be IncFII. S1 pulsed-field gel electrophoresis (PFGE) confirmed that plasmids of two different sizes ( 80 kb and 45 kb) were detectable in both isolates, with the conjugative plasmid being the larger one (Fig. 1A). Southern hybridization showed that the fosA3 gene was present in the 80-kb conjugative plasmid (Fig. 1A). Genetic-environment analysis showed that the fosA3 gene was surrounded by two IS26 elements with a genetic structure of IS26-fosA3-orf1-orf2-orf3-IS26. An identical structure was first reported for plasmids from clinical E. coli isolates in Japan, South Korea, and Hong Kong (3, 4, 6, 11). Importantly, a PCR assay revealed the presence of a -lactamase gene, blaCTX-M-55, on the conjugative plasmids recovered from strains S76 and S79; the result was confirmed by Southern hybridization (Fig. 1B). Primers targeting IS26 and blaCTX-M-55 identified a gene cassette, IS26-blaTEM-1-orf20-blaCTX-M-55-IS26, in the conjugative plasmids pS76 and pS79 (Fig. 1C). The genetic structures of fosA3 and blaCTX-M-55 were very similar to that of a previously identified plasmid, pHK23a, except that the blaCTX-M-55 and blaCTX-M-4 elements were found in pHK23a and pS76/pS79, respectively (11). However, the length or nature of linkage between fosA3 and blaCTX-M-55 genetic structures in pS76 and pS79 was different from that of pHK23a, since an attempt to amplify the linkage region between the fosA3 and blaCTX-M-55 structures in