Prevalence and molecular epidemiology of plasmid-mediated fosfomycin resistance genes among blood and urinary Escherichia coli isolates

A total of 1878 non-duplicate clinical Escherichia coli isolates (comprising 1711 urinary isolates and 167 blood-culture isolates), which were collected from multiple centres in Hong Kong during 1996-2008, were used to investigate the prevalence and molecular epidemiology of plasmid-mediated fosfomycin (fos) resistance genes. Eighteen of the 1878 clinical E. coli isolates were fosfomycin resistant, of which six were fosA3 positive and two were positive for another fosA variant (designated fosKP96). No isolates had the fosC2 gene. The clones of the eight isolates were diverse: sequence type (ST) 95 (n=2), ST118 (n=1), ST131 (n=1), ST617 (n=1), ST648 (n=1), ST1488 (n=1) and ST2847 (n=1). In the isolates, fosA3 and bla(CTX-M) genes were co-harboured on conjugative plasmids with F2:A-:B- (n=2), N (n=1), F-:A-:B1 and N (n=1) and untypable (n=2) replicons. Both fosKP96-carrying plasmids belonged to replicon N. RFLP analysis showed that the two F2:A-:B- plasmids carrying fosA3 and bfa(CTX-M-3) genes shared the same pattern. Complete sequencing of one of the two F2:A-:B- plasmids, pFOS-HK151325 (69 768 bp) demonstrated it to be >99% identical to the previously sequenced plasmid pHK23a originating from a pig E. coli isolate in the same region. This study demonstrated the dissemination of fosA3 genes in diverse E. coli clones on multiple bla(CTX-M)-carrying plasmid types, of which F2:A-:B- plasmids closely related to pHK23a were shared by isolates from human and animal sources.