Detection of the plasmid-encoded fosfomycin resistance gene fosA3 in Escherichia coli of food-animal origin

Objectives: To investigate the occurrence of plasmid-mediated fosfomycin resistance genes among Escherichia coli from food animals in China. Methods: A total of 892 E. coli isolates collected from individual pigs (n=368), chickens (n=196), ducks (n=261), geese (n=35), pigeons (n=20) and partridges (n=12) in Guangdong Province during 2002-08 were screened for the presence of fosA3, fosA and fosC2 by PCR amplification and sequencing. The clonal relationship of fosA3-positive isolates, plasmid content and other associated resistance genes were also characterized. Results: Twelve (1.3%) E. coli isolates showed resistance to fosfomycin and 10 (1.1%) isolates (4 from pigs, 2 from chickens, 2 from ducks, 1 from a goose and 1 from a pigeon) were positive for fosA3. None of the E. coli isolates was positive for fosA or fosC2. All of the isolates carrying fosA3 were CTX-M producers, and three of them carried rmtB. Most of the fosA3-harbouring isolates were found to be clonally unrelated. The fosA3 genes were flanked by IS26. Two fosA3 genes co-localized with rmtB and bla(CTX-M-65) on indistinguishable F33:A-:B- plasmids that carried three addiction systems (pemI/pemK, hok/mok/sok and srnB). Four, one and one fosA3 genes were found to be associated with IncN (ST8 type), Inch 1 and F2:A-:B- plasmids, respectively. Conclusions: We discovered that fosA3 is always associated with blo(CTX-M), which facilitates its quick dispersal. The emergence of fosA3 in food animals could impact on human medicine by the potential transfer of resistance through the food chain.