Background: Polymyxins are currently regarded as a possible last-resort therapy to eradicate multidrug-resistant (MDR) gram-negative bacteria. Meanwhile, the old antimicrobial agent fosfomycin has recently been reintroduced into clinical use for the treatment of extended-spectrum β-lactamase (ESBL)-producing and carbapenem-resistant Enterobacteriaceae . This study investigated a multidrug-resistant Salmonella  4,[5],12:i:- strain from a food catering handler, which had the potential to act as a vehicle for transmitting MDR foodborne pathogens.
Methods: A Salmonella  4,[5],12:i:- YZU1189 strain was isolated from the fecal sample of a food catering worker according to the standard protocol of the Salmonella  detection method from World Health Organization in 2003. Serotyping of YZU1189 was performed according to the Kauffmann–White scheme. The antimicrobial resistance phenotype of the strain was determined by the agar dilution method according to the instruction from Clinical and Laboratory Standards Institute (CLSI). Plasmid conjugation was performed between the donor strain Salmonella  4,[5],12:i:- YZU1189 and the recipient strain Escherichia coli  C600. The genetic locations of mcr-1, bla _CTX-M-14 and fosA3  genes were determined by the whole genome sequence analysis.
Results: Salmonella  4,[5],12:i:- YZU1189 was an ESBL-producing stain isolated from a healthy catering worker. The strain displayed resistance to aminoglycosides, beta-lactams, polymyxins, fosfomycins, phenicols, trimethoprims, sulfonamides, tetracyclines and fluoroquinolones. Whole genome sequence analysis and plasmid conjugation revealed that the strain had a transferable IncHI2 plasmid carrying the mcr-1, bla _CTX-M-14 and fosA3  genes. Sequence homology analysis showed that this plasmid possessed high sequence similarity to previously reported mcr-1, bla _CTX-M-14 and fosA3  positive plasmids in China.
Conclusion: This study reported a the multidrug-resistant Salmonella  4,[5],12:i:- isolate harboring mcr-1, bla _CTX-M-14 and fosA3  from human for the first time in China. The occurrence of mcr-1  and fosA3  genes in the transferable IncHI2 plasmid pYZU1189 from the ESBL-producing Salmonella  4,[5],12:i:- isolate showed a potential threat to public health. Great concern should be taken for the spread of multidrug-resistant ESBL-producing Salmonella  isolates from food catering workers to consumers.
Keywords: Salmonella  4[5] 12:i:-, whole genome sequencing analysis, multidrug resistant, ESBL, colistin resistance, fosA3