IncI1 plasmids encoding various blaCTX-Ms contributed to ceftriaxone resistance in Salmonella Enteritidis in China.
Resistance to extended spectrum ß-lactams in Salmonella, in particular serotypes such as S. Enteritidis that are frequently associated with clinical infections, is a serious public health concern. In this study, phenotypic characterization of 433 clinical S. Enteritidis strains obtained from a nationwide collection of China CDC during the period of 2005~2010 depicted an increasing trend of resistance to ceftriaxone from 2008 onwards. Seventeen (4%) of the strains were found to be resistant to ceftriaxone, 7% to ciprofloxacin and 0.7% to both ciprofloxacin and ceftriaxone. Most of the ceftriaxone-resistant S. Enteritidis strains (15/17) were genetically unrelated, and originated from Henan province. The complete sequence of an IncI1 plasmid pSE115 which belonged to a novel Sequence Type was obtained. This 87,255bp IncI1 plasmid was found to harbour a blaCTX-M-14 gene located in a novel Multidrug Resistance Region (MRR) within the tra locus. Although the majority of strains were also found to contain conjugative IncI1 plasmids of similar size to pSE115(~90kb) and harbor a variety of blaCTX-MGroup 1 and Group 9 elements, the novel MRR site at the tra locus in pSE115 was not detectable in the other IncI1 plasmids. Findings in this study show that cephalosporin resistance in S. Enteritidis strains collected in China was mainly due to dissemination of blaCTX-M-encoding IncI1 plasmids, resembling the situation in which IncI1 plasmids serve as major vectors of blaCTX-M variants in other members of Enterobacteriaceae. Copyright © 2015, American Society for Microbiology. All Rights Reserved.