Emergence of clinical Salmonella enterica serovar Typhimurium isolates with concurrent resistance to ciprofloxacin, ceftriaxone, and azithromycin

Abstract

Salmonella infection is an important public health issue for which the needs of antimicrobial treatment are increasing. A total of 546 human clinical S. enterica serovar Typhimurium isolates were recovered from patients in hospitals in China during the period of 2005 to ∼ 2011. Twenty percent of the isolates exhibited resistance to ciprofloxacin, and 4% were resistant to ceftriaxone. Importantly, for the first time, 12 (2%) S. Typhimurium isolates resistant to both ciprofloxacin and ceftriaxone were recovered; among these 12 isolates, two were also resistant to azithromycin, and one was resistant to all other drugs tested. The combined effects of various transferrable extended-spectrum β-lactamase determinants and a novel efflux-based ciprofloxacin resistance mechanism encoded by the mobile efflux gene oqxAB were responsible for the emergence of these extremely (highly) drug-resistant (XDR) S. Typhimurium isolates. The dissemination of resistance genes, such as those encoding ESBLs and the OqxAB pump, among Salmonella organisms will speed up the selection of XDR Salmonella, posing a huge threat to public health and Salmonella infection control.

FIG 1

Plasmid profiles of representative S. Typhimurium isolates and transconjugants determined by S1-PFGE. Transmissible plasmids from S. Typhimurium and transconjugants (T) are highlighted. The same symbol in the parental isolate and transconjugant indicates the position of the conjugative plasmid. The marker is a phage lambda concatemer with different bands labeled.

FIG 2

PFGE patterns of clinical S. Typhimurium isolates that are resistant to both ciprofloxacin and ceftriaxone. The isolate identity (ID), year of isolation, origin, and location of isolation, as well as the ESBL, PMQR gene(s), and multilocus sequence types, are also indicated.