Analysis of antimicrobialresistant mechanisms and genetic homogeny of Salmonella from community acquired infectionsin Shenzhen

Abstract

：Objective To investigatethe antimicrobial resistance mechanisms and genetic homogeny of Salmonella from communityacquired infections in Shenzhen,China.Methods Ninety-three of Salmonella were isolatedfrom 2002 to 2007 at Shenzhen People's Hospital,China.PCR and DNA sequencing were used toinvestigate the mutation in QRDR of the gyrA,gyrB,parC and parE.Plasmid mediated quinoloneresistance genes including qnr and aac(6')-Ib-cr,β-lactamase genes includingblaTEM,blaSHV,blaOXA, blaCTX-M, and class 1 integron were detected. All isolates weretyped by PFGE. Results S. enterica typhi and S. enterica paratyphi A were susceptible toampicillin, chloramphenicol, trimethoprim/sulfamethoxazole, ceftriaxone and ciprofloxacin,with the susceptible rate of 96%-100%. Fifty-two percent (13/25) of S. enterica typhi and95% (61/64) of S. enterica paratyphi A were resistant to nalidixic acid. Twenty-fourpercent (6/25) of nalidixic acid-resistant S. enterica typhi and 94% (60/64) of nalidixicacid-resistant S. enterica paratyphi A showed decreased susceptibility to ciprofloxacin(MIC of 0. 125-1 μg/ml).All nalidixic acid-resistant (susceptible tociprofloxacin ) Salmonella (NARS) isolates had a single substitution in the QRDR of GyrA,and 91% (68/75) of these isolates carried the substitution Ser83Phe in GyrA. Two mutationsin the QRDR of GyrA were detected in both of two ciprnfloxacin-resistant Salmonella,withthe additional one mutation in the QRDR of parC. Plasmid mediated quinolone resistancegenes including qnr and aac(6')-lb-cr were not detected in any isolate. The blaCTX-M-14gene was detected in a ceftriaxoneresistant isolate of S. enterica paratyphi A, withISEcpl located on the upstream of it. Three muhidrugresistant strains of Salmonella allcarried one 1 900 bp classⅠ integron gene cassette dhfrⅫ-orfF-aadA2,withthe additional one β-lactamase gene of blaTEM-1, or blaOXA-30. Twenty-two distinct PFGE patterns wereobserved among twenty-five S. enterica typhi. The PFGE patterns of sixty-four S. entericaparatyphi A showed limited genetic diversity (average similarity of 91% ). Ninetyinvestigated inpatients were infected in the community. Six patients infected by S.enterica paratyphi A had a travel history before infection. Conclusions Nalidixicacid-resistant S. enterica typhi and S. enterica paratyphi A are highly prevalent inShenzhen,China. The mutation in the QRDR of GyrA is the prevalent mechanism responsiblefor the resistance to nalidixic acid in Slmonella. The great genetic similarity among S.enterica paratyphi A isolates indicates endemic disease from the presence of a singleclone over 6-year period. Key words: Salmonella typhi; Salmonella paratyphi A ; Drugresistance, bacterial; Nalidixic acid; beta-Lactamases; Electrophoresis, gel,pulsed-field