Study On The Antimicrobial Resistance And Its Molecular Mechanism Of Salmonella Enterica Serovar Typhimurium From Clinical Infections In Henan Province

Objective The aim of this study was to analyze the antimicrobial resistance phenotypes and to elucidate the molecular mechanisms underlying resistance to first-line treatment agents cephalosporins,ciprofloxacin and azithromycin among S.Typhimurium isolates in Henan,China.Meanwhile,the genetic polymorphism of the resistant isolates was measured by multilocus sequence typing?MLST?and pulsed-field gel electrophoresis?PFGE?to explore their genetic relationships,which will provide scientific basis for the monitoring and clinical treatment for drug-resistan of S.Typhimurium.Methods Minimum inhibitory concentration?MICs?were determined by broth microdilution,with results being interpreted according to the recommendations of the Clinical and Laboratory Standards Institute.We selected the related isolates to amplify the resistance genes of cephalosporins?ceftriaxone,ceftiofur,cefoxitin?,ciprofloxacin,and azithromycin by PCR to analyze the molecular mechanisms underlying resistance to the three drugs.Genetic relationship and genetic polymorphism of were assessed by MLST and PFGE.Results In this study,the S.Typhimurium isolates examined exhibited high rates of resistance to older-generation antimicrobials.There were 89.8%of the isolates which were resistant to ampicillin,tetracycline or sulfisoxazole,followed by nalidixic acid?78.2%?,trimethoprim/sulfamethoxazole?77.6%?,chloramphenicol?76.9%?,gentamicin?69.3%?,streptomycin?68.7%?,and amoxicillin/clavulanic acid,2:1 ratio?66.0%?.Fourthmore,of the 147 isolates,91.1%were multidrug resistant?MDR?,with4.1%being resistant to all antibiotic classes tested.Importantly,some isolates demonstrated strong resistance to at least one of five first-line treatment agents,namely,ciprofloxacin?29.2%?,ceftiofur?17.0%?,ceftriaxone?16.3%?,azithromycin?10.2%?,and cefoxitin?6.1%?.Of concern,13 MDR isolates were co-resistant to the first-line treatments cephalosporins and ciprofloxacin,while three were also resistant to azithromycin.Of the 43 ciprofloxacin-resistant isolates,39 carried double gyr A mutations?Ser83Phe,Asp87Asn/Tyr/Gly?and a single parC?Ser80Arg?mutation,including 1 isolate with four mutations?gyrA:Ser83Phe,Asp87Gly;par C:Ser80Arg;parE:Ser458Pro?.In addition,12 isolates not only carried mutations in gyrA and parC but also had at least one plasmid-mediated quinolone resistance?PMQR?gene.Among the 32 cephalosporin-resistant isolates,the most common extended-spectrum?-lactamase?ESBL?gene was bla_OXA-1,followed by bla _CTX-M,bla _TEM-1,and bla_CMY-2.Moreover,the mphA gene was identified in 5 of the 15 azithromycin-resistant isolates.Four MDR isolates contained ESBL and PMQR genes,and one of them also had carried mphA in addition.The MLST results show the ST19 and ST34 were the most commonly encountered.This study analyze 147 isolates S.Typhimurium by PFGE,the result showed that the genotypic diversities.There was a close genetic relationship between the strains resistant to ciprofloxacin.Conclusion S.Typhimurium isolates in Henan not only exhibited high rates of resistance to traditional antimicrobials but also show resistance rates to the first-line treatments for Salmonella infection.More importantly,we identified certain MDR isolates co-resistant to cephalosporins,ciprofloxacin,and azithromycin.Among the isolates,we detected various plasmid-encoded antimicrobial-resistance genes,including PMQR,ESBL,and mph A genes,with some isolates even carrying two or more types,posing a serious threat to global public health.Therefore,more comprehensive surveillance is essential to prevent further spread of resistant clonal strains.