| Salmonella is one of the most common pathogens that causes foodborne diseases.Every year,it leads to about 93.8 million cases of severe dysentery,among which about 155,000 died.Most of the outbreaks arise from Salmonella-contaminated food or animal transmission,which severely threatens public health and results in huge economic losses.Among the more than 2500 serotypes of Salmonella,Salmonella enterica serovar Typhimurium is one of the predominant serotypes,and the global outbreak of food-borne diseases due to infection by S.Typhimurium is impressive.Therefore,this study explore the surveillance of S.Typhimurium in shanghai.Between January 2011 and December 2014,641 S.Typhimurium isolates were collected in Shanghai,including 82 foodborne strains and 559 strains of clinical origin.Children under 5 years of ages,and especially less than 1-year-old children,were highly susceptible to infection,which accounted for 49.4% of all the patients.The ratio of the male to female patients was 1.3:1.Infections occurred mainly in summer and autumn.In order to understand the antimicrobial characteristics of S.Typhimurium in Shanghai,susceptibility test was performed to all strains.The result showed that S.Typhimurium exhibited high rates of resistance to traditional antimicrobials,including tetracycline(85%),nalidixic acid(81.6%),ampicillin(81.1%),sulfisoxazole(79.3%),chloramphenicol(58.2%),streptomycin(54.6%),trimethoprim/sulfamethoxazole,gentamicin(50%),and gentamicin(39.8%).In addition,12.8% of the isolates displayed resistance to ciprofloxacin;resistance to ceftiofur,ceftriaxone,and cefoxitin was found in 17.6%,17.6%,and 4.4% of isolates,respectively;and 5% of isolates were showed resistance to azithromycin.Compared to the resistance characteristics between the isolates from foodborn and human infections,the resistance of food-borne strains to nalidixic acid,tetracycline,gentamicin,chloramphenicol,sulfamethoxazole,trimethoprim /sulfamethoxazole was higher than the clinical isolates,but the clinical isolates were more resistant to ceftiofur,ceftriaxone,and cefoxolin than food-borne strains,and there was no significant difference between ciprofloxacin and azithromycin.It is noteworthy that only 26 isolates were susceptible to all 14 antimicrobials.MDR was observed in 86.1% of the isolates,in which ?3,?4,?5,?6,and ?7 classes of antimicrobials were found in 86.1%,79.9%,64.3%,41.3% and 0.9% of the MDR isolates,respectively.Among the MDR isolates,30.4% showed the ACSSu T resistance pattern(defined as resistance to ampicillin,chloramphenicol,streptomycin,sulfamethoxazole,and tetracycline).More importantly,22 of the isolates showed co-resistance to cephalosporins and ciprofloxacin,and 6 of them were co-resistant to azithromycin.If these MDR strains are as prevalent as S.Typhimurium DT104 all over the world,it will pose a great threat to global public health.This research finding provided the guide for appropriate clinical antimicrobial treatment for patients with S.Typhimurium infection and reminded people to use antimicrobal rationally so as to aviod the incresing number of resistant strainsTo analyze the molecular mechanisms underlying resistance to cephalosporins,ciprofloxacin,and azithromycin,we selected the related isolates to amplify the resistance genes by PCR.Of all the 82 isolates with resistance to ciprofloxacin,76 showed one mutation(D87Y or D87N)or double mutations(S83F/D87 N or D87G)in gyrA.Among them,6 simultaneously had a single mutation(S80R)in parC gene and 1 isolate had two mutations(T57S/S80R)in parC gene.No point mutations in gyrB and parE were found.Here,three species of PMQR determinants including acc(6')-Ib-cr,qnrB,and qnrS were detected.Fifty-four(65.9%)isolates contained acc(6')-Ib-cr,6(7.3%)contained qnrB(qnrB2,n=2;qnr B4,n=4),and 1 contained qnrS1.Moreover,among the 53 isolates with no mutation or only one mutation in gyrA gene,one or two PMQR genes were detected.It is proved that PMQR gene has potential to prevail in mediating ciprofloxacin resistance.Among the 116 cephalosporin-resistant isolates,64.7%,44.8%,37.1%,and 2.6% isolates contained the blaCTX-M,blaTEM-1,blaOXA-1,and blaSHV-12 genes,respectively.BlaCTX-M included eight subtypes: blaCTX-M-14,blaCTX-M-15,blaCTX-M-27,blaCTX-M-55,blaCTX-M-65,blaCTX-M-104,blaCTX-M-123,and blaCTX-M-125.blaCTX-M-14(29%)and blaCTX-M-55(41%)were found to be dominant.No blaCTX-M-2/8/25 genes were found.This phenomenon proved that the blaCTX-M-1 group and blaCTX-M-9 groups are popular types in S.Typhimurium isolates in Shanghai.To the best of our knowledge,this is the first report of the presence of blaCTX-M-123 and blaCTX-M-125 in S.Typhimurium.Among the 32 cephalosporin-resistant isolates,15 high-level azithromycin-resistant isolates(MIC?128?g/mL)harbored the mphA gene,while other 17(MIC: 16-64?g/mL)did not.All azithromycin-resistant isolates were negative for mphB,ermA,ermB,ereA,mefA,and msrA.These results indicate that the mphA gene may mediate a high level of resistance to azithromycin in S.Typhimurium.Among the 22 isolates with co-resistance to cephalosporins and ciprofloxacin,15 contained ESBL and PMQR genes.In addition,2 out of the 15 isolates co-resistant to azithromycin concurrently harbored ESBLs,PMQR,and mphA genes: aac(6')-Ib-cr/ blaCTX /blaTEM/ mphA.Antimicrobial-resistance genes are usually located on plasmids and therefore can be easily disseminated.Coexistence of these genes leads to the emergence of MDR.Our findings indicate that it is imperative to continue monitoring the prevalence of the above-mentioned resistant genes in S.Typhimurium isolates.Future studies should be focused on identifying ways to prevent the dissemination of these antimicrobial-resistance genes.To further understand the genetic relationship and genetic polymorphism of S.Typhimurium in Shanghai,this study analyze 257 isolates S.Typhimurium by pulsed field gel electrophoresis(PFGE).The results showed that the food-borne strains were closely related to the clinical strains.The band similarity between the strains of two source in the A cluster was 85%,indicating that animals infected with S.Typhimurium may be the source of human infection with S.Typhimurium.The genotypic diversities were found in isolates with resistance to cephalosporins,ciprofloxacin,and azithromycin.The strains with the type CEP.022 contained blaCTX-M and blaTEM genes and were distributed in 2011 to 2014.This phenomenon indicated that these isolates were closely related and derived from the same clone group.As these strains are resistant to more than seven kinds of antimicrobial,it is easy for them to cause widespread dissemination.Thus,monitoring of this type of strain should be strengthened.There was no significant correlation between the other resistance genes and genotyping.In summery,the study analyzed the antimicrobial resistance phenotypes and elucidated the molecular mechanisms underlying resistance to cephalosporins,ciprofloxacin,and azithromycin in S.Typhimurium isolates in Shanghai from 2011 to 2014.Meanwhile,the genotype of the resistant isolates was analyed by pulsed field gel electrophoresis(PFGE)and explored the genetic relationship between food-borne strains and clinical strains,which provided a solid foundation for the prevention and control of S.Typhimurium. |
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