| BackgroundSalmonella is a common zoonotic pathogen,which mainly infects humans and animals by contaminating food and water,and is one of the most important pathogens causing food poisoning and causes a serious disease burden in many countries and regions.More than 2700 serotypes have been detected,and Salmonella Typhimurium variants(S.1,4,[5],12:i:-)are antigenically similar to Salmonella Typhimurium,showing only the loss of flagellum antigen expression in phase II,which is highly similar to Salmonella Typhimurium in genetic characteristics.Therefore,it is considered to be a monophasic variant evolved from Salmonella Typhimurium.In recent years,the detection rate of S.1,4,[5],12:i:-in food,animal and human has risen sharply,and it has become one of the main serotypes leading to human Salmonella infection in Europe,America,Asia and other countries,and is closely related to many international outbreaks of foodborne Salmonellosis.S.1,4,[5],12:i:-in Guangdong has been the dominant serotype of Salmonella isolates from humans since 2015,when it surpassed other serotypes.The high prevalence of this serotype is closely related to its multi-drug resistance characteristics,especially the mechanism of horizontal transfer of drug-resistant genes between strains mediated by mobile genetic elements,which poses a serious threat to human health and global public health security.At present,there are few studies on the epidemiological characteristics and evolutionary origin of this variety,and there is no clear evidence yet.Objective1.To understand the antibiotic resistance of S.1,4,[5],12:i:-in this region,analyzing the molecular characteristics related to drug resistance and virulence of S.1,4,[5],12:i:-,and explore the mechanism of antibiotic resistance and pathogenicity,so as to provide evidence support for drug-resistance monitoring and antibiotic use in clinical treatment of this serotype.2.The homology between S.1,4,[5],12:i:-and Salmonella Typhimurium was investigated by analyzing the flora structure and genetic characteristics.3.To analyze the genetic and evolutionary relationship between S.1,4,[5],12:i:-and Salmonella Typhimurium,and the global epidemic strains,and to clarify the evolutionary origin of S.1,4,[5],12:i:-.MethodA total of 228 strains of S.1,4,[5],12:i:-and 102 strains of Salmonella Typhimurium collected in Guangdong Provincial Sentinel Hospital for Surveillance of Diarrhea Pathogenic Agents were used as the research subjects to analyze the drugresistant phenotypes,drug-resistant genotypes,virulence characteristics,molecular typing,phylogenetic and evolutionary relationships.1.Minimal inhibitory concentration(MIC)of the strain against 27 kinds of antibiotics was determined by microbroth dilution method,and the drug resistance and drug resistance spectrum characteristics of the strain were analyzed.2.Molecular typing and homology analysis of S.1,4,[5],12:i:-and Salmonella Typhimurium were performed by PFGE technique to compare the genetic differences between the two strains.3.The whole genome sequencing technology combined with bioinformatics was used to detect drug resistance,virulence and related molecular characteristics.Multi-locus sequence typing(MLST)and genome-wide single nucleotide polymorphisms(wgSNPs)were used to analyze the bacterial community structure and phylogenetic relationships among the strains.Result1.Drug sensitivity test results showed that S.1,4,[5],12:i:-showed high level of resistance to tetracycline,doxycycline,ampicillin and sulfamisoxazole.Sensitivity to quinolones decreased obviously.The multi-drug resistance was serious,and the resistance spectrum was diversified.Only 44.7%isolates showed the same characteristics as the international epidemic clones.For S.1,4,[5],12:i:-,the resistance rate of antibiotics with high level of resistance is higher than that of Salmonella Typhimurium.2.Multiple antibiotic resistant genes were detected,and the overall coincidence rate of drug-resistance genotypes and phenotypes were well.Two types of mobile genetic elements,plasmid and insertion sequence,were detected.Among them,there were 19 plasmid types,mainly IncHI2/IncHI2a,IncQ plasmid.Some resistant genes were found to have a large number of insertion sequences.3.There were significant differences in the distribution of virulence factor between the two serotypes.Salmonella Typhimurium strains carried high virulence plasmid genes of spv,pef,rck and mig-5,while S.1,4,[5],12:i:-were not found.4.PFGE showed that both serotypes showed genetic polymorphism with highly sporadic epidemic characteristics.The similarity between their fingerprints were not high,and the polymorphism of S.1,4,[5].12:i:-was lower than that of S.Typhimurium.5.MLST typing and genomic single nucleotide polymorphism analysis showed that S.1,4,[5],12:i:-was prevalent with ST34 clones,while S.Typhimurium was prevalent with ST 19,ST34 and ST36 clones simultaneously.In terms of ST34 S.1,4,[5],12:i:-,its genetic distance was close to the native ST34 S.Typhimurium(SNPs<100).The genetic distance between the ST19 S.1,1,[5],12:i:-and endemic S.Typhimurium was relatively long(500-1000SNPs).There were close genetic relationships b etween the main prevalent clones of S.1,4,[5],12:i:-and the internationally prevalent strains.Conclusion1.The phenomenon of multiple drug resistance for S.1,4,[5],12:i-isolates in Guangdong is serious.Acquired drug-resistance gene transmission mediated by mobile genetic elements is the main mechanism of multiple drug resistance.2.The genetic polymorphism of S.1,4,[5],12:i:-shows highly sporadic epidemic characteristics,resulting in a low risk of large-scale outbreaks.3.Different from Salmonella Typhimurium in molecular characteristics,It is likely that the high virulence factor is lost in the evolutionary process,and more drug-resistant genes are obtained,showing an important molecular evolution characteristic of multidrug resistance and low virulence of S.1,4,[5],12:i:-.And the flora diversity of the S.1,4,[5],12:i:-was lower than that of Salmonella Typhimurium.ST34 S.1,4,[5],12:i:was the dominant clone,which consistent with the global epidemic trend,while different from that of Salmonella Typhimurium.4.The phylogenetic branch of local S.1,4,[5],12:i:-endemic strains were clear,which probably evolved from the ST34 clone lineage of Salmonella Typhimurium,and no mainstream clone replacement has occurred since the local epidemic. |
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