| Salmonella is an important pathogen that may contaminate animal foods such as poultry,eggs and dairy products.Salmonella may carry virulence factors such as enterotoxin and virulence protein and other virulence factors,thus causing diseases.At present,due to the extensive use of antimicrobial drugs in the prevention and treatment of human and animal diseases,the antimicrobial resistance of Salmonella has rapidly increased.The existence of multiple antimicrobial resistance increases the difficulties to prevent and control of Salmonella.Studies have shown that the emergence of antimicrobial resistance is not only related to the selective pressure of multiple antimicrobials in the environment and the wide spread of drug-resistant genes through movable genetic elements,but also related to their virulence.Therefore,in this paper,the Salmonella strains previously isolated from foods by our laboratory were used to study epidemiological characteristics,antimicrobial resistance,distribution of virulence genes and antimicrobial resistant genes.Typical strains were selected to perform whole genome sequencing to screen antimicrobial resistant genes and plasmids.The phenotypic features and genotypic correlation of antimicrobial resistant Salmonella isolates were studied.The results of our study provide information for studies on the prevalence,virulence genes and antimicrobial resistance of Salmonella.A total of 120 Salmonella strains previously isolated from foods were chosen as subjects of this study.The sensitivity of Salmonella isolates to 15 antibiotics such as ampicillin,amoxicillin,cefotaxime,ceftriaxone and chloramphenicol etc were detected by K-B disc diffusion method.The results showed that the antimicrobial resistance of Salmonella was very common.The Salmonella strains had maximum resistance to doxycycline(80%),while resistance to ceftazidime(3.3%)was minimum.Multi-drug resistant strains reached 78.3%.According to antimicrobial susceptibility analysis,28 out of 120 Salmonella isolates were selected to detect 5 antimicrobial resistant genes(blaTEM-1,Sul-?,strB,floR,tetA)by PCR amplification.The results showed that barTEM-1 was commonly present in the isolates(39.3%).However,sul-II gene was only detected positive in 17.9%of isolates.The resistance genes of aminoglycosides antibiotics displayed the highest consistency(85.7%)with the phenotypic features of antimicrobial resistance of the isolates,while only 67.9%of Salmonella isolates carrying associated resistant genes showed resistance to sulfonamides.According to the above results,18 out of 28 Salmonella were chosen for serotyping.Of the 18 isolates,4 strain was non-typable.The rest Salmonella strains consisted of 4 serotypes.Salmonella Bridain was the predominant one(10/18,55.6%),followed by Kentucky(2/18,11.1%),Newport(1/18,5.6%)and Tombusson(1/18,5.6%).Among the four non-typable strains,one was chosen for multilocus sequence typing(MLST).Its ST was identified as ST236 and it was determined to be Salmonella Arechavaleta.The 18 Salmonella strains were then detected virulence island genes(sseL,hilA,mgtC),enterotoxin gene(stn)and invasion protein coding gene(invA).The detection rate of the five virulence factors was 100%.Among the 18 isolates,5 isolates were selected to evaluate the expression levels of five virulence genes by reverse transcription quantitative PCR.The results showed that the expression levels of mgtC in the tested isolates were significantly higher than the standard strain ATCC14028(P<0.05).A test strain 045 showed significantly higher expression in genes seeL,mgtC and invA compared with ATCC14028(P<0.05),indicating that some food-borne Salmonella isolates pose potentially high pathogenicity.A total of 4 Salmonella strains were further selected for whole genome sequencing analysis.The results showed that the antimicrobial resistance phenotype and genotype basically matched.The fluoroquinolones resistance-associated genes gyrA,gyrB,parC,parE presented a few mutations.The genomes contain chromosomal point mutations in gyrA gene(p.S83F and p.D87N)and parC gene(p.T57S,p.S80I and p.T57S).Three unkown mutations also occurred in parC gene(parC p.T255S,parC p.E292K and parC p.V657I).Whether the unknown point mutations in parC gene found in this study will cause fluoroquinolones resistance need study further.A total of 15 bacterial plasmids were found among the 4 sequencing strains,strain 62 had a drug-resistant plasmid,which was located on the 46th scaffold.The plasmid contained aadA and lnuB_F.Strain 4 has two drug resistance plasmids,which are located on the 33rd scaffold and the 40th scaffold respectively.The plasmid which was on the 33rd scaffold contains cmlA and the sull,The plasmid which was on 40th scaffold contains sul3,and the rest of the plasmids are bacterial plasmids.In this study,serotyping,MLST molecular typing,drug resistance analysis and virulence genes were performed on Salmonella from different samples and different hosts,revealing the distribution of Salmonella in different samples and host sources,epidemiological characteristics,It is aim to provide a basis for early warning and control of Salmonella outbreaks. |
PDF Full Text Request