Isolation,Identification And Antibiotic Resistance Analysis Of Salmonella From Pigs In Yangzhou Area

Salmonella is an important food-borne pathogen,which has been listed as one of the food-borne pathogens by the World Health Organization.Pigs are one of the major hosts of Salmonella,and China is a major producer and consumer of pork and pork products.Therefore,it is very important to strengthen the surveillance and control of pig Salmonella.In this study,the contamination and serotype distribution of Salmonella from pig in Yangzhou area were investigated,and the antibiotic sensitivity and molecular typing of the isolates were studied,in order to provide the material basis for the establishment of control measures of Salmonella in pig production chain.1.Contamination condition and distribution characteristics of Salmonella from pigs in Yangzhou area.From May 2020 to April 2021,sample collection,Salmonella isolation and serotype identification were carried out at the A slaughterhouse in Yangzhou area and the corresponding 7 farmer's markets downstream.The results showed that a total of 76 Salmonella strains were isolated from the 438 samples collected from the slaughterhouse,with a total isolation rate of 17.4%.A total of 184 Salmonella strains were isolated from 740 fresh pork samples collected from the farmers' market,with a total isolation rate of 24.9%.Statistical analysis found that the isolation rate of Salmonella in slaughterhouses and pork is significantly affected by seasonal factors,with the highest isolation rate in summer and the lowest isolation rate in winter.Serotype identification showed that the dominant serotype of Salmonella in slaughterhouses was Salmonella Rissen(53.9%),followed by Salmonella Derby(32.9%),and the dominant serotype in pork was Salmonella London(37.5%),followed by Salmonella Rissen(25%).2.Antibiotic sensitivity analysis and molecular typing of Salmonella from pigs in Yangzhou area.Detecting the susceptibility of 260 Salmonella isolates to 16 kinds of antibiotics,and conduct resistance gene detection based on the resistance phenotype.The results showed that Salmonella from slaughterhouses had the highest resistance rate to ampicillin,98.7%;the resistance rates to amoxicillin,tetracycline and doxycycline were 97.4%,94.7%and 92.1%,respectively.97.4%of the isolates showed multi-drug resistance,of which the proportion of strains resistant to 7-9 antibiotics was the highest,at 43.4%.There are 31 multi-drug resistance spectrums in 74 multi-drug resistant Salmonella strains,the most common multi-drug resistance spectrum is AMP-AMX-CHL-DOX-FFC-GEN-KAN-STR-TET.The 184 strains of Salmonella from pork had the highest resistance rate to tetracycline,87.5%;the resistance rate to ampicillin,amoxicillin and doxycycline were all 75%and above,91.3%of the isolates showed multi-drug resistance,of which the strains resistant to 7-9 kinds of antibiotics accounted for the highest proportion,55.4%.There are 52 multi-drug resistance profiles in 168 multi-drug resistant Salmonella strains,and the most common multi-drug resistance profile is AMP-AMX-CHL-DOX-FFC-GEN-KAN-STR-TET.The results of drug resistance gene detection showed that the four drug resistance genes,tetA,blaTEM and floR,had a high detection rate,accounting for 92.7%,77.3%and 61.2%of the tested samples respectively,the three genes mcr-1,tet(X4)and blaCMY were not detected.According to the result of whole-genome sequencing analysis,it was found that two strains with polymyxin-resistant phenotype did not carry other mcr-resistant genes,and no pmrCAB mutations were identified.A total of 13 ST types were identified by MLST typing of 260 Salmonella isolates.Among them,ST469,ST155,ST40,ST34,ST358 and ST19 were all detected in the pig production chain,accounting for 33.5%,27%,17.7%,4.6%,2.3%and 1.5%.After comparison,it was found that there was a one-to-one correspondence between the ST type of the isolate and the serotype.The PFGE typing of 37 strains of Salmonella from slaughterhouses and farmers' markets showed that the similarities of from different slaughter links and market sources were greater than 85%,and some were as high as 100%,which proved that Salmonella may be along the pig industry chain,meanwhile,colonizing bacteria may exist in the slaughterhouse,causing continuous pollution to the carcass.The PFGE typing results of 29 Salmonella London strains from the market showed that the homology of Salmonella London from different farmers' markets was higher than 85%,indicating that the strains of the same PF type may have the same source.In summary,Salmonella contamination in the pig production chain is relatively serious,most of the isolates show multi-drug resistance,and there are clonal transmission and cross-contamination in the production chain.Therefore,the slaughter process should be strictly regulated to control the spread of Salmonella.