| Salmonella is one of the most important zoonotic bacterial pathogens,and the contaminated pork can cause human foodborne disease.Pig farms and slaughterhouses are the main sites for contamination and spread of porcine Salmonella.The unreasonable use of veterinary drug in pig husbandry and veterinary clinics frequently leads to the development of bacterial antimicrobial resistance,which poses a serious threat to human health.Therefore,it is of great significance for food safety and public health to enhance the monitoring and prevention of Salmonella contamination and drug resistance in pork production chain.In the first part of this study,different samples were collected from three pig slaughterhouses in Wuhan and used to isolate and identify Salmonella strains.Then the isolates were determined by drug sensitivity test and multiple locus sequence typing(MLST),and the contamination law of Salmonella in pig slaughtering were discussed.In the second part,249 Salmonella strains restored from diseased or healthy pigs during 2008-2015 were determined for antimicrobial resistance phenotype and resistance gene detection.Through the comparison of the antimicrobial resistance of Salmonella from different sources,and analysis of the correlation between resistance genes detection and drug resistance phenotype,we try to clarify the regulation to develop drug resistance in porcine Salmonella.The main results were described as follows.1.Investigation on contamination law of Salmonella in pig slaughteringA total of 1962 pig samples and environmental samples were collected from three pig slaughterhouses in Wuhan city.261 Salmonella strains were identified and the overall detectable percentage was 16.4%.A total of 12 serotypes were identified,among which S.derby,S.risen and S.typhimurium were predominant.A total of 226 strains were performed by MLST typing and classified as 18 kinds of different sequence types(STs),among which 14 STs belonged to 9 homologous complexes,and ST34,ST40 and ST469 were the predominant STs.In addition,two new STs(STN1 and STN2)were identified.Cluster analysis revealed that there were four ancestral ST models,ST19,ST29,ST32 and ST40.The results of cladogram showed that there was a close relationship among the STs from the B slaughterhouse,and among the ST469 and STN1 of strains isolated from the A slaughterhouses.The minimal inhibitory concentrations(MICs)of antimicrobial to 226 Salmonella strains were determined using the microdilution broth method.The results showed that all isolates exhibited a high resistance percentage of tetracycline(>90%).There was no significant difference in the resistance to gentamicin,trimethoprim/ sulfamethoxazole and chloramphenicol among three slaughterhouses,105 strains(46.6%)were resistant to ciprofloxacin,in which the strains from the B(63.7%)and C slaughterhouses(72.5%)showed higher resistance percentages.The resistance percentage of cefotaxime was the lowest(12.6%).67% of the strains exhibited the multidrug resistant phenotype,and 26.1% of the strains were resistant to more than seven drugs.Furthermore,the study showed that the strains of ST34 restored from part of anal swabs,splitting swabs and pork at the C slaughterhouse exhibited the same resistance spectrum,indicating that they might come from the same origin and spread along the slaughtering chain.2.Antimicrobial resistance of Salmonella isolated from healthy and diseased pigs A total of 249 porcine Salmonella isolates collected during 2008–2015 were used in this study,including 155 clinical isolates from diseased pigs and 94 nonclinical isolates from healthy pigs.Based on the MICs of seven antimicrobial agents,96.4% of the isolates were resistant to at least one of the tested drugs and 81.0% of them showed multidrug resistance.The highest resistance was observed for tetracycline(85.9%),and the lowest was found for cefotaxime(13.3%).The isolates from diseased pigs exhibited significantly higher levels of resistance than those from healthy pigs.Cefotaxime resistance of the strains isolated from diseased pigs during 2013-2015 was significantly higher compared with the strains isolated during 2008-2010.Twenty-two isolates from healthy pigs were resistant to ciprofloxacin.Further study showed that the correlation between phenotypic and genotypic resistance varied among the isolates from different sources,and in many cases,the presence of resistance genes was not consistent with the resistance to the corresponding antimicrobials,indicating that the antimicrobial resistance mechanism may be different among the isolates from different sources. |
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