Prevalence, Antimicrobial Resistance And Molecular Subtype Of Salmonella Isolated From Broiler Chicken Farms In Beijing

Salmonella is one of the most important foodborne pathogenic bacteria in the world, of which chicken is a major infection source. For broiler chicken farms are the source of infection, it has great significance to broiler chicken industry and public health to study the prevalence, antimicrobial resistance and molecular subtype of salmonella. As there are more than2,500kinds of salmonella serotypes and the prevalence is different between countries, areas and even farms, in our study, the prevalence, antimicrobial resistance and molecular subtype of salmonella isolated from broiler chicken farms in Beijing were studied.As the isolation and identification methods for salmonella were a lot and their effectiveness were varied from one another, thus, in our study, salmonella of different serotypes were cultured and isolated with different combinations of selective enrichment broth and selective culture mediums, and then the enrichment and isolation effect was detected and evaluated and the best isolation and culture method for salmonella isolated from chicken was concluded in the end.According to the distribution of broiler chicken farms in Beijing,1231samples (including cloaca, yolk sac and environmental samples) were collected from19different broiler chicken farms and abattoirs and106strains of salmonella were isolated by the definitive method. The serotyping results indicated that:salmonella prevalence of different farms showed a significant deviation (P<0.05); S. enteritidis and S. gallinarum were the most prevalent serovars; Although the serotypes varied to different farms, only one or two dominate serotypes would be found in the same farm; In addition, the main serotypes in isolates from broiler chicken’s yolk sac were S. enteritidis and S. gallinarum, while that in isolates from broiler chicken were S. enteritidis and S. javiana.106strains of salmonella collected in our study were examined for antibiotic resistance patterns by broth dilution method and the carrying rates of drug resistance genes were also detected according to the resistance results. The results indicated that polymyxin and spectinomycin-resistance was the most common, which the resistance rates were88.68%and82.08%respectively. The resistance rates of salmonella isolates to other antibiotics were all low than20%, of which amoxicillin/clavulanic acid showed the highest sensitivity with a sensitive rate of100%. The resistance of different serotypes of salmonella was different. Two dominant serotypes of salmonella (S. enteritidis and S. gallinarum) both showed high resistance rate to polymyxin E and spectinomycin, but they showed significant deviation in the resistance rate to enrofloxacin, sulfisoxazole and ampicillin (P<0.05). Strains resistant to two antimicrobial agents (polymyxin E and spectinomycin) predominated in multidrug resistance. Corresponding resistance genes of stains resistant to spectinomycin, tetracycline and polymyxin E were amplified by PCR approach. The results showed carrying rate of aadA2gene in spectinomycin-resistance isolates was very low (1.15%) with no aadAl gene be found, let (A) and tet(M) genes in tetracycline-resistance isolates were71.43%and14.29%respectively, and pmrA and pmrB genes in polymyxin-resistance isolates were63.83%and73.40%respectively. It indicated that initiative excretion effect predominated in the resistance mechanism of tetracycline and polymyxin-resistance isolates.Molecular subtypes of the same serotype salmonella isolated in our study were analysised by ribotyping and pulsed-field gel electrophoresis (PFGE) analysis. The typing results of the two approaches were consistent with each other, while PFGE showed a better discriminatory. Limited genetic diversity were found in S. enteritidis, S. javiana and S. ndolo isolates, which indicated prevalence of these serotypes of salmonella were very stable in Beijing. Much genetic diversity was found in S. gallinarum and S. nitra isolates, which indicated prevalence of these serotypes of salmonella had much diversity and complexity in this area.