| Salmonella is widespread in the environment.Salmonellosis caused by Salmonella is a global zoonosis,among which the main source is the animal-derived food-borne Salmonella.Over the years,the overall drug-resistance spectrum of Salmonella is expanding,especially the third generation cephalosporin resistant strains,which is likely to lead to treatment failure and other risks to low immune population.This study investigated the presence of third-generation cephalosporin resistant Salmonella from duck slaughter chain and investigated the resistance,resistance genes and virulence genes of the strains,and classified them into clusters,providing reference for production management,clinical treatment and epidemiological investigation.The main results are as follows:According to the sequence of slaughtering process,867 samples were obtained from the slaughter chain of duck,of which 242 Salmonella strains were isolated,with an overall pollution rate of 27.9%.The susceptibility test of ceftazidime and cefotaxime to Salmonella isolates was carried out by disk diffusion method.Of 242 isolates,65 were resist to at least one of the third-generation cephalosporin,namely 3GC-RS isolates,with a separation rate of 26.9%.The main serotypes were S.Indiana(n=23)and S.Give(n=11),followed by S.London(n=6)and S.Typhimurium(n=5).In addition,35 isolates of extended-spectrum β-lactamase producing phenotypes were identified by double-disk synergy test method(DDST),mainly from the first sampling period between 2016 and2017;21 strains of Amp C producing phenotypes were identified by Tris-EDTA Amp C disk test method,mainly from the second sampling period between 2018 and 2019.3GC-RS resistance rates to doxycycline,tetracycline,streptomycin and ampicillin were90.8%,86.2%,84.6% and 80%,respectively.The resistance rates to aztreonam,sulfamethoxazole-trimethoprim,ciprofloxacin and gentamicin were 63.1%,63.1%,60%and 55.4%,respectively.The resistance rates to amoxicillin-clavulanic acid(21.5%),meropenem(13.8%)and amikacin(12.3%)were relatively low.The multi-drug-resistance of 3GC-RS isolates is very high,95.4% can resist more than 5 antimicrobials and the highest can resist 12 antimicrobials at the same time.The multi-drug-resistance of 3GC-RS isolates in different sampling time shows an upward trend.PCR test method was used to detect drug-resistance genes and virulence genes.In the test results of drug resistance genes,the detection rate of sul-2 gene was 73.8%(n=48),followed by tet C(64.6%),str A(60%),aac(6`)-Ib(55.4%),CTX-M-1(46.2%),tet A(44.6%),CTX-M-9(41.5%),sul-3(40%),The detection rates of aad A2,tet G and CMY-2genes were relatively low with 30.8%,29.2% and 23.1%,respectively.The detection rates of qnr S genes were only 4.6%.CTX-M-2,CTX-M-8,CTX-M-25 and tet B genes were not detected in all isolates.In the test results of virulence genes,the detection rate of Mgt C gene was 96.9%,followed by Avr A(87.7%),Sip C(86.2%),Spv R(47.7%),Spv C(47.7%),Pef A(46.7%),Sod C1(30.8%),only one Sop E gene(1.5%)was detected.65 isolates of 3GC-RS were tested by PFGE,of which 64 strains formed clear and recognizable fingerprints,41 PFGE patterns and three main subgroups were constructed by cluster analysis;40 representative isolates of 3GC-RS were selected for MLST test,ST-17(n=20),ST-516(n=13),ST-19(n=6)and ST-463(n=1)were detected.PFGE method has a good clustering effect on Salmonella of different sampling period. |
