| Salmonella is an important zoonotic pathogen commonly found in cultured animals.In recent years,there have been many reports about monophasic variants of Salmonella typhimurium(S.Typhimurium).Since the 21st century,Salmonella 4,[5],12:i:-(S.4,[5],12:i:-)has become one of the most common serotypes causing human salmonellosis in China.It is also one of the most common serotypes isolated from human and food in several other countries.The detection rate has increased significantly in many countries around the world,and has caused many serious outbreaks of salmonellosis.The serotype has similar antigenicity and genotype to S.Typhimurium,and lacks the H2 phase flagellum antigen gene compared to S.Typhimurium.At present,the rate of separation in the country is getting higher and higher,and most of them are in the form of multi-drug resistance.In order to analyze the antimicrobial resistance of S.4,[5],12:i:-isolates,antimicrobial susceptibility testing to 19 antimicrobial agents of 225 strains was performed using the broth microdilution method,and the drug resistance and multidrug resistance of S.4,[5],12:i:-were analyzed.Then whole-genome sequence were performed for all of the 123 strains of 5.4,[5],12:i:-and 114 strains of S.Typhimurium,analysis of S.4,[5],12:i:-and S,Typhimurium in the difference in the type and quantity of drug resistance genes,and then the resistance genes and the drug-resistant phenotypes of S.4,[5],12:i:-were compared,the correspondence was analyzed,and the reasons for the partial strains were not corresponded.This study will help to further understand the drug resistance,drug resistance characteristics,multi-drug resistance status and major molecular resistance mechanisms of S.4,[5],12:i:-.1 Antimicrobial resistance of 5.4,[5],12:i:-isolatesAntibiotic susceptibility of 255 strains of S.4,[5],12:i:-was tested by the broth microdilution method.The results showed that the resistance rate to tetracycline was the highest,reaching 90.6%,followed by ampicillin(86.7%),amoxicillin/clavulanic acid(85.5%);the resistance rate of the strains to streptomycin,nalidixic acid,chloramphenicol and trimethoprim-sulfamethoxazole was above 50%;the resistance rate of cephazolin,gentamicin,kanamycin,olaquindox and aztreonam.is between 19.6%and 42.7%.About 90%of the isolates showed sensitive to nitrofurantoin,colistin and fosfomycin.Almost all of the strains showed sensitive to amikacin and meropenem.S.4,[5],12:i:-isolates in different origins of breeding,slaughter,market and human beings showed similar resistance to amoxicillin/clavulanic acid,tetracycline,ampicillin,above 80%,which was the same as the overall drug resistance.For kanamycin,gentamicin and trimethoprim-sulfamethoxazole,the resistance rate has gradually declined from the breeding-slaughter-market-human beings.For nitrofurantoin,drug-resistant strains were detected only in the slaughter process.For colistin and fosfomycin,drug-resistant strains were detected only in the breeding and slaughter,but no meropenem and amikacin-resistant strains were detected in the four process.Generally speaking,the resistance rates of S.4,[5],12:i:-in the breeding process and slaughter process were higher than market process and human process..The multi drug resistance of S.4,[5],12:i:-was more common,and the overall multidrug resistance rate reached 91.8%.Among them,the isolates from chicken,cat,goose and human were multidrug-resistant,and the multidrug resistance rate of pigs origin reached 91.0%.In different processes,the highest rate of multi-drug resistance was 96.4%in human process,and the overall rate of multi-drug resistance decreased first and then increased from the breeding to human.Among the multidrug-resistant strains,43 were similar to Spanish clone(ACSuGSTTm),38 were similar to American clone(ACSSuT)and 6 were similar to European clone(ASSuT).In addition,there were more multidrug-resistant phenotypes of AST and ACSxT,with 54 and 42 strains respectively.2.Drug resistance genes and drug resistance of S.4,[5],12:i:-and S.typhimuriumIn this study,123 S.4,[5],12:i:-and 114 S.Typhimurium strains were sequenced to detect the distribution of drug-resistant genes.The sequence was compared with ResFinder database by blast software.Six types of drug-resistant genes were detected,which were β-lactam resistance gene(blaTEM-1A、blaTEM-1B、blaTEM-18、blaCTX-M-27、blaCTX-M-14、blaCTX-M-55、blaCTX-M-65、blaOXA-1、blaOXA-10、blaDHA-1、blaNDM-5 and blaCMY-2),sulfonamide resistance genes(sul1、.sul2、dfrA12、dfrA14、dfrA17、dfrA27 and sul3),phosphomycin-resistant genes only detect fosA3 and fosA,tetracycline resistance genes(tetA,tetB and tetM),amidol resistance genes(catB3、catB4、floR、cmlA1、catA1 and catA2),quinolone resistance genes(qnrS1、qnrS2、qnrB6、oqxA、oqxB and aac(6’)-Ib-cr),aminoglycoside resistance genes(strA、strB、aadA1、aadA2、aadA5、aadA8、aadA16、aadA17、aadA24、aac(6)-Iaa、aac(3)-IVa、aac(3)-IId、aph(3)-IIa、aph(3)-Ia、aph(3’)-Ib、aph(3"’)-Ib、aph(4)-Ia、aph(6)-Id、aph(3)-Iva and aac(6’)Ib-cr).Overall,50 resistance genes were detected in S.4,[5],12:i:-and 51 resistance genes were detected in S.Typhimurium.In terms of the number of drug-resistant genes carried,27.6%of S.4,[5],12:i:-carries seven drug-resistant genes,40.7%of S.4,[5],12:i:-carries more than 15 drug-resistant genes,while 37.7%of S,Typhimurium strains do not carry or only carry one drug-resistant gene,and 24.6%of S.Typhimurium carries more than 15 drug-resistant genes.Therefore,S.4,[5],12:i;-carry far more resistant genes than S.Typhimurium,suggesting that S.4,[5],12:i:-poses a greater potential threat to food safety and human health than S.Typhimurium.According to the correspondence between resistance genes and phenotypes,blaTEM-1B gene with penicillin-based AMP and AMC accorded 98.6%and 100%,strA and strB gene with aminoglycoside-based STR accorded 92.5%and 93.8%,tetB gene with tetracycline-based tetracycline accorded 100%,the coincidence rate of sul3 and dfrA12 genes with trimethoprim-sulfamethoxazole was 97.9%,floR gene with amidol chloramphenicol was 98.3%,and fosA3 gene with fosmycin was 87.5%,which showed good correspondence.The resistance of strains to quinolones was mainly caused by gyrA point mutation,and the presence of aac(6‘)-Ib-cr and oqxAB genes also reduced the sensitivity of strains to quinolones."phantom resistance ome",which is sensitive to antibiotics while carrying resistance genes,this implies that there are still some unknown resistance genes or mechanisms that have not been excavated,which pose a certain threat to human health and food safety.The dominant multi-drug resistant strains were AST and ACTSxT,and the corresponding multi-drug resistant genotypes were blaTEM-1B-strA-strB-tetB and blaTEM-1B-floR-sul2-tetB-dfrAl2.There is also ACSGTSxT similar to Spanish strain,and its corresponding genotype is blaTEM-1B-floR-sul2-aph(4)-Ia-strA-strB-tetB-dfrA12,and ACSTSxT similar to American strain,corresponding genotype is blaTEM-1B-floR-sul2-strA-strB-tetB,similar to European strain is ASTSxT,corresponding genotype is blaTEM-1B-strA-strB-sul2-tetBThe plasmid types of 123 S.4,[5],12:i:-were analyzed and 12 plasmid types were detected,including IncQ1,IncY,IncHI2/IncHI2A,p0111,IncFIA,IncFIB,IncHI1A/IncHI1B,Incl1,IncI2,IncFII,IncA/C2 and IncR.IncQ 1 and IncHI2/IncHI2A were dominant in quantity.Most strains carried 1-2 plasmid types,which was different from S.Typhimurium.Among S.Typhimurium,IncFIB and IncFII were dominant in quantity,and most strains carried 2-3 plasmid types.For MLST typing,98.4%of S.4,[5],12:i:-are ST34,and there are two new ST types.The ST type of S.Typhimurium is widely distributed,among which ST 19 is the main type. |
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