Prevalence And Antimicrobial Resistance Of Salmonella Isolates From Food Animals In Zhejiang,China

Salmonella is one of the most important foodborne pathogens that can cause Salmonella infections such as diarrhea and bacteremia,which are harmful to human health.In recent years,antibiotics are widely used in livestock and poultry because of their role in promoting growth and preventing diseases.While improving the production and quality of food animal products,serious bacterial drug resistance has been caused.As an important reservoir of antibiotic-resistant genes,Salmonella can carry a variety of antibiotic-resistant genes to spread in the food chain,posing a great threat to food safety and human health.Therefore,the surveillance study of the prevalence and antimicrobial resistance of Salmonella isolates from food animals is of great significance for food safety risk assessment and prevention and treatment of Salmonella infections.This paper collected fecal samples,anal swab samples and cecal samples from 22 chicken,duck,pig farms(households)and abattoirs in six cities including Hangzhou,Jiaxing,Shaoxing,Wenzhou,Huzhou and Qvzhou in Zhejiang Province.The Salmonella in samples was selectively cultured and identified.Subsequently,the serotyping,MLST and MIC to 16 antibiotics were tested to analyze the Salmonella isolates,and two Salmonella isolates carrying mcr-1 were performed whole-genome sequencing and genomics analysing.The main findings are as follows:(1)From March 2018 to October 2018,a total of 1145 fecal samples,anal swab samples and cecal samples were collected,including 596 duck samples,419 chicken samples and 130 pig samples.A total of 193 Salmonella were isolated and identified,and the overall isolation rate was 16.86%.The isolation rates of positive Salmonella in chicken,duck and pig samples were 14.32%,18.79% and 16.15%,respectively.(2)125 Salmonella isolates were selected for serotyping and MLST typing.The serotype identification results showed that there were 15 serotypes in all isolates,of which the largest number of serotypes was Salmonella Indiana,followed by Salmonella Typhimurium.The Salmonella from chicken included three serotypes,the main serotypes were Salmonella Enteritidis and Salmonella Indiana;The Salmonella from duck included eleven serotypes,the main serotypes were Salmonella Typhimurium and Salmonella Indiana;The Salmonella from pig included three serotypes,the main serotypes was Salmonella Rissen.The MLST results showed that 125 Salmonella were divided into 14 STs,and the dominant sequence types were ST17 and ST19.The study found that MLST typing results were basically consistent with serotyping results.(3)The antibiotic resistance results of Salmonella showed that 90.4% of the Salmonella isolates showed at least one resistant phenotype,53.6% had four or more resistant phenotypes,and 10.4% could be resistant to ten or more antibiotics.The antibiotic with the highest rate of resistance was ampicillin,and all Salmonella isolates showed sensitivity to meropenem.There were 30 different antibiotic resistance phenotypes in antibiotic-resistant Salmonella,among which triple antibiotic-resistant phenotype was most common.Among major Salmonella serotypes,S.Typhimurium and S.Enteritidis had the largest proportion of Salmonella with 1-3 resistant phenotypes;the most common Salmonella serotype with 4-6 resistant phenotypes was S.Rissen;the most common Salmonella serotypes with 7-9 resistant phenotypes were S.Meleagridis and S.Indiana.In addition,only S.Indiana has ten or more resistant phenotypes.(4)Two Salmonella isolates carrying mcr-1 were detected in this study,and their whole-genome sequence were obtained by Pac Bio RS II platform.The SSDFZ54 isolate belonged to ST5399 S.Ngor and carrying 11 plasmids;the SSDFZ69 isolate belonged to ST2529 S.Goldcoast and carrying 2 plasmids.The mcr-1 gene were detected in Inc HI2 p TB501(188,527 bp)and Inc X4 p TB602(33,303 bp),respectively.The p TB501 did not have the ability to transfer to recipient strain.Compared with plasmid p HNSHP45-2,p TB501 lacked a large region of 44,299 bp harboring the tra region,some antimicrobial resistance genes such as cml A,sul1,sul3,and some insertion sequences.The p TB602 could transfer to recipient strain successfully.Plasmid p TB602 had a high similarity with other Inc X4 plasmids and was a prevalent kind of plasmid carrying mcr-1.A phylogenetic tree based on Inc HI2-type plasmids harboring mcr-1 genes with complete sequences available in Gen Bank.The results showed that plasmids isolated from different hosts could be clustered into the same branch,indicating that there occured clone spread.In addition,all plasmids from non-Chinese sources were clustered together,indicating that these plasmids have a a common ancestor.A backbone,tra-and AMR genes-relative variable regions were proposed based on BRIG comparison analysis.mcr-1 gene is located in the backbone;a lot of antibiotic resistance genes and IS sequences are located in the AMR genes-relative variable region which is closely related to the resistant phenotypes of strains and the acquisition of drug resistance genes;a number of key tra-genes are located in the tra-relative variable region,which deciding that the conjugation and transfer ability of the plasmid.