Distribution and Transmission of Colistin Resistance Genes mcr-1 and mcr-3 among Nontyphoidal Salmonella Isolates in China from 2011 to 2020.

Mobile colistin resistance (mcr) genes are present mainly in plasmids and can disseminate clonally or horizontally via either plasmids or insertion sequences in different genomic locations among the Enterobacteriaceae. A nationwide large-scale study on mcr prevalence and transmission in nontyphoidal Salmonella isolates is still lacking. Here, we identified 140 mcr-positive Salmonella isolates out of 7,106 isolates from 29 provinces in China from 2011 to 2020. We aligned short reads to putative plasmids from long-read hybrid assemblies and predicted the plasmid backbones of non-long-read sequencing isolates to elucidate mcr transmission patterns. The mcr-1 and mcr-3 genes are transmitted on similar high-risk clones (sequence type 34 [ST34]) but through plasmids of various replicon types. Furthermore, the ban on colistin use in food animals can lead to a decrease in the mcr-positive Salmonella prevalence among diarrheal patients, related mainly to IncHI2A_IncHI2 plasmids. We provide a framework for plasmid data incorporation into genomic surveillance systems, contributing to a better understanding of mcr spread and transmission. IMPORTANCE Nontyphoidal Salmonella is one of four major causative agents of diarrheal diseases globally, with most cases of salmonellosis being mild. Antimicrobial treatments are required for cases of life-threatening infections, and colistin is one of the last-line antibiotics for the treatment of multidrug-resistant Salmonella infections. However, the efficacy of colistin has been compromised by the emergence of various mcr genes. To elucidate the transmission of mcr genes in Salmonella isolates, our study analyzed 7,106 Salmonella strains from 29 provinces in China from 2011 to 2020. The results showed that mcr genes are transmitted on similar high-risk clones (ST34) but through plasmids of various replicon types. In addition, our data illustrated that the ban on the use of colistin in food animals led to a significant decrease in mcr-positive isolates. Our findings offer an essential step toward a more comprehensive understanding of the spread and transmission of mcr genes.

[Preliminary quantitative risk assessment of the effect of Salmonella on human health in retail fresh pork].

OBJECTIVE: A quantitative microbiological risk assessment was conducted for Salmonella spp. in fresh pork from retail stores to consumers in China in order to provide evidence for adopting effective risk management measures. METHODS: The national food safety and risk monitoring data was used to estimate the initial contamination level of Salmonella in the retail fresh pork. The growth model and the cross-contamination model of Salmonella were referenced from the ComBase data base and scientific literature, respectively. Then the human health risk was predicted by the consumption of Salmonella-contaminated fresh pork and the Beta-Poisson dose-response model. RESULTS: The number of Salmonellosis per year among 1 million residents due to fresh pork was estimated to be 4748.The sensitivity analysis showed that the risk of Salmonellosis was mainly and positively correlated with the preparation rate of ready to eat food, the consumption quantity of pork, the initial contamination level of Salmonella in fresh pork, and the proportion of improper cleaning of knives and chopping boards. When the contamination level of Salmonella in fresh pork is controlled below 1 CFU/g, the average probability of illness will be reduced by 59.39%; the average probability of illness will be reduced by 58.97% by increasing the ratio of complete cleaning of chopping boards or the separation ratio of chopping boards for raw and ready-to-eat food to 90%. CONCLUSION: The risk could be effectively reduced by controlling the concentration of Salmonella contamination in retail pork or improving hygiene practices.

Prevalence and antimicrobial susceptibility of Salmonella in the commercial eggs in China.

Salmonellosis is a challenge to public health globally, and many infections have been principally linked to the consumption of contaminated eggs. The objective of this study is to estimate the prevalence of Salmonella in commercial eggs and susceptibilities of isolates to a panel of 14 antimicrobial agents which were determined according to Clinical and Laboratory Standards Institute (CSLI) procedures. A total of 33,288 eggs (5548 pooled samples of six eggs) were collected across China in 2016 and the prevalence of Salmonella was 0.5% (27/5548). The most predominant serotype was S. enteritidis. No significant differences were observed on the basis of the egg component tested, shell condition, packaging type, sampling site or sampling season. However, there were significant differences among provincial regions. About 64.3% (n = 18) isolates were resistant to nalidixic acid, followed by ampicillin (39.3%) and ampicillin/sulbactam (39.3%). All isolates were susceptible to ceftazidime, cefalothin, ciprofloxacin, cefepime, cefotaxime, imipenem and meropenem. Three Salmonella isolates exhibited resistance to multiple antibiotics. This study provides valuable baseline data of the occurrence of Salmonella in eggs, which will be used for risk assessments of possible human foodborne infections associated with the consumption of contaminated eggs.

[Epidemiological characteristics of Salmonella in animal source foods in Hunan].

OBJECTIVE: To study the molecular epidemiological characteristics of Salmonella in animal source foods in Hunan. METHODS: The fair trade markets and supermarkets of ten cities were chosen to sample animal source foods for isolating Salmonella in Hunan province in 2010. A total of 692 samples were collected by aseptic sampling, included 159 livestock meats, 152 poultry meats, and 381 aquatic products.Salmonella strains isolated were subjected to stereotyping, antimicrobial susceptibility testing and pulsed field gel electrophoresis (PFGE). RESULTS: Salmonella was detected in 93 of 692 animal food samples with the detection rate of 13.4%. The detection rates of Salmonella in poultry meats, livestock meats and aquatic products were 23.0% (35/152), 22.6% (36/159) and 5.8% (22/381) respectively. Therefore, the detection rate in aquatic products was lower than that of poultry meats and livestock meats (χ(2) = 33.86, P < 0.05; χ(2) = 33.29, P < 0.05, respectively). The serotypes of isolates showed diversity, and Salmonella Derby (33/94, 35.1%) was the predominant serotypes.79.8% (75/94) strains showed resistant to more than one antibiotic used in the test, 31.9% (30/94) strains showed resistant to more than 5 antibiotics. A significant difference was observed for multidrug resistance between Salmonella isolated from poultry (47.2%, 17/36) and livestock meats (22.2%, 8/36) (χ(2) = 4.96, P < 0.05). And the highest resistant rate was found in tetracycline, as high as 62.8% (59/94). All the strains were divided into 69 PFGE subtypes.Furthermore the dominating subtypes were type 7 (6 strains), type 15 (4 strains), type 22 (6 strains). CONCLUSION: Inspection results showed that Salmonella contamination in animal source foods were serious in Hunan province, and the isolates expressed high level resistance to the antibiotics.Furthermore the PFGE results indicated that there were epidemic strains of Salmonella in Hunan.