Salmonella Infection in Pigs: Disease, Prevalence, and a Link between Swine and Human Health

The genomic characteristics of dominant Salmonella enterica serovars from retail pork in Sichuan province, China

Foodborne Salmonella is the main cause of salmonellosis in China. Porcine animals are a reservoir for this bacterium consequently posing a threat to food safety and public health. In this study, 157 out of 240 pork samples (65.42 %) were identified as Salmonella-positive. From these, after isolation and deduplication, 376 Salmonella isolates were collected. Twenty four serovars were identified based on WGS, among which S. London/ST155 (24.47 %), S. Rissen/ST469 (23.40 %), S. Derby/ST40 (13.56 %), and S. 4,[5],12:i:- (monophasic S. Typhimurium)/ST34 (13.30 %) were dominant. In all, 69.68 % (262/376) of these isolates expressed multidrug resistance (MDR, defined as resistance to compounds in three or more antimicrobial classes) phenotypes with S. London (54.35 %, 50/92) accounting for the highest proportion of these. Notably, the resistance to front-line critically important antimicrobial agents (CIA), including cephalosporins, ciprofloxacin, and azithromycin was 0.80 %. Based on in silico analysis, antimicrobial resistant-encoding genes (ARG) identified in the MDR isolates included aac(3)-IId, aac(6')-Iaa, blaTEM-1B, mph(A), qnrB6, aac(6')-Ib-cr, sul1, sul2, and tet(A), which expressed resistance to aminoglycosides, β-lactams, macrolides, quinolones, sulfonamides, and tetracyclines. Furthermore, diverse biocide and heavy metal resistance-encoding genes were distributed across different serovars with triC encoding triclosan resistance being identified exclusively in S. London. Moreover, monophasic S. 4,[5],12:i:- carried the greatest number of virulence factors and heavy metal resistance genes among the dominant serovars. This study extended our understanding of the genomic epidemiology and multidrug resistance of Salmonella derived from pork and highlighted the potential risk to human health, posed by commonly encountered serovars.

Anatomo-pathological investigations in pigs of the Piedmont region (Northern Italy) for infectious diseases surveillance in an antimicrobial resistance perspective

Intensive breeding responds to the expanding market demand and animal health must be guaranteed to safeguard human and Public Health. Pig farming has grown in the last decades, leading to increased animal stress, pathogens dissemination, and the consequent use of antibiotics and the spread of antimicrobial-resistant bacteria. The present study aimed to describe the anatomopathological findings observed during necropsies performed from 2019 to 2021 on deceased pigs from five intensive breeding farms in northwestern Italy also reporting bacterial isolations, antimicrobial susceptibility testing, and virological results. Weaned pigs (WP) (N = 143) represented the majority of the carcasses, while the remaining part (N = 49) were fattening pigs (FP). Most of WP were affected by systemic disease (N = 79, 55.2 %), whereas 49 % of FP were affected by respiratory disease (N = 24). Streptococcus suis was the most frequently isolated microorganism in respiratory (around 20 % in both WP and FP) and systemic cases (25.3 % in WP and 33.3 % in FP). Enteric disorders were attributable to Brachyspira spp. and Escherichia coli in WP and FP (44.5 % and 55.5 %, respectively). Escherichia coli was considered to be the second causative agent of systemic disease (22.8 %) in WP. Streptococcus suis and monophasic Salmonella Typhimurium were isolated and demonstrated a high rate of multi-drug resistance (84 % and 100 %, respectively): the first showed resistance mainly against tetracyclines (100 %), lincosamides (88 %) and macrolides (84 %), whereas the latter to tetracyclines, sulfametox-trimethoprim, ampicillin, florfenicol, and enrofloxacin. Necropsy and microbiological assays are powerful tools for disease surveillance programs, highlighting potential risks for public health.

The prevalence, distribution, and diversity of Salmonella isolated from pork slaughtering processors and retail outlets in the Shandong Province of China

Salmonella is a foodborne pathogen of global significance and is highly prevalent in pork. This study investigated the prevalence, contamination distribution, virulence genes and antibiotic resistance of Salmonella in 3 pork processors in the Shandong Province of China. Samples were collected from 13 different sampling sources across the slaughter procedures (600 samples) as well as at retail outlets supplied by these processors (45 samples). The prevalence was 18.9 % among all the samples, with the highest prevalence observed in feces (40.0 %), lairage pens (38.0 %), and hides (34.0 %). A total of 6 serotypes were identified, with S. Rissen (46.3 %) and S. typhimurium (32.0 %) found to be the most prevalent serotypes. 86.8 % of Salmonella isolates were multi-drug resistant, with the majority of strains resistant to erythromycin, sulfisoxazole, and ampicillin. The multilocus sequence typing analysis revealed 6 STs were obtained from 45 isolates, with the dominant type ST469 accounting for 40.0 % of the total, which suggested a high possibility of cross-contamination between the plant processing chain and retail outlets. This work reveals the prevalence and correlation of Salmonella isolates between pork slaughter and retail outlets and acts as a case-study for stakeholders wishing to improve pork supply chain hygiene, control cross-contamination between the various slaughtering processes, and obtain continuous updates on Salmonella surveillance.

Characterization of novel mutations involved in the development of resistance to colistin in Salmonella isolates from retail pork in Shanghai, China

Salmonella is an important foodborne pathogen that poses a significant threat to food safety. This study aims to assess the prevalence, genomic features, and colistin-resistant mechanisms of Salmonella isolates collected from 118 retail pork samples from January 2021 to January 2022 in Shanghai, China. Overall, 46 (39.0 %, 46/118) Salmonella isolates were collected, which were identified as 12 serotypes by genomic analysis, including Salmonella Typhimurium (n = 17) and Salmonella London (n = 6). Antimicrobial resistance profiling revealed that the resistance rate of these isolates to colistin was 13.0 % (6/46), while 60.9 % (28/46) exhibited multidrug-resistant. It was found that there were 51 distinct antimicrobial resistance genes in these 46 isolates, which were predominantly associated with resistance to aminoglycosides, fluoroquinolones, and β-lactams. More importantly, among six colistin-resistant isolates, two isolates (Salmonella Schwarzengrund and Salmonella Indiana) were found to carry the mcr-1 gene. The mechanism of resistance in the remaining four colistin-resistant isolates was further studied, and it was found that there were nine amino acid substitutions in PmrAB. It was demonstrated by site-directed mutagenesis that novel substitutions G53W in PmrA and I83V in PmrB led to colistin resistance in Salmonella (MIC = 2 or 4 μg/mL). Analysis results by real-time quantitative PCR and mass spectrometry indicated that the mutants PmrAG53W and PmrBI83V displayed higher expression levels of the gene pmrE than in the parental strain. This upregulation resulted in an increase in the production of 4-amino-4-deoxy-l-arabinose (L-Ara4N) that modified lipid A, thereby conferring resistance to colistin. These findings demonstrated that there was a high prevalence of MDR Salmonella isolates in retail pork in Shanghai, and the substitution G53W in PmrA and I83V in PmrB were independent factors contributing to the development of resistance to colistin in Salmonella via modification of lipid A with L-Ara4N.

Recombinant Live-Attenuated Salmonella Vaccine for Veterinary Use

Vaccination is essential for maintaining animal health, with priority placed on safety and cost effectiveness in veterinary use. The development of recombinant live-attenuated Salmonella vaccines (RASVs) has enabled the construction of balanced lethal systems, ensuring the stability of plasmid vectors encoding protective antigens post-immunization. These vaccines are particularly suitable for production animals, providing long-term immunity against a range of bacterial, viral, and parasitic pathogens. This review summarizes the progress made in this field, with a focus on clinical trials demonstrating the efficacy and commercial potential of RASVs in veterinary medicine.

Examination of Salmonella Prevalence in Pigs Through Rye-Based Feeding and Coarser Feed Structure Under Field Conditions

INTRODUCTION

Salmonellosis is the second most commonly occurring bacterial zoonosis in Germany. Rye in pig feeding offers new possibilities for addressing that issue due to its high content of non-starch polysaccharides (NSPs). These are fermented in the intestinal tract to specific fermentation products, which seem to have bacteriolytic effects against Salmonella. A coarse feed structure can display synergistic effects.

METHODS

Seven conventional pig fattening farms increased the rye content (40%-70%) while limiting the amount of fine particles (maximum of 20% ≤0.25 mm). Samples from pigs were tested for Salmonella antibodies and compared with samples from 167 farms without any changes to the feed.

RESULTS

Rye-based diets had a significant (p < 0.05) impact on Salmonella antibody (percentage optical density [OD%]) detection. In this study, it became apparent that significantly fewer positive OD% values could be detected due to the increase in rye compared to farms that did not change the feed (Farm 6 P0: 35.45 ± 36.18; P1: 15.48 ± 16.98; P2: 9.36 ± 8.17). An elimination of Salmonella could not be achieved, but especially farms with high antibody counts were able to strongly reduce those in both phases consecutively (Farm 5 P0: 35.17 ± 35.53; P1: 18.56a ± 20.96; P2: 13.38a ± 18.99). That was different on farms without adapted feeding, where an increase in Salmonella antibodies was observed (P0: 17.38 ± 22.21; P1: 20.12 ± 25.39; P2: 18.12 ± 25.44).

CONCLUSION

By increasing the proportion of rye and limiting the proportion of fine particles in the feed, Salmonella antibodies (OD% values) in meat juice and blood can be significantly reduced, especially on farms with an initially high incidence of Salmonella. If that is implemented in feeding across the board on farms, an improvement in food safety and a decreased risk of zoonosis can be expected.

Evaluation of Multidrug Resistance of Salmonella Isolated from Pork Meat Obtained from Traditional Slaughter Systems in Romania

Antibiotic resistance among pathogenic bacteria in humans and animals poses a critical public health challenge, leading to diminished effectiveness of existing antimicrobial treatments. Notably, animal-derived food products are significant vectors for the transmission of resistant bacteria to humans, with Salmonella species being predominant culprits in foodborne illnesses. This study investigates the prevalence and antibiotic resistance patterns of Salmonella serovars isolated from traditionally sourced pork meat in Romania. Over a four-year period, 208 pork samples were collected using standardized protocols during traditional slaughtering practices. Salmonella spp. were isolated following ISO 6579:2002 guidelines and confirmed using biochemical assays and PCR. Serotyping was performed using specific antisera, and antimicrobial susceptibility testing was conducted through the standard disk diffusion method, assessing 11 antibiotics. Results indicated a 23.07% prevalence of Salmonella, with 48 isolates categorized into eight serovars, primarily S. Infantis (n = 15), S. Typhimurium (n = 15), and S. Derby (n = 11). PCR results confirmed the presence of Salmonella by detecting the hilA and ompC genes, with 31.25% of isolates being positive for the Typhimurium-specific sequence. Notably, 93.75% of the isolates were multidrug-resistant (MDR), exhibiting high resistance rates against streptomycin (91.66%) (>10 µg), tetracycline (83.33%) (>30 µg), and sulfamethoxazole (68.75%) (>300 µg). More than 60% of MDR isolates displayed resistance to five or more antibiotics. These findings underscore the need for coordinated control measures in the pork production chain to combat the spread of Salmonella and protect public health. Enhanced surveillance and intervention strategies are crucial for addressing antibiotic resistance and reducing the risk of foodborne illnesses linked to contaminated animal products.

Effect of single parenteral administration of marbofloxacin on bacterial load and selection of resistant Enterobacteriaceae in the fecal microbiota of healthy pigs

BACKGROUND

Antimicrobial resistance (AMR) is a global concern impacting both humans, animals and their environment. The use of oral antimicrobials in livestock, particularly in pigs, has been identified as a driver in the selection of AMR bacteria. The aim of the present study was to evaluate the effects of a single intramuscular (IM) dose of marbofloxacin (8 mg/kg) on Enterobacteriaceae and E. coli populations, as well as on fluoroquinolone resistance within the fecal microbiota of pigs. Twenty healthy pigs, 60-days old, were divided into two groups: a treated group (n = 13) and a control group (n = 7) and were monitored over a 28-day experimental period. Fecal samples were collected from all animals for the isolation of E. coli and Salmonella strains. The minimum inhibitory concentration (MIC) of marbofloxacin for the isolates recovered on MacConkey agar supplemented with 1 or 4 µg/mL of marbofloxacin and for some generic E. coli isolates (recovered from MacConkey agar not supplemented with marbofloxacin) was determined using the broth microdilution method. Genomic DNA was extracted from the confirmed bacterial strains and sequenced using the Sanger method to identify mutations in the quinolone resistance determining regions (QRDRs) of the gyrA and parC genes.

RESULTS

The single IM administration of marbofloxacin resulted in a significant decrease in Enterobacteriaceae and E. coli fecal populations from days 1 to 3 post- treatment. No Salmonella isolates were detected in either group, and no marbofloxacin-resistant E. coli isolates were identified. The MIC of the selected generic E. coli strains (n = 100) showed an increase to up to 0.5 µg/mL between days 1 and 3 post-treatment but remained below the clinical breakpoint of marbofloxacin resistance (4 µg/mL). Sequencing of these isolates revealed no mutations in gyrA and parC genes.

CONCLUSIONS

The present study showed that this dosing regimen of marbofloxacin significantly decreases the fecal shedding of Enterobacteriaceae and E. coli populations in pigs, while limiting the selection of marbofloxacin-resistant E. coli isolates. These findings warrant validation in sick pigs to support the selective use of this antibiotic solely in cases of clinical disease, thereby minimizing the reliance on conventional (metaphylactic) group treatments in pigs.

Immunopeptidomics of Salmonella enterica Serovar Typhimurium-Infected Pig Macrophages Genotyped for Class II Molecules

Salmonellosis is a zoonotic infection that has a major impact on human health; consuming contaminated pork products is the main source of such infection. Vaccination responses to classic vaccines have been unsatisfactory; that is why peptide subunit-based vaccines represent an excellent alternative. Immunopeptidomics was used in this study as a novel approach for identifying antigens coupled to major histocompatibility complex class II molecules. Three homozygous individuals having three different haplotypes (Lr-0.23, Lr-0.12, and Lr-0.21) were thus selected as donors; peripheral blood macrophages were then obtained and stimulated with Salmonella typhimurium (MOI 1:40). Although similarities were observed regarding peptide length distribution, elution patterns varied between individuals; in total, 1990 unique peptides were identified as follows: 372 for Pig 1 (Lr-0.23), 438 for Pig 2 (Lr.0.12) and 1180 for Pig 3 (Lr.0.21). Thirty-one S. typhimurium unique peptides were identified; most of the identified peptides belonged to outer membrane protein A and chaperonin GroEL. Notably, 87% of the identified bacterial peptides were predicted in silico to be elution ligands. These results encourage further in vivo studies to assess the immunogenicity of the identified peptides, as well as their usefulness as possible protective vaccine candidates.

Relevant foodborne bacteria associated to pork production chain

Foodborne diseases affect millions of people globally, resulting in a huge number of hospitalizations and deaths. In this context, laboratory-based research is crucial to identify the major pathogens as well as the relevance of each one for distinct food production chains. Pork meat is very popular, being the most consumed meat in many countries and its inspection at the slaughterhouse is the main component of surveillance to protect consumers. Healthy pigs may carry pathogenic and antibiotic resistant bacteria that can be subsequently transferred to humans through the consumption of contaminated meat. Further, the food processing environment can harbor pathogenic persistent bacteria, representing a risk of cross-contamination to pork meat, demanding strict slaughtering procedures. Among these foodborne bacteria, Salmonella, Yersinia enterocolitica, Escherichia coli, Campylobacter spp., Listeria monocytogenes and Staphylococcus aureus are the most relevant in the pork production chain. Molecular subtyping has been fundamental for pathogen detection and also to track transmission, and nowadays it is a key component of the efforts to prevent and control foodborne diseases. In this chapter, characteristics of these major foodborne bacteria associated to pork meat will be addressed, including their occurrence and importance along the pork production chain, worldwide distribution, typing, as well as control and prevention measures from farm to fork.

Assessing antimicrobial resistance profiles of Salmonella enterica in the pork production system

Introduction. Salmonella enterica is a significant enteric pathogen affecting human and livestock health. Pork production is a common source of Salmonella contamination, with emerging multidrug resistance (MDR) posing a global health threat.Gap statement. Salmonella contamination and antimicrobial resistance (AMR) profiles in the pig production chain are underreported.Aim. To investigate the prevalence of S. enterica in the pig production chain and characterise their AMR profiles.Methodology. We collected 485 samples from pig farms, a standard pig abattoir and retail markets in Patthalung and Songkhla provinces in southern Thailand. Antimicrobial susceptibility testing was performed on these samples, and AMR profiles were determined.Results. S. enterica was detected in 68.67% of farm samples, 45.95% of abattoir samples and 50.67% of retail market samples. Analysis of 264 isolates, representing 18 serotypes, identified S. enterica serotype Rissen as the most prevalent. The predominant resistance phenotypes included ampicillin (AMP, 91.29%), tetracycline (TET, 88.26%) and streptomycin (STR, 84.47%). Over 80% of isolates showed resistance to three or more antimicrobial classes, indicating MDR. The AMP-STR-TET resistance pattern was found in nearly 70% of all MDR isolates across the production chain.Conclusions. The high prevalence of MDR is consistent with extensive antimicrobial use in the livestock sector. The presence of extensively resistant S. enterica highlights the urgent need for antimicrobial stewardship. Strengthening preventive strategies and control measures is crucial to mitigate the risk of MDR Salmonella spreading from farm to fork.

A Single Base Change in the csgD Promoter Resulted in Enhanced Biofilm in Swine-Derived Salmonella Typhimurium

Pathogenic Salmonella strains causing gastroenteritis typically can colonize and proliferate in the intestines of multiple host species. They retain the ability to form red dry and rough (rdar) biofilms, as seen in Salmonella enterica serovar Typhimurium. Conversely, Salmonella serovar like Typhi, which can cause systemic infections and exhibit host restriction, are rdar-negative. In this study, duck-derived strains and swine-derived strains of S. Typhimurium locate on independent phylogenetic clades and display relative genomic specificity. The duck isolates appear more closely related to human blood isolates and invasive non-typhoidal Salmonella (iNTS), whereas the swine isolates were more distinct. Phenotypically, compared to duck isolates, swine isolates exhibited enhanced biofilm formation that was unaffected by the temperature. The transcriptomic analysis revealed the upregulation of csgDEFG transcription as the direct cause. This upregulation may be mainly attributed to the enhanced promoter activity caused by the G-to-T substitution at position -44 of the csgD promoter. Swine isolates have created biofilm polymorphisms by altering a conserved base present in Salmonella Typhi, iNTS, and most Salmonella Typhimurium (such as duck isolates). This provides a genomic characteristics perspective for understanding Salmonella transmission cycles and evolution.

Tackling salmonellosis: A comprehensive exploration of risks factors, impacts, and solutions

Salmonellosis, caused by Salmonella species, is one of the most common foodborne illnesses worldwide with an estimated 93.8 million cases and about 155,00 fatalities. In both industrialized and developing nations, Salmonellosis has been reported to be one of the most prevalent foodborne zoonoses and is linked with arrays of illness syndromes such as acute and chronic enteritis, and septicaemia. The two major and most common Salmonella species implicated in both warm-blooded and cold-blooded animals are Salmonella bongori and Salmonella enterica. To date, more than 2400 S. enterica serovars which affect both humans and animals have been identified. Salmonella is further classified into serotypes based on three primary antigenic determinants: somatic (O), flagella (H), and capsular (K). The capacity of nearly all Salmonella species to infect, multiply, and survive in human host cells with the aid of their pathogenic and virulence arsenals makes them deadly and important public health pathogens. Primarily, food-producing animals such as poultry, swine, cattle, and their products have been identified as important sources of salmonellosis. Additionally, raw fruits and vegetables are among other food types that have been linked to the spread of Salmonella spp. Based on the clinical manifestation of human salmonellosis, Salmonella strains can be categorized as either non-typhoidal Salmonella (NTS) and typhoidal Salmonella. The detection of aseptically collected Salmonella in necropsies, environmental samples, feedstuffs, rectal swabs, and food products serves as the basis for diagnosis. In developing nations, typhoid fever due to Salmonella Typhi typically results in the death of 5%-30% of those affected. The World Health Organization (WHO) calculated that there are between 16 and 17 million typhoid cases worldwide each year, with scaring 600,000 deaths as a result. The contagiousness of a Salmonella outbreak depends on the bacterial strain, serovar, growth environment, and host susceptibility. Risk factors for Salmonella infection include a variety of foods; for example, contaminated chicken, beef, and pork. Globally, there is a growing incidence and emergence of life-threatening clinical cases, especially due to multidrug-resistant (MDR) Salmonella spp, including strains exhibiting resistance to important antimicrobials such as beta-lactams, fluoroquinolones, and third-generation cephalosporins. In extreme cases, especially in situations involving very difficult-to-treat strains, death usually results. The severity of the infections resulting from Salmonella pathogens is dependent on the serovar type, host susceptibility, the type of bacterial strains, and growth environment. This review therefore aims to detail the nomenclature, etiology, history, pathogenesis, reservoir, clinical manifestations, diagnosis, epidemiology, transmission, risk factors, antimicrobial resistance, public health importance, economic impact, treatment, and control of salmonellosis.

Construction and Mechanism Exploration of Highly Efficient System for Bacterial Ghosts Preparation Based on Engineered Phage ID52 Lysis Protein E

Bacterial ghosts (BGs) are hollow bacterial cell envelopes with intact cellular structures, presenting as promising candidates for various biotechnological and biomedical applications. However, the yield and productivity of BGs have encountered limitations, hindering their large-scale preparation and multi-faceted applications of BGs. Further optimization of BGs is needed for the commercial application of BG technology. In this study, we screened out the most effective lysis protein ID52-E-W4A among 13 mutants based on phage ID52 lysis protein E and optimized the liquid culture medium for preparing Escherichia coli Nissle 1917 (EcN). The results revealed a significantly higher lysis rate of ID52-E-W4A compared to that of ID52-E in the 2xYT medium. Furthermore, EcN BGs were cultivated in a fermenter, achieving an initial OD600 as high as 6.0 after optimization, indicating enhanced BG production. Moreover, the yield of ID52-E-W4A-induced BGs reached 67.0%, contrasting with only a 3.1% yield from φX174-E-induced BGs. The extended applicability of the lysis protein ID52-E-W4A was demonstrated through the preparation of Salmonella pullorum ghosts and Salmonella choleraesuis ghosts. Knocking out the molecular chaperone gene slyD and dnaJ revealed that ID52-mediated BGs could still undergo lysis. Conversely, overexpression of integral membrane enzyme gene mraY resulted in the loss of lysis activity for ID52-E, suggesting that the lysis protein ID52-E may no longer rely on SlyD or DnaJ to function, with MraY potentially being the target of ID52-E. This study introduces a novel approach utilizing ID52-E-W4A for recombinant expression, accelerating the BG formation and thereby enhancing BG yield and productivity.

Characterization of the Effects of a Novel Probiotic on Salmonella Colonization of a Piglet-Derived Intestinal Microbiota Using Improved Bioreactor

The carriage of Salmonella in pigs is a major concern for the agri-food industry and for global healthcare systems. Humans could develop salmonellosis when consuming contaminated pig products. On the other hand, some Salmonella serotypes could cause disease in swine, leading to economic losses on farms. The purpose of the present study was to characterize the anti-Salmonella activity of a novel Bacillus-based probiotic using a bioreactor containing a piglet-derived intestinal microbiota. Two methods of probiotic administration were tested: a single daily and a continuous dose. Salmonella enumeration was performed using selective agar at T24h, T48h, T72h, T96h and T120h. The DNA was extracted from bioreactor samples to perform microbiome profiling by targeted 16S rRNA gene sequencing on Illumina Miseq. The quantification of short-chain fatty acids (SCFAs) was also assessed at T120h. The probiotic decreased Salmonella counts at T96 for the daily dose and at T120 for the continuous one. Both probiotic doses affected the alpha and beta diversity of the piglet-derived microbiota (p < 0.05). A decrease in acetate concentration and an increase in propionate proportion were observed in the continuous condition. In conclusion, the tested Bacillus-based product showed a potential to modulate microbiota and reduce Salmonella colonization in a piglet-derived intestinal microbiota and could therefore be used in vivo.

Diversity of evolution in MDR monophasic S. Typhimurium among food animals and food products in Southern China from 2011 to 2018

The monophasic variant of Salmonella enterica serovar Typhimurium with the antigenic formula 1,4,[5],12:i:- is one of the most common pathogenic bacteria causing global food-borne outbreaks. However, the research on molecular characteristics and evolution of monophasic S. typhimurium in China is still lacking. In the current study, 59 monophasic S. typhimurium strains were isolated from food animals and food products in South China between 2011 and 2018. A total of 87.5 % of monophasic S. typhimurium isolates were grouped into one independent clade with other monophasic S. typhimurium strains in China distinct from other countries by phylogenomic analysis. These isolates possess variable genotypes, including multiple ARGs on plasmid IncHI2, diverse evolutions at the fljAB locus, and virulence factors. Our results suggest that the monophasic S. typhimurium isolates currently circulating in China might be an independent epidemic subtype.