Dairy cows can be reservoirs of foodborne pathogens such as Salmonella that pose serious public health risks to humans. The study was designed to examine the molecular epidemiology and antimicrobial resistance profiles of Salmonella isolates from dairy heifer calves and adult lactating cows in the pasture-based system of Australia. A total of 838 animals (328 heifer calves and 510 lactating cows) from 22 farms were sampled. Overall, 54 Salmonella isolates were recovered (calves 28/328 and cows 26/510). A herd-level Salmonella prevalence of 50% (95% confidence interval: 31%-69%) was recorded. Within-herd prevalence for Salmonella ranged between 4%-29% and 4%-45% among the heifer calves and adult lactating cows, respectively. Three different serovars were identified with Salmonella Infantis being the most common serovar (n = 33, 61%) followed by Salmonella Kiambu (n = 20, 37.0%) and one isolate of Salmonella Cerro (2%). The highest antimicrobial resistance prevalence of Salmonella isolates was found against streptomycin (n = 31, 57%), followed by cefoxitin (n = 12, 22%), ceftriaxone (n = 2, 4%), and chloramphenicol (n = 1, 2%). Multiple class resistance was observed on 4 isolates against cefoxitin, chloramphenicol, and streptomycin. Multilocus sequence types ST32 (61%), ST309 (37%), and ST367 (2%) were strongly linked to the serovars Salmonella Infantis, Salmonella Kiambu, and Salmonella Cerro, respectively. Whole genome sequencing of Salmonella isolates detected only 2 resistance genes: aac(6\') gene that confers resistance against aminoglycosides among 40.7% of the isolates, and a single isolate positive for the blaDHA-16 gene. Two distinct clusters among the serovars were observed suggesting 2 independent sources of spread. Despite the low prevalence of antimicrobial resistance among Salmonella from the dairy farms, our findings contribute to the regional and national understanding of antimicrobial resistance in dairy herds in Australia. There is need for continued antimicrobial resistance stewardship and surveillance programs to ensure the production of high-quality food products and the long-term protection of both animal and human health.

Salmonella Infantis has emerged as a major clinical pathogen causing gastroenteritis worldwide in recent years. As an intracellular pathogen, Salmonella has evolved to manipulate and benefit from the cell death signaling pathway. In this study, we discovered that S. Infantis inhibited apoptosis of infected Caco-2 cells by phosphorylating Akt. Notably, Akt phosphorylation was observed in a discontinuous manner: immediately 0.5 h after the invasion, then before peak cytosolic replication. Single-cell analysis revealed that the second phase was only induced by cytosolic hyper-replicating bacteria at 3-4 hpi. Next, Akt-mediated apoptosis inhibition was found to be initiated by Salmonella SopB. Furthermore, Akt phosphorylation increased mitochondrial localization of Bcl-2 to prevent Bax oligomerization on the mitochondrial membrane, maintaining the mitochondrial network homeostasis to resist apoptosis. In addition, S. Infantis induced pyroptosis, as evidenced by increased caspase-1 (p10) and GSDMS-N levels. In contrast, cells infected with the ΔSopB strain displayed faster but less severe pyroptosis and had less bacterial load. The results indicated that S. Infantis SopB-mediated Akt phosphorylation delayed pyroptosis, but aggravated its severity. The wild-type strain also caused more severe diarrhea and intestinal inflammatory damage than the ΔSopB strain in mice. These findings revealed that S. Infantis delayed the cells\' death by intermittent activation of Akt, allowing sufficient time for replication, thereby causing more severe inflammation.

Salmonella is important as both a cause of clinical disease in swine and as a source of food-borne transmission of disease to humans. Lactobacillus and Bacillus are often used as antibiotic substitutes to prevent Salmonella infection. In this study, we evaluated the effects of a select mixture of Lactobacillus johnsonii L531, Bacillus licheniformis BL1721 and Bacillus subtilis BS1715 (LBB-mix) in prevention of Salmonella enterica serovar Infantis infection in a pig model. LBB-mix was orally administered to newly weaned piglets for seven days before S. Infantis challenge. LBB-mix pretreatment ameliorated S. Infantis-induced fever, leukocytosis, growth performance loss, and ileal inflammation. Pre-administration of LBB-mix reduced the number of Salmonella in the feces but increased the number of goblet cells in the ileum. S. Infantis infection resulted in an increase in cell death in the ileum, this increase was attenuated by LBB-mix consumption. Claudin 1 and cleaved caspase-1 expression was decreased in the ileum of pigs challenged with S. Infantis, but not in pigs pretreated with LBB-mix. In conclusion, our data indicate that a select LBB-mix has positive effects on controlling S. Infantis infection via alleviating inflammation and maintaining the intestinal mucosal barrier integrity in pigs.

In the current study, we screened Lactobacillus strains isolated from the colon of clinically healthy weaned piglets for potential probiotic properties and isolated Lactobacillus. johnsonii L531, which produced high levels of beneficial metabolites (butyric, acetic, and lactic acid) in vitro. We also evaluated the efficacy of this metabolites-producing probiotic in treating Salmonella. Infantis infection. Oral administration of L. johnsonii L531 to newly weaned piglets significantly decreased levels of Salmonella colonization in colonic and jejunal contents, accelerated the clearance of Salmonella in feces after infection, and reduced S. Infantis translocation to the spleen. Pretreatment with SCFAs-promoting probiotic L. johnsonii L531 significantly ameliorated the depletion of SCFAs induced by S. Infantis infection and led to significantly greater weight gain and better feed conversion ratios compared to piglets challenged only with S. Infantis. These data provide further evidence that SCFAs-promoting probiotic L. johnsonii L531 treatment could be a suitable nonantibiotic alternative for controlling Salmonella infection and maintaining metabolic homeostasis, thereby enhancing the gut health of piglets during the critical weaning period.

Salmonella enterica serovar Infantis (S. Infantis) is a common source of foodborne gastroenteritis worldwide. Here, Lactobacillus rhamnosus GG (LGG) was administrated to weaned piglets for 1 week before S. Infantis challenge. S. Infantis caused decreased ileal mucosal microbiota diversity, a dramatic Lactobacillus amylovorus bloom, and decreased abundance of Arsenicicoccus, Janibacter, Kocuria, Nocardioides, Devosia, Paracoccus, Psychrobacter, and Weissella. The beneficial effect of LGG correlated with the moderate expansion of L. amylovorus, L. agilis, and several members of the phyla Proteobacteria, Firmicutes, and Bacteroidetes. S. Infantis translocation to the liver was decreased in the LGG-pretreated piglets. An in vitro model of LGG and S. Infantis co-incubation (involving the porcine intestinal epithelial cell line IPEC-J2) was established, and nalidixic acid was used to kill the extracellular S. Infantis. LGG suppressed the initial S. Infantis invasion in the IPEC-J2 cells and deceased the rate of cell death. LGG inhibited S. Infantis-induced autophagy and promoted epidermal growth factor receptor (EGFR) and Akt phosphorylation in both the ileum and IPEC-J2 cells. Our findings suggest that LGG inhibited S. Infantis-induced autophagy by promoting EGFR-mediated activation of the negative mediator Akt, which, in turn, suppressed intestinal epithelial cell death and thus restricted systemic S. Infantis infection. LGG can restore the gut microbiota balance and preserve the autophagy-related intestinal epithelial barrier, thereby controlling infections.

The high rate of Salmonella enterica serovar Infantis (S. Infantis) infection poses significant risk for the development of non-typhoidal Salmonella gastroenteritis. However, efficient strategies to prevent or treat the infection remain elusive. Here, we explored the effect of the probiotic Lactobacillus rhamnosus GG (LGG) administration in preventing S. Infantis infection in a pig model. Probiotic LGG (1.0 × 1010 CFU/day) was orally administered to newly weaned piglets for 1 week before S. Infantis challenge. LGG pretreatment reduced the severity of diarrhea and alleviated intestinal inflammation caused by S. Infantis. Pre-administration of LGG excluded Salmonella from colonization of the jejunal mucosa but increased the abundance of Bifidobacterium in the feces. LGG promoted the expansion of CD4+ T-bet+ IFNγ+ T cells but attenuated S. Infantis-induced increases in the percentage of CD4+ IFNγ+ T cells and serum interleukin (IL)-22 levels in peripheral blood after S. Infantis challenge. In the small intestine, LGG pretreatment upregulated expression of the transcription factor T-bet but downregulated the S. Infantis-induced increase of CD4+ IFNγ+ T cells in Peyer\'s patches and IL-7Rα expression in the jejunum. Notably, LGG-treated pigs had enhanced expression of IL-22 and activated STAT3 in the ileum in response to S. Infantis infection. Pretreatment of pigs with LGG also elevated intestinal IL-22-binding protein production in response to S. Infantis challenge. In contrast, LGG consumption reduced the S. Infantis-induced increase in the number of CCL20-expressing cells in the jejunum. Our results suggest that the mechanism by which LGG ameliorates the intestinal inflammation caused by S. Infantis involves the upregulation of T-bet, activation of STAT3, and downregulation of CCL20.