Calf Gut Microbes Show Powerful Resilience Against Salmonella

Researchers at the University of Florida and Kyung Hee University have isolated strains of Bifidobacterium longum from healthy dairy calves that display remarkable resilience in the harsh conditions of the gastrointestinal tract and a striking ability to suppress Salmonella, one of the most economically significant zoonotic pathogens in cattle production. The study, published in Current Research in Food Science, offers a detailed genomic and functional portrait of bacteria that could form the basis of host-specific probiotics designed for livestock rather than adapted from human sources. The work arrives amid growing pressure to find sustainable alternatives to antibiotics in animal agriculture, a search that has intensified as concerns about antimicrobial resistance and consumer demand for drug-free production continue to reshape the industry.

The rationale behind the study rests on a persistent mismatch in the probiotic marketplace. Although Bifidobacterium longum is a well-characterized Gram-positive, anaerobic bacterium with a long history of safe use in humans, holding Qualified Presumption of Safety status from the European Food Safety Authority and Generally Recognized as Safe classification from the U.S. Food and Drug Administration, the vast majority of commercial strains were isolated from infant feces, adult intestines, and breast milk. Host specificity matters. Differences in gut anatomy, physiology, resident microbial communities, and diet between humans and livestock can undermine the adhesion capacity and survival of human-derived strains when they are administered to animals, limiting their efficacy in the very settings where they are increasingly needed.

Previous attempts to deploy human strains in animals have produced mixed results. An infant-derived B. longum strain reduced Campylobacter jejuni counts in poultry without improving growth performance, and piglets given a probiotic mixture containing an infant-derived B. longum subspecies alongside Lactobacillus rhamnosus failed to recover feed intake and body weight after infection with enterotoxigenic Escherichia coli. These observations led the research team to hypothesize that B. longum strains recovered from healthy calves would carry host-specific traits that enhance their survival and persistence in the bovine gut, and they set out to test this idea with an unusually comprehensive combination of whole-genome sequencing and in vitro functional assays.

Working under University of Florida Institutional Animal Care and Use Committee approval, the team collected rectal fecal samples from 47 healthy pre-weaning dairy calves between three and 28 days of age. Using Bifidus Selective Medium under anaerobic conditions, they recovered 225 presumptive isolates, from which 16 Bifidobacterium candidates were selected for whole-genome sequencing on an Illumina NextSeq platform. Rigorous taxonomic classification confirmed 12 of these as B. longum, alongside three Bifidobacterium pseudocatenulatum isolates and one Ligilactobacillus salivarius. Core-genome phylogenetic analysis revealed that the 12 B. longum isolates, while genetically close overall, split into two distinct clades, and no identical genomes were detected, underscoring the strain-level diversity circulating within a single herd. Intriguingly, all strains isolated from nine-day-old calves grouped within the same clade, hinting at age-related colonization patterns during early life.

Genome annotation painted a picture of bacteria well equipped for gastrointestinal life. Every strain carried the atpA-H gene cluster encoding the ATP synthase complex, which maintains intracellular pH homeostasis under acidic conditions, a fundamental requirement for surviving gastric transit. Roughly twenty loci tied to exopolysaccharide biosynthesis and biofilm assembly were distributed across the genomes, including multiple epsF variants showing strain-specific sequence variation, glycosyltransferases that influence adhesion and surface colonization. Conserved ribonucleotide reductase components including nrdH, nrdI, and nrdE2 support genome stability under oxidative stress, while ATP-dependent chaperone-protease systems such as clpX, clpB, and clpP, together with heat shock proteins dnaK, dnaJ, and the groL/groS chaperonin system, provide the protein quality control machinery needed to weather environmental shocks. Critically, screening with the BAGEL4 platform detected no known bacteriocin biosynthetic gene clusters, a finding that would later shape interpretation of the antimicrobial results.

To place the calf isolates in broader evolutionary context, the researchers constructed a core-genome phylogeny incorporating 792 publicly available B. longum genomes, of which 759 originated from humans. The resulting tree showed clear host-associated clustering: livestock strains, including the calf isolates, cows, and pigs, grouped together in a distinct clade, while companion animal and food-derived strains were more interspersed among human genomes. The 12 calf-derived isolates formed their own tight cluster within the livestock-associated branch, providing compelling genomic evidence of host-associated diversification. This lineage-level divergence supports the central premise of the study, that probiotic strains should ideally be sourced from the host species they are meant to benefit.

Functional testing then put the isolates through a gauntlet of gastrointestinal stressors. In simulated gastric fluid containing pepsin at pH 3, all strains except two maintained greater than 10 percent viability after two hours, and every strain held viable counts above 10^7 CFU/mL. Under postprandial acid conditions at pH 3, all isolates exceeded 10 percent survival, with eight strains retaining more than half their viable cells after two hours. Bile salt challenge at 0.2 percent concentration, which mimics the antimicrobial environment of the small intestine, left all strains above 10^7 CFU/mL, consistent with the conserved bile salt hydrolase gene bsh found in every genome. Lysozyme, an innate immune enzyme that cleaves the peptidoglycan of Gram-positive bacteria, failed to meaningfully reduce viability in any isolate. Only osmotic stress at 2 and 3 percent sodium chloride proved broadly inhibitory. Three strains, KCJ2K3504, KCJ2K3532, and KCJ2K3574, consistently outperformed their relatives across every tolerance assay, marking them as standouts for survival during oral administration and gut transit.

Colonization potential was assessed through four complementary phenotypes. Cell surface hydrophobicity, measured by adhesion to xylene, exceeded 50 percent in eight strains, with two isolates reaching values above 95 percent, a trait linked to initial contact with mucosal surfaces. Mucin adhesion, quantified in mucin-coated microplates, was measurable in all strains except one, confirming capacity to engage the intestinal mucus layer that serves as the primary interface between host and microbe. Every strain formed biofilms under mucus-associated conditions, and auto-aggregation assays revealed strain-dependent variation, with one isolate, KCJ2K3577, aggregating at a rate of 94 percent within five hours while the others ranged from 45 to 66.2 percent. Together these properties suggest the isolates could establish persistent populations in the calf gut, forming microcolonies that resist mechanical flushing and competitive exclusion of invaders.

The antagonism results were the study’s most striking. Cell-free supernatants from all 12 B. longum isolates produced clear inhibition zones against both Salmonella enterica serovar Typhimurium and serovar Dublin in agar well diffusion assays, and this activity was significantly reduced when supernatants were neutralized to pH 7, indicating that the inhibition was primarily pH-dependent and driven by organic acid production rather than bacteriocins, consistent with the absence of bacteriocin gene clusters. Co-culture competition assays delivered even more dramatic outcomes: three strains completely suppressed S. Typhimurium growth over 48 hours, while seven strains completely inhibited S. Dublin. Given that Typhimurium represents a broad-host-range pathogen and Dublin is cattle-adapted and invasive, the breadth of inhibition across both serovars carries real significance for preharvest food safety.

Safety screening rounded out the evaluation. Antimicrobial resistance gene analysis against the CARD database detected no resistance genes in two isolates, KCJ2K3532 and KCJ2K3574, while the remaining strains carried mostly intrinsic or genus-associated determinants such as rpoB and ileS, predominantly chromosomal and considered low risk for horizontal transfer. Integrating all the evidence, the authors conclude that KCJ2K3532 and KCJ2K3574, which combine strong Salmonella inhibition, robust stress tolerance, colonization traits, and clean genomic safety profiles, are the most promising candidates for in vivo validation. The team cautions that the safety assessment was computational and that future work must include phenotypic safety testing, host response studies, colonization trials in calves, and characterization of the antimicrobial compounds themselves. Still, the study makes a persuasive case that the next generation of livestock probiotics will come not from human microbiome catalogs but from the guts of the animals they are meant to protect.

Subject of Research: Probiotic characterization of calf-derived Bifidobacterium longum and their inhibition of Salmonella

Article Title: Host-associated Bifidobacterium longum exhibit gastrointestinal resilience and antagonistic activity against Salmonella serovars

Article References: Host-associated Bifidobacterium longum exhibit gastrointestinal resilience and antagonistic activity against Salmonella serovars. (n.d.). https://doi.org/10.1016/j.crfs.2026.101580

Image Credits: AI Generated

DOI: 10.1016/j.crfs.2026.101580

Keywords: Bifidobacterium longum, probiotics, dairy calves, Salmonella, gut microbiome, livestock health, whole-genome sequencing, antimicrobial resistance genes, organic acids, host specificity, gastrointestinal resilience, food safety