Impact of lactic acid bacteria and Bifidobacterium intervention on pathogenic bacteria and short-chain fatty acid production in diarrheic calves: an in vitro fermentation study

• The bacterial strains work differently: acidification or production of metabolites. • The mixed of lactic acid bacteriais is most effective against pathogenic bacteria. • Pathogen suppression is linked to elevated short-chain fatty acid production. • This research identifies promising probiotics to curb antibiotic use in calves. To evaluate the inhibitory effects of lactic acid bacteria and Bifidobacterium interventions on pathogenic bacteria in the gut microbiota of diarrheic calves and assess their impact on the production of short-chain fatty acids, this study examined the effects of Lactiplantibacillus plantarum LP45, Bifidobacterium animalis subsp. animalis L531, Streptococcus thermophilus S131, Lactobacillus delbrueckii subsp. bulgaricus L8, and their combination using an in vitro YCFA medium model. Fecals of ten diarrheic female Holstein calves were randomly selected, and each sample was subjected to the intervention experiment, independently. The relative abundances of four pathogenic bacterial species associated with diarrhea were quantified before and after intervention using qPCR, and SCFAs production was analyzed by GC. Additionally, correlation analyses between the abundances of these pathogenic bacteria and SCFAs levels were conducted. The results indicated that interventions with LAB and Bifidobacterium strains were associated with the reduction of four target pathogens. Strain-specific effects were observed: LP45 and L8 were effective against Campylobacter , S131 strongly inhibited Salmonella and Campylobacter , and L531 was effective against Escherichia coli . The mixed intervention of LAB and Bifidobacterium strains demonstrated the broadest and strongest inhibitory effect, suggesting functional synergy. Interventions with S131 and Mix significantly increased concentrations of the health-promoting SCFAs, such as acetic acid, propionic acid and butyric acid. A negative correlation was observed between pathogen abundance and SCFA concentrations, suggest that the antimicrobial activity of the LAB and Bifidobacterium strains may be linked to their ability to enhance SCFA production.