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

To evaluate the inhibitory effects of lactic acid bacteria and <i>Bifidobacterium</i> 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 <i>Lactiplantibacillus plantarum</i> LP45, <i>Bifidobacterium animalis subsp. animalis</i> L531, <i>Streptococcus thermophilus</i> S131, <i>Lactobacillus delbrueckii subsp. bulgaricus</i> L8, and their combination using an <i>in vitro</i> 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 <i>Bifidobacterium</i> strains were associated with the reduction of four target pathogens. Strain-specific effects were observed: LP45 and L8 were effective against <i>Campylobacter</i>, S131 strongly inhibited <i>Salmonella</i> and <i>Campylobacter</i>, and L531 was effective against <i>Escherichia coli</i>. The mixed intervention of LAB and <i>Bifidobacterium</i> 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 <i>Bifidobacterium</i> strains may be linked to their ability to enhance SCFA production.