In vitro antagonistic activities of animal intestinal strains against swine-associated pathogens

A wide range of enteropathogens cause costly diarrhoeal diseases in fattening piglets and account for food-related infections in humans. The objective of this study was to screen beneficial bacterial strains from the gastrointestinal tract of various animal sources for antagonistic activity against diverse pathogens associated with hazardous pig production times. Using agar spot assays, 15 well-characterized strains belonging to Lactobacillus, Enterococcus, Bifidobacterium and Bacillus were studied for inhibition of Clostridium perfringens type A, various serovars of enterotoxigenic Escherichia coli and Salmonella enterica, as well as Brachyspira pilosicoli. Strong antagonists were further analyzed by studying their cell-free supernatants with and without pH neutralization, proteinase K and catalase treatment. Enterobacteriaceae were effectively inhibited by Lactobacillus salivarius and Lactobacillus reuteri strains, independent from the animal source, and on a lower level by single strains belonging to Lactobacillus mucosae, Lactobacillus amylovorus and Bifidobacterium thermophilum, due to organic acid production. The Bacillus subtilis strain was found to produce an anti-clostridial and anti-Brachyspira metabolite of proteinaceous nature. Homofermentative lactobacilli and B. thermophilum could suppress the growth of B. pilosicoli, the causative agent of intestinal spirochaetosis, whereas heterofermentative strains belonging to L. reuteri and L. mucosae had no effect. The lactic acid bacteria exerted their activity primarily by organic acid release, except one Enterococcus faecium and L. amylovorus strain, which exhibited antagonism through joint activity of lactate and hydrogen peroxide. The findings of this study provide a basis for further in vitro studies and encourage feeding studies to evaluate the antagonistic potential of promising strains in pig production.