Quantification and determination of antagonistic activity of bifidobacteria and lactobacilli in faeces of breast-fed and mixed-fed infants

This study aimed to determine the effect of feeding method on the quantity of bifidobacteria and lactobacilli in infants and investigate the antagonistic of these bacteria against enteric pathogens such as enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), enteropathogenic E. coli (EPEC), enterohemorrhagic E. coli (EHEC), Salmonella Typhimurium, Shigella flexneri and Vibrio cholerae. The bifidobacteria and lactobacilli counts were compared between the 30 exclusively breast-fed and 33 mixed-fed infants (infants who received breast milk and formula milk) by real-time, quantitative PCR assays. The number of bifidobacteria in breast-fed and mixed-fed infants (mean values 8.26 and 8.42 log16S rRNA genes/g wet weight faeces, respectively; p = 0.67) and lactobacilli in these two groups (mean values 6.84 and 7.20 log16S rRNA genes/g wet weight faeces, respectively; p = 0.14) were not significantly different. One hundred and eleven Bifidobacterium and 39 Lactobacillus isolates from infants faeces were tested for their antagonistic activities against seven gastrointestinal pathogens by agar spot method. One hundred and six isolates of bifidobacterium had inhibitory activities. Interestingly, 10 Bifidobacterium isolates demonstrated strong antagonistic activities with clear zones against V. cholerae and Sh. Flexneri and weak activities against EHEC, EPEC, ETEC, EIEC and S. Typhimurium. Fourteen isolates of Lactobacillus had inhibitory activities, 7 isolates demonstrated weak antagonistic activities against V. cholerae with small clear zone. Genotypic identification by DNA sequencing demonstrated that the Bifidobacterium and Lactobacillus species in faeces of Thai infants at the age of one month included B. longum, B. bifidum, B. adolescentis, B. pseudocatenulatum, L. gasseri, L. salivarius, L.fermentum, L. ruminis, L. mucosae, L. vaginalis, L. oris, L. rhamnosus และ L. casei, respectively. Breast feeding and mixed feeding did not have effect on the number of bifidobacteria and lactobacilli in Thai infants. Bifidobacterium isolates possessed stronger antagonistic activities than Lactobacillus isolates in term of the varities of inhibited enteric pathogens and the magnitude of inhibition. Bifidobacterium isolates with the most potent antagonistic activities 90% DNA similarity with the most matched B. adolescentis. This indicated that these isolates might be new strains or new species and thus the potential sources of novel probiotic strains.