| Summary: | Recent studies have shown that metabolites produced by microbes can be considered as mediators of host-microbial interactions. In this study, we examined the production of tryptophan metabolites by Bifidobacterium strains found in the gastrointestinal tracts of humans and other animals. Indole-3-lactic acid (ILA) was the only tryptophan metabolite produced in bifidobacteria culture supernatants. No others, including indole-3-propionic acid, indole-3-acetic acid, and indole-3-aldehyde, were produced. Strains of bifidobacterial species commonly isolated from the intestines of human infants, such as <i>Bifidobacterium longum</i> subsp. <i>longum</i>, <i>Bifidobacterium longum</i> subsp. <i>infantis</i>, <i>Bifidobacterium breve</i>, and <i>Bifidobacterium bifidum</i>, produced higher levels of ILA than did strains of other species. These results imply that infant-type bifidobacteria might play a specific role in host−microbial cross-talk by producing ILA in human infants.
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