Probiotic foods can help boost human health by promoting the growth of beneficial bacteria, but although probiotic bacteria can protect against infections with pathogens, little is known about the mechanisms underlying this interaction. In a new study, researchers evaluated the microbiome of a unique fermented kefir mixture. One of the predominant microbes found was the fungus Kluyveromyces marxianus, and a metabolite secreted by fungi – tryptophol acetate, which inhibits bacterial communication and virulence – was identified in the probiotic drink. Tryptophol acetate blocked the ability of the gut pathogen Vibrio cholerae to chemically sense bacterial density and form biofilms and altered the expression of genes associated with virulence. Although further studies are needed to fully understand the effects of probiotics on harmful gut bacteria, these results uncover a new cross-kingdom inhibition mechanism, where probiotic yeast prevent the growth of pathogenic bacteria. Understanding this mechanism will help researchers tap into the health benefits of probiotics to develop new strategies to treat gut microbial dysbiosis.