Although microorganisms form mixed communities, the exchange of signals between fungal and bacterial species has not been extensively investigated. Now, the production of ethanol by Saccharomyces cerevisiae has been found to stimulate the growth of Acinetobacter species and enhance the virulence of this pathogen towards Caenorhabditis elegans, according to research published in Molecular and Cellular Biology.

In niches including soil and immunocompromised human hosts, it is likely that S. cerevisiae, the best-characterized fungal species, and Acinetobacter species could be found together and exchange signals. Smith et al. used co-culture to show that S. cerevisiae enhanced the growth of several Acinetobacter species. Contact between the yeast and bacterial cells was not required for growth enhancement, and the possibility that S. cerevisiae simply supplied a bacterial growth substrate was ruled out. Instead, a diffusible molecule produced during yeast growth was responsible.

Crude biochemical analyses revealed that ethanol satisfied the characteristics of the diffusible molecule and exogenous addition of ethanol was subsequently shown to enhance bacterial growth. Using gene knockouts the disruption of S. cerevisiae genes that encode alcohol dehydrogenases was correlated with a reduced ability of the mutant strains to produce ethanol and a concomitant reduction in their ability to promote bacterial growth.

Ethanol causes stress in bacterial cells and Smith et al. found that ethanol pre-treatment protected Acinetobacter baumannii against salt stress, but not heat or oxidative stress. Ethanol might trigger a signal-transduction cascade that results in the activation of both ethanol and salt tolerance genes. Because pathogens are subjected to stress during host infection, ethanol-treated A. baumannii was tested for virulence in C. elegans. A. baumannii is a food source for the worm, but ethanol pre-treatment rendered this species lethal to C. elegans. Elucidating the mechanism of worm killing — which might be due to increased bacterial virulence, or ethanol-mediated compromise of host immunity — is a key goal of further studies. The authors speculate that yeast signalling to Acinetobacter species could enhance bacterial virulence for worms — and ultimately worm death removes a predator of both yeast and bacterial species.

Humans often extol the benefits of consuming alcohol to enhance communication. Now it seems that ethanol signalling between yeast and bacterial species might benefit microbial communities too.