Editors' ChoiceMicrobiology

Toxin-induced reproduction

Sci. Signal.  20 Jan 2015:
Vol. 8, Issue 360, pp. ec14
DOI: 10.1126/scisignal.aaa6943

Biofilms secrete compounds that inhibit the growth of competing microbes. Many microbial infections of human tissues and medical implants are mixed-species biofilms composed of bacteria and fungi. Zheng et al. found that compounds secreted by mixed-species biofilms also mediate interspecies communication. Biofilms composed of the bacterium Pseudomonas aeruginosa and the filamentous fungus Aspergillus fumigatus often colonize the lungs of cystic fibrosis patients, where they secrete redox-active secondary metabolites such as phenazines (P. aeruginosa) and gliotoxin (A. fumigatus), both of which can stimulate the production of reactive oxygen species (ROS). In coculture experiments using mutant strains of P. aeruginosa that produced different amounts of phenazines, phenazines produced by P. aeruginosa differentially affected the growth and development of A. fumigatus depending on amount of phenazines produced. P. aeruginosa–derived phenazines inhibited A. fumigatus growth at high concentration, stimulated vegetative growth of A. fumigatus at moderate concentration, and stimulated asexual sporulation of A. fumigatus at low concentration. Culture conditions that favored the formation and stability of phenazine free radicals stimulated A. fumigatus sporulation, and the presence of free radical scavengers inhibited sporulation of A. fumigatus. The transcription factor NapA mediates the oxidative stress response and is involved in sporulation in the related fungus A. nidulans. NapA was required for phenazine-induced sporulation in A. fumigatus. Furthermore, gliotoxin also promoted NapA-dependent sporulation in A. nidulans, indicating that the oxidative stress pathway also mediates sporulation in response to this endogenous signal. Thus, as in humans and yeast, small amounts of oxidative stress can stimulate an adaptive biological response in biofilm-associated pathogenic fungi, whereas larger amounts are toxic.

H. Zheng, J. Kim, M. Liew, J. K. Yan, O. Herrera, J. W. Bok, N. L. Kelleher, N. P. Keller, Y. Wang, Redox metabolites signal polymicrobial biofilm development via the NapA oxidative stress cascade in Aspergillus. Curr. Biol. 25, 29–37 (2015). [PubMed]