Editors' ChoiceBiofilms

Making Microbes Multicellular

Sci. Signal.  07 May 2013:
Vol. 6, Issue 274, pp. ec101
DOI: 10.1126/scisignal.2004307

Like many species of bacteria, Bacillus subtilis forms biofilms to improve survival under environmental conditions that would be too stressful for individual cells. These multicellular communities are bound together by an extracellular matrix containing exopolysaccharides and the amyloid-like protein TasA. Phosphorylation of the master sporulation regulator SpoOA leads to derepression of the operons that encode TasA and the exopolysaccharide biosynthesis enzymes. Several histidine kinases that participate in the phosphorelay that activates SpoOA have been identified, and Kolodkin-Gal et al. report that two of these, KinA and KinB, sense impaired respiration by independent mechanisms. Under normoxic conditions, B. subtilis formed smooth colonies, but hypoxia promoted expression of tasA and the deposition of extracellular matrix, which produces wrinkled colonies. KinA and KinB were required for this response, and maximal matrix production required high iron concentrations, suggesting the involvement of heme-containing cytochromes. Genetics experiments also suggested that the ability of KinB to detect changes in respiration required cytochromes, and a tagged version of this integral membrane kinase coimmunoprecipitated with cytochromes of the respiratory complex. In contrast, KinA is cytoplasmically localized and therefore unlikely to associate with the respiratory complex. Instead, KinA has PAS domains, which often function in ligand binding; thus, the authors performed mass spectrometry on a small molecule extracted from affinity-purified KinA and identified it as NAD+. Deletion analysis indicated that two of the three PAS domains of KinA were required for a robust hypoxia-induced colony-wrinkling response, and a mutant KinA lacking the first PAS domain failed to bind NAD+. Reduced aerobic respiration was accompanied by a decrease in the NAD+:NADH ratio, suggesting that KinA activity might be inhibited by NAD+ under normal respiratory conditions and that reduced respiration relieves this inhibition. This study identifies two independent mechanisms by which B. subtilis can sense reduced respiration, an indicator of limited oxygen availability, and respond by initiating biofilm formation.

I. Kolodkin-Gal, A. K. W. Elsholz, C. Muth, P. R. Girguis, R. Kolter, R. Losick, Respiration control of multicellularity in Bacillus subtilis by a complex of the cytochrome chain with a membrane-embedded histidine kinase. Genes Dev. 27, 887–899 (2013). [Abstract] [Full Text]