Bacterial quorum-sensing systems coordinate cellular behaviors—such as virulence and biofilm formation—across a population and are based on the release of small molecules that, when present in sufficient abundance, flip gene expression switches. HQNO (2-n-heptyl-4-hydroxyquinoline-N-oxide), which inhibits bacterial respiration by inhibiting the cytochrome bc1 complex (also called complex III), is a quorum factor secreted by Pseudomonas aeruginosa that inhibits the growth of competing bacteria. Although wild-type planktonic P. aeruginosa undergo spontaneous autolysis when cultured for more than 48 hours, Hazan et al. found that P. aeruginosa mutants that were incapable of synthesizing HQNO and several related compounds did not undergo autolysis. Adding physiologically relevant concentrations of HQNO to the culture medium restored autolysis in a dose-dependent manner. The ability of HQNO to induce autolysis required the quinol-binding site and the electron-acceptor site of the cytochrome bc1 complex. Inhibition of the cytochrome bc1 complex induces a burst of reactive oxygen species (ROS). Cultures undergoing autolysis exhibited a ROS burst, and the addition of ROS scavengers to the culture medium inhibited autolysis. In eukaryotes, inhibition of the cytochrome bc1 complex disrupts the mitochondrial membrane and membrane potential. Experiments with a membrane-impermeable dye and a fluorescent indicator of membrane potential indicated that HQNO treatment increased membrane permeability and reduced membrane potential in P. aeruginosa. Experiments with P. aeruginosa using compounds that target the cytochrome bc1 complex in mitochondria suggested that membrane permeabilization and depolarization in bacterial autolysis occur through a mechanism similar to that of eukaryotic cells undergoing apoptosis (see Zemke and Bomberger). DNA is a major component of the biofilm matrix, a structure that protects microbes from antibiotics. Exogenously supplied HQNO partially rescued biofilm formation in quorum-sensing mutants and promoted tolerance to the antibiotic meropenem. The addition of DNAse to the culture medium impaired HQNO-enhanced biofilm formation and antibiotic tolerance, suggesting that autolysis was important for releasing DNA that could be used in forming the matrix. These results demonstrate that in addition to releasing spores and eliminating phage-infected cells, bacterial autolysis can benefit the population by promoting the formation of biofilms.
R. Hazan, Y. A. Que, D. Maura, B. Strobel, P. A. Majcherczyk, L. R. Hopper, D. J. Wilbur, T. N. Hreha, B. Barquera, L. G. Rahme, Auto poisoning of the respiratory chain by a quorum-sensing-regulated molecule favors biofilm formation and antibiotic tolerance. Curr. Biol. 26, 195–206 (2016). [PubMed]
A. C. Zemke, J. M. Bomberger, Microbiology: Social suicide for a good cause. Curr. Biol. 26, R80–R82 (2016). [PubMed]