Switching on virulence factors at the right time and in the right place is a prerequisite for successful pathogens. However, pinpointing the signals that infecting bacteria respond to is difficult, owing to the complexity of the host environment. A new report published in Cell has revealed a mechanism by which bacteria switch on virulence genes in response to one component of the host innate immune armoury, the antimicrobial peptides.

A plethora of antimicrobial peptides function in host innate immunity. Although it was already known that selected antimicrobial peptides activated Salmonella enterica serovar typhimurium virulence through the PhoPQ two-component signal-transduction system, the mechanism by which the PhoQ sensor kinase detected these peptides was not clear. Antimicrobial peptides can kill pathogens by punching holes in their membranes, so it was plausible that PhoQ was sensing membrane damage rather than the antimicrobial peptide, or that another protein relayed detection of the peptide signal to the PhoQ sensor kinase.

Bader et al. sought to clarify how the antimicrobial peptide signal was detected, using a set of complementary approaches. Initially, experiments with a PhoQ-regulated gene fusion revealed that the periplasmic domain of the PhoQ sensor is required to respond to antimicrobial peptides. Using an in vitro assay, they confirmed that PhoQ alone is required to sense peptides and that peptide sensing only occurs when the peptide can contact the PhoQ periplasmic domain. This ruled out a mechanism in which PhoQ sensed membrane damage caused by the peptide. Plus, sensing peptide stimulated both the autokinase function of PhoQ and the transfer of phosphate groups from PhoQ to the PhoP response regulator protein — sensing peptides therefore stimulates the signal-transduction cascade that is required to switch on PhoPQ-regulated virulence genes.

Biophysical studies with the purified periplasmic domain of the PhoQ sensor were used to map the interaction between PhoQ and antimicrobial peptides to a negatively charged acidic surface on the protein. Presumably, non-covalent interactions occur between positively charged antimicrobial peptides and this negative patch. Divalent cations also bound to the same surface. The mechanism favoured by the authors is that peptide binding to PhoQ disrupts cation bridges that have formed between the sensor protein and the membrane, and that this stimulates conformational changes that result in autophosphorlyation of PhoQ, phosphotransfer to PhoP and activation of the cascade that ultimately results in virulence gene activation.

Two-component regulatory systems are present in all bacteria. While PhoPQ is essential for S. typhimurium virulence, the PhoPQ two-component system isn't restricted to this species and coordinates virulence-factor production in different Gram-negative pathogens that infect animals, plants and insects. So understanding how PhoPQ is activated could enable researchers to understand the pathogenic strategies of many important bacteria.

The virulence genes of S. typhimurium are activated after host macrophages have engulfed the invading bacterium into a phagosome. Acidification of the phagosome and production of antimicrobial peptides kills most microbial invaders, but PhoQ sensing of these signals enables the pathogen to subvert this basic host defence mechanism. As antimicrobial peptides are ubiquitous in eukaryotic hosts, subverting their function to activate virulence is another clever tactic of bacterial pathogens.