Bacterial landlines: contact-dependent signaling in bacterial populations

Abstract

Bacterial populations utilize a variety of signaling strategies to exchange information, including the secretion of quorum-sensing molecules and contact-dependent signaling cascades. Although quorum sensing has received the bulk of attention for many years, contact-dependent signaling is forging a niche in the research world with the identification of novel systems and the emergence of more mechanistic data. Contact-dependent signaling is probably a common strategy by which bacteria in close contact, such as within biofilms, can modulate the growth and behavior of both siblings and competitors. Ongoing work with diverse bacterial systems, including Myxococcus xanthus, pathogenic Escherichia coli strains, Bacillus subtilis, and dissimilatory metal-reducing soil bacteria, is providing increasingly detailed insight into the dynamic mechanisms and potential of contact-dependent signaling processes.

Figure 1. Model of The Contact-Dependent Inhibition System

Contact with CdiBA-positive E. coli cells triggers decreased membrane potential (Δp), ATP levels, and respiration in susceptible target cells (sad face). This metabolic downregulation coincides with activation of the phage shock response (pspA). BamA acts as the receptor for CDI, with AcrB as a downstream target (close-up inset). Other targets may potentially include BamA homologues expressed by mitochondria and chloroplasts. Expression of capsule, P or S pili, or CdiI renders bacterial cells (happy faces) resistant to CDI. Growth arrest by CDI is reversible by induced expression of CdiI in the presence of suitable carbon sources.