Macrophages destroy bacteria by engulfing them in intracellular compartments, which they then acidify to kill or neutralize the bacteria. However, some pathogenic bacteria, such as Salmonella enterica, have evolved to exist and even grow while within these acidified compartments. How Salmonella responds to the acidic environment and how that environment affects the virulence of this pathogen are unclear.
A new paper in in PLOS Biology demonstrate thats, instead of combating the acidification of the Salmonella-containing vacuole, Salmonella acidifies its own cytoplasm in response to the extracellular low pH (A FRET-Based DNA Biosensor Tracks OmpR-Dependent Acidification of Salmonella during Macrophage Infection. doi: 10.1371/journal.pbio.1002116). The acidic cytoplasm then acts as a signal to stimulate the secretion of a particular class of Salmonella virulence proteins. These virulence proteins, or effectors, are released into the host cell, where they are able to perturb the immune response.
The findings of this paper contradict other previous reports that suggest that a neutralization step is required for secretion of the virulence proteins. The authors show that Salmonella has adapted what was once an antibacterial response by the macrophage into a signal for when it is in the correct time and place to secret its virulence proteins and establish an infection.