The role of acid stress in Salmonella pathogenesis.

After uptake by epithelial cells or engulfment by macrophages, Salmonella resides in an acidic vacuole. Salmonella senses this acidic compartment through the action of the EnvZ/OmpR two-component regulatory system. OmpR, in turn, represses the cadC/BA system, preventing neutralization of the bacterial cytoplasm. New, single cell techniques now enable us to observe that in response to acid stress, the pH is low in bacterial cells and acidification is critical for infection. Instead of recovering from acid stress, Salmonella uses acid pH as a signal to drive pathogenesis. The relevant molecular mechanisms employed by Salmonella to couple acid stress with the expression of virulence genes that promote intracellular survival are explored.