The role of the type III secretion system in the intracellular lifestyle of enteric pathogens

Many bacterial pathogens have evolved to infect host cells from the inside. In fact, some bacteria, such as Rickettsia spp. and Coxiella spp., are entirely reliant on host intracellular resources to propagate (1). The adaptation of bacteria to an intracellular lifecycle is thought to confer a means to avoid the harsh extracellular milieu (low pH, physical stress, host defenses), to gain access to a nutrient-rich environment, and to facilitate the spread of the pathogen to neighboring host tissues (2, 3).