Intracellular pathogens face many daunting problems, among them how to obtain enough energy and nutrients for active growth while, preferably, keeping the host cell alive for as long as possible. This issue is especially acute for pathogens that grow at a fast rate and reach large numbers. When they succeed at this task, however, these fast-growing pathogens often cause serious infectious disease. A fine example of an organism with marked strategic skills is Shigella flexneri, the agent of life-threatening bloody diarrhea. This is the endemic form of shigellosis or bacillary dysentery that is prevalent among children in developing countries and results in the highest mortality rate for diseases caused by members of this genus. Infection begins when a small dose of 10 ‒ 100 bacteria reaches the human colon epithelial cells, where it progresses to very high loads. This rapid proliferation is achieved in part via a plasmid that delivers several virulence factors through a Type 3 secretion system. These factors include the Invasion Plasmid Antigen Proteins (Ipa's) that facilitate entrance of the bacteria into host cells, and IcsA, which triggers actin polymerization in the host cell, helping cell-to-cell spread of the pathogen.
Shigella Steals Host Nutrients... Economically
Source: Small Things Considered