The enteric pathogen, Salmonella enterica serovar Typhimurium, causes food poisoning resulting in gastroenteritis. The S. Typhimurium effector protein, SipA, promotes gastroenteritis by functional motifs that either trigger mechanisms of inflammation or bacterial entry. During infection of intestinal epithelial cells, SipA was found to be responsible for the early activation of caspase-3, an enzyme that is required for SipA cleavage at a specific recognition motif that divided the protein into its two functional domains, and activated SipA in a manner necessary for pathogenicity. Other caspase-3 cleavage sites identified in S. Typhimurium appeared to be restricted to secreted effector proteins, indicating this may be a general strategy employed by this pathogen for processing of its secreted effectors.

Shigella flexneri is the causative agent of dysentery, and its pathogenesis is mediated by a type III secretion system (T3SS). S. flexneri secretes effector proteins into the eukaryotic cell via the T3SS, and these proteins usurp host cellular functions to the benefit of the bacteria. OspF and OspC1 are known to be secreted by S. flexneri, but their functions are unknown. We transformed S. flexneri with a plasmid that expresses a twohemagglutinin tag (2HA) in frame with OspF or OspC1 and verified that these proteins are secreted in a T3SS-dependent manner. Immunofluorescence of HeLa cells infected with S. flexneri expressing OspF-2HA or OspC1-2HA revealed that both proteins localize in the nucleus and cytoplasm of host cells. To elucidate the function of these T3SS effectors, we constructed ⌬ospF and ⌬ospC1 deletion mutants by allelic exchange. We found that ⌬ospF and ⌬ospC1 mutants invade host cells and form plaques in confluent monolayers similar to wild-type S. flexneri. However, in the polymorphonuclear (PMN) cell migration assay, a decrease in neutrophil migration was observed for both mutants in comparison to the migration of wild-type bacteria. Moreover, infection of polarized T84 intestinal cells infected with ⌬ospF and ⌬ospC1 mutants resulted in decreased phosphorylation of extracellular signal-regulated kinase 1/2 in comparison to that of T84 cells infected with wild-type S. flexneri. To date, these are the first examples of T3SS effectors implicated in mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathway activation. Ultimately, OspF and OspC1 are essential for PMN transepithelial migration, a phenotype associated with increased inflammation and bacterial access to the submucosa, which are fundamental aspects of S. flexneri pathogenesis.Shigella species are responsible for dysentery (shigellosis) in humans, which starts as an acute infection in the large intestine, which is followed by cramps, diarrhea, and fever. The infection is usually self-limiting, but it can also cause damage to the colonic mucosa, intestinal bleeding, and death if untreated. Worldwide, Shigella spp. infections are responsible for approximately 163 million cases of dysentery and 1 million deaths each year (23). The majority of infections occur in third-world countries, where contaminated food and drinking water are common (23); however, even developed nations still have multiple Shigella outbreaks every year (47). Therefore, studying the pathogenesis of these gram-negative bacteria is of utmost importance, particularly in light of emerging antibiotic resistance, the lack of an appropriate vaccine, and the potential for use of Shigella as a bioweapon (23,33,42).Following ingestion, Shigella flexneri eventually reaches the large intestine, the target site for infection, where access to the basolateral membrane is a prerequisite for the invasion of epithelial cells (see reference 48 for a review). M cells in the large intestine phagocytose and subsequently transcytose the bacteria from the lumen to t...