Omptins are outer membrane proteases found in gram negative bacteria that cause diseases in humans, such as pathogenic Escherichia coli, Shigella flexneri, Salmonella typhimurium, and Yersinia pestis. Bacterial species that express omptins cause diseases such as highly fatal plague and severe diarrhea and dysentery. The genes that encode these proteases are ompT, icsP, pgtE, and pla, respectively. These proteases are highly related in structure and share approximately 50% sequence identity. In S. flexneri, IcsP has been shown to cleave a key virulence determinant, IcsA (Egile et al., 1997). IcsA recruits host actin and allows for intracellular movement within host cells (Steinhauer et al., 1999). In S. typhimurium, PgtE has been shown to cleave a human a-helical cationic antimicrobial peptide (CAMP), LL-37 (Guina et al., 2000). LL-37 is a major component of the innate immune defense system (Zasloff, 1992). It functions by permeabilizing the bacterial membrane, which ultimately results in bacterial cell lysis. PgtE cleaves LL-37, thereby protecting Salmonella against the bactericidal effects of CAMPs. In E. coli, OmpT has been shown to cleave protamine, an antimicrobial peptide that acts on the bacterial membrane and causes problems in cellular energy transduction and nutrient accumulation (Aspedon and Groisman, 1996) (Figure 2). Like PgtE, OmpT circumvents the immune defense by cleaving protamine into smaller fragments
