Chromobacterium violaceum is a Gram-negative bacterium that infects humans and animals with fatal
sepsis. The
infection with C. violaceum is rare in case of those who are healthy, but once established, C. violaceum causes sever disease accompanied by
abscess formation in the lungs, liver and spleen. Furthermore, C. violaceum is resistant to a broad range of
antibiotics, which in some cases renders the antimicrobial
therapy for this
infection difficult. Thus, the
infection with C. violaceum displays high mortality rates unless initial proper antimicrobial
therapy. In contrast, the
infection mechanism had completely remained unknown. To this end, we have tried to identify
virulence factors-associated with C. violaceum
infection. Two distinct
type III secretion systems (TTSSs) were thought to be one of the most important
virulence factors, which are encoded by Chromobacterium pathogenicity island 1/1a and 2 (
Cpi-1/-1a and -2) respectively. Our results have shown that
Cpi-1/-1a-encoded TTSS, but not Cpi-2, is indispensable for the virulence in a mouse
infection model. C. violaceum caused
fulminant hepatitis in a
Cpi-1/-1a-encoded TTSS-dependent manner. We next have identified 16 novel effectors secreted from
Cpi-1/-1a-encoded TTS machinery. From these effectors, we found that CopE (Chromobacterium outer
protein E) has similarities to a
guanine nucleotide exchange factor (GEF) for
Rho GTPases. CopE acts as GEF for Rac1 and Cdc42, leading to induction of actin cytoskeletal rearrangement. Interestingly, C. violaceum invades cultured human epithelial cells in a CopE-dependent manner. Finally, an inactivation of CopE by disruption of copE gene or
amino acid point mutation leading to loss of GEF activity attenuates significantly the mouse virulence of C. violaceum. These results suggest that
Cpi-1/-1a-encoded TTSS is a major virulence determinant for C. violaceum
infection, and that CopE contributes to the virulence in part of this pathogen.