Role of new virulence mechanisms/factors (type 3 secretion system and TOX-R regulated lipoprotein [TAGA]) in the pathogenesis of the emerging human pathogen Aeromonas hydrophila

Aeromonas hydrophila, a gram-negative bacterium that causes gastroenteritis, wound infections, septicemia, and other diseases in humans, produces many different virulence factors. A clinical isolate SSU of A. hydrophila possesses a cytotoxic enterotoxin Act, a potent virulence factor that is secreted into the environment through the bacterium’s type 2 secretion system (T2SS) and possesses several biological activities, including cytotoxicity, enterotoxicity, and lethality in a mouse model. The purpose of this study was to identify new virulence factors that contribute to the pathogenesis of this bacterium. We identified and characterized a type 3 secretion system (T3SS) in A. hydrophila SSU. By marker-exchange mutagenesis of the aopB gene, a crucial gene involved in the formation of the translocon apparatus, the functionality of the T3SS was elucidated, both in in vitro and in vivo models. Further, the characterization of the regulatory gene DNA adenine methyltransferase (Dam) from SSU and its role in modulating the function of both the T3SS and Act was investigated. The role of the T3SS in influencing the phenomenon of quorum sensing (QS) in A. hydrophila SSU was also conducted. This study highlights a unique link between the T3SS and Act of A. hydrophila and the production of QS molecules or lactones. While searching for potential effector proteins secreted through the T3SS of A. hydrophila SSU, the identification of a new virulence factor, ToxR regulated lipoprotein (TagA), was revealed. TagA is a zinc metalloprotease which has only been identified in the gram-negative pathogens, Escherichia coli O157:H7 and Vibrio cholerae. In A. hydrophila, TagA has been shown to play a role in the inhibition of complement by binding to and cleaving the serpin C1-INH. By recruiting C1-INH to the surface of the bacteria and cleaving it, TagA is able to significantly prevent the activation of complement at the cell surface, ultimately increasing the serum resistance of the pathogen. TagA can also target C1-INH to erythrocyte surfaces and decrease the lysis that occurs in the presence of serum. Confocal fluorescence microscopy revealed that the serpin C1-INH binds to TagA on the surface of the bacteria.