Microbiology

Aeromonas salmonicida is a Gram-negative bacterial pathogen that is the causal agent of furunculosis in salmonid fishes, a debilitating and lethal disease encountered in aquaculture. Current vaccines offer limited effectiveness and epizootics are common in farmed fish. Development of broadly effective control measures requires a more complete understanding of the interactions between pathogen and host factors that contribute to the disease state.

Similar to our previous work with A. hydrophila, we have cloned several genes required by A. salmonicida to express a specific class of pili, designated type IV, which are known virulence determinants in other pathogenic bacteria and which have been used as vaccines against some animal infections. Pili (or fimbriae) are proteinaceous filaments that extend from many bacterial cells that mediate adherence and colonization of host cells by interactions with specific cell receptors (the thin fibers shown in the scanning transmission electron micrograph of an Aeromonas hydrophila cell at left). We propose to characterize the role of A. salmonicida type IV pili in colonization and virulence by first determining whether the pili are expressed during infections of salmon. The induction of specific anti-pilin antibody will be measured in A. salmonicida-challenged fish to determine whether the fish immune system reacts to pili. We will then determine whether the pili play a role in pathogenesis by comparing the virulence of wild-type and pili-negative strains during experimental infections. If pili are important for A. salmonicida to infect fish, it is possible they could form the basis of an easily administered vaccine.