A molecular approach to understanding the pathogenesis ofAeromonas salmonicida: relevance to vaccine development

Aeromonas salmonicidahas been the focus of a number of molecular genetic studies, designed to characterise the pathogenesis of this microorganism. In particular, the paracrystalline surface protein array (A-layer) has received considerable attention. The gene encoding the A-layer subunits,vapA, has been cloned and sequenced and factors influencing its regulation have also been described. The secretion of the A-layer subunits has also been studied and a specific designated secretion pathway has been identified. The lipopolysaccharide O-polysaccharide side chains ofA. salmonicidaare homogeneous in length and their synthesis and secretion require the product of theabcAgene. AbcA is unusual in that it is involved in O-chain biogenesis and also plays a role in the regulation of thevapAgene. The correct organisation of the outer surface ofA. salmonicidais important for virulence. Mutagenesis of theasoAgene results in the disruption of the outer membrane and A-layer, which resulted in an increase in systemic virulence when the bacteria were administered by intraperitoneal injection. Mutagenesis of theasoBgene, on the other hand, resulted in pleiotropic effects on A-protein translocation across the cytoplasmic membrane and the outer membrane protein profile.asoBmutants were found to exhibit reduced virulence by bath immersion, but not by intraperitoneal injection. A highly conserved porin fromA. salmonicidawas characterised and shown to elicit protective immunity when administered by intraperitoneal injection to rainbow trout. These studies and new ones dealing with the determination of the genomic map ofA. salmonicidaand the detailed characterisation of the virulence factors should facilitate the rational design of vaccines and other preventative measures againstA. salmonicida-induced disease and can also serve as a model system for other bacterial pathogens of fish.