The molecular biology of Aeromonas salmonicida

The fish pathogen, Aeromonas salmonicida, has been the focus of a number of molecular genetic studies designed to characterize the microorganism and its pathogenesis. The paracrystalline surface protein layer (A-layer) of A. salmonicida has been studied in considerable detail. The A-layer gene, vapA, has been cloned and sequenced and studies have been performed on its regulation. The secretion pathway specific for the A-layer subunits has also been partially characterized as has the general protein secretion pathway. Other genes involved in the biogenesis of the A. salmonicida surface include abcA, asoA and asoB. The abcA gene encodes a protein which is involved in lipopolysacharide O-chain synthesis and secretion and may have a role in the regulation of vapA gene expression. A. salmonicida also possesses plasmids of various sizes which exhibit a high degree of conservation and can encode antibiotic resistance elements. Insertion sequence elements have been identified in two strains of A. salmonicida and are capable of transposing within a cell to cause mutations that affect virulence. Molecular biology techniques have also been applied to the problem of detection of low levels of A. salmonicida in natural environments and carrier fish. The development of tools such as specific DNA probes and PCR primer pairs allows the detection of extremely low numbers of A. salmonicida even in the presence of high numbers of other bacteria. The development of vaccines against A. salmonicida has incorporated some new techniques such as the generation of specific mutations in the chromosome or the production of large quantities of particular proteins, such as the outer membrane porins, in expression systems. Another approach involves the use of an avirulent A. salmonicida strain as a shuttle system to express fragments of genes from viral pathogens with a view to providing protective immunity against multiple diseases with a single vaccine.