Atypical strains of Aeromonas salmonicida contain multiple copies of insertion element ISAsa4 useful as a genetic marker and a target for PCR assay.

The species Aeromonas salmonicida includes a quite complex group of pathogens that cause a variety of diseases in fishes. Best studied strains of this species are those of the subspecies salmonicida also referred to as 'typical' A. salmonicida, which cause furunculosis in salmonids. Less completely understood are bacteria assigned to other subspecies, e.g. achromogenes and masoucida, or those that cannot be assigned to a recognized subspecies. These strains are referred to collectively as 'atypical' A. salmonicida and cause diseases distinct from furunculosis, primarily affecting non-salmonids. In the course of a study to investigate the suitability of the gene product of tapA as a subunit vaccine, we discovered several atypical strains of A. salmonicida in which the tapA gene was interrupted by an insertion sequence (IS). Subsequent Southern blot analyses indicated that nearly all atypical strains (27 of 29) examined carry many copies of this IS, which we named ISAsa4. Genetic characterization of this IS element revealed it to be a member of the IS5 family, subgroup IS903. Aside from the presence of ISAsa4 in several atypical strains, the nucleotide sequence of tapA was virtually identical to that found in typical strains. This finding suggests that ISAsa4 might be a major source of genetic diversity among atypical strains which, unlike typical strains, are genetically heterogeneous. The presence of ISAsa4 in atypical strains may also help explain the host tropism of atypical strains of this bacterium. Using information on the nucleotide sequences of ISAsa4 from atypical strains of A. salmonicida, primers were designed to selectively amplify genomic DNA from most atypical strains.