Contribution of Type IV Pili to the Virulence of Aeromonas salmonicida subsp. salmonicida in Atlantic Salmon (Salmo salar L.)

National Research Council of Canada Institute for Marine Biosciences, 1411 Oxford St., Halifax, NS; Department of Physics and Atmospheric Science, Dalhousie University, Halifax, NS

^* To whom correspondence should be addressed. Email: jessica.boyd{at}nrc-cnrc.gc.ca.

	Abstract

Aeromonas salmonicida subsp. salmonicida, a bacterial pathogen of Atlantic salmon, has no visible pili, yet its genome contains genes for three type IV pili systems. One system, Tap, is similar to the Pseudomonas aeruginosa Pil, a second, Flp, resembles the Actinobacillus actinomycetemcomitans Flp pilus while the third has homology to the MSHA pilus of Vibrio cholerae. The latter system is likely non-functional since eight genes, including the main pilin subunit, are deleted, compared with the orthologous V. cholerae locus. The former two systems were characterized to investigate their expression and role in pathogenesis. The pili of A. salmonicida were imaged using atomic force microscopy on Tap and Flp over-expressing strains. The Tap pili appeared to be polar while the Flp pili appeared peritrichous. Strains deficient in tap and/or flp were used in live bacterial challenges of Atlantic salmon which showed that the Tap pilus made a moderate contribution to virulence while the Flp pilus made little or no contribution. Delivery of the tap mutant by immersion resulted in reduced cumulative morbidity compared with the wild type strain; however, delivery by intraperitoneal injection resulted in cumulative morbidity similar to wild type. Unlike other piliated bacterial pathogens, A. salmonicida type IV pili are not absolutely required for virulence in Atlantic salmon. Significant differences in the behavior of the two mutant strains indicated that these two pili systems are not redundant.