Aeromonas salmonicida type I pilus system contributes to host colonization but not invasion

The host-adherence strategies employed by Aeromonas salmonicida subsp. salmonicida, the etiological agent of an infectious bacteremia of salmonids, are poorly understood. In addition to the outer protein coat or S-layer, A. salmonicida has both Type I and Type IV pili loci. The A. salmonicida Type I or Fim pilus is encoded by an operon with genes for a chaperone, an usher, and 3 pilus subunits and is predicted to be similar to the Pap fimbriae of uropathogenic Escherichia coli, which are considered significant virulence factors. A Fim-deficient strain of A. salmonicida strain A449, Δfim, was created by deleting this operon. Virulence of Δfim was unchanged in direct live challenges of Atlantic salmon Salmo salar L., a natural host for A. salmonicida. A measure of clinically inapparent (covert) infections suggested Fim was required to establish or maintain a covert infection. This was confirmed by an ex vivo adherence and invasion assay using freshly excised salmon gastrointestinal (GI) tract, which showed that, compared to the parental strain, the ability of the isogenic Δfim mutant strain to adhere to the salmon GI tract was reduced but, once adhered, its ability to invade was unchanged. Thus the Fim pilus functions as an adhesin in A. salmonicida and the presence of a functional Fim improved the efficiency of A. salmonicida infection of Atlantic salmon.