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

Abstract

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. KEY WORDS: Pilin · Adhesin · Furunculosis · VirulenceResale or republication not permitted without written consent of the publisher Dis Aquat Org 88: 199-206, 2010 epithelium by a trans-cellular route and that this translocation requires bacteria with intact and functional surface structures. Our continued interest in A. salmonicida adhesins leads us to ask if we could identify factors important for adherence to salmon intestinal tissue.The most well studied Aeromonas salmonicida adhesin is its outer protein coat, the S-layer, the major protein constituent of which is VapA. While the S-layer has long been demonstrated to be both an important virulence factor and an adhesin (Ishiguro et al. 1981, Trust et al. 1983, Garduño et al. 1992, Kay & Trust 1997, Garduño et al. 2000, VapA-deficient strains may still be infectious (Ellis et al. 1988, Olivier 1990. In addition to the S-layer, A. salmonicida strain A449 also has genes for at least 4 specific adhesion systems: Tap, Flp, MSHA, and Fim (Reith et al. 2008). The first 3 are Type IV pili and have been characterized elsewhere (Masada et al. 2002). The latter system, Fim, is predicted to encode a Type I pilus, similar to the fimbriae of pathogenic Escherichia coli that are considered to be important virulence factors in that organism. The present study reports on the contribution of Fim to A. salmonicida strain A449 virulence as assessed by a Fim-deficient isogenic laboratory strain. MATERIALS AND METHODSBacterial strains and growth conditions. Bacteria and plasmids used in the present study are listed in Table 1. The parental strain for all knockouts was Aeromonas salmonicida subsp. salmonicida strain ...

Aeromonas salmonicida Growth in Response to Atlantic Salmon Mucins Differs between Epithelial Sites, Is Governed by Sialylated and N -Acetylhexosamine-Containing O -Glycans, and Is Affected by Ca 2+

“…In Escherichia coli, Neu5Ac has been shown to promote type I fimbria production, while GlcNAc has an inhibitory role (41). As type I fimbriae are important adhesion and virulence factors in A. salmonicida (42), one can speculate that Atlantic salmon mucins exert similar changes in fimbria production. The presence of GlcNAc may signal to the pathogen that it is in the mucus niche, and downregulating such factors may in turn free up more resources for growth.…”

Aeromonas salmonicida Binds Differentially to Mucins Isolated from Skin and Intestinal Regions of Atlantic Salmon in an N -Acetylneuraminic Acid-Dependent Manner

“…The A. salmonicida S-layer and type I pilus system have previously been shown to contribute to A. salmonicida adhesion to the salmon intestine ex vivo (57).…”

Determination of Microbial Diversity of Aeromonas Strains on the Basis of Multilocus Sequence Typing, Phenotype, and Presence of Putative Virulence Genes

“…Several recent studies reported the involved virulence factors in fish infections (11,17,36). The primary challenge in establishing an unequivocal link between the Aeromonas genus and pathogenesis is the extremely complicated taxonomy.…”