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

Abstract

Aeromonas salmonicida subsp. salmonicida, a bacterial pathogen of Atlantic salmon, has no visible pili, yet its genome contains genes for three type IV pilus systems. One system, Tap, is similar to the Pseudomonas aeruginosa Pil system, and a second, Flp, resembles the Actinobacillus actinomycetemcomitans Flp pilus, while the third has homology to the mannose-sensitive hemagglutinin pilus of Vibrio cholerae. The latter system is likely nonfunctional since eight genes, including the gene encoding the main pilin subunit, are deleted compared with the orthologous V. cholerae locus. The first two systems were characterized to investigate their expression and role in pathogenesis. The pili of A. salmonicida subsp. salmonicida were imaged using atomic force microscopy and Tap-and Flp-overexpressing strains. The Tap pili appeared to be polar, while the Flp pili appeared to be 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 cumulative morbidity observed with the wild-type strain; however, delivery by intraperitoneal injection resulted in cumulative morbidity similar to that of the wild type. Unlike the pili of other piliated bacterial pathogens, A. salmonicida subsp. 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 the two pilus systems are not redundant.Aeromonas salmonicida subsp. salmonicida is a gram-negative, nonmotile, rod-shaped bacterium that is the etiologic agent of an infectious bacteremia-septicemia of salmonids known as furunculosis. Furunculosis is a complex disease that exists in different forms depending on the health, age, and species of fish. Many cell-associated and secreted factors have been implicated as virulence determinants in this bacterium (for reviews, see references 5 and 7). Despite this, much of the pathogenesis of A. salmonicida subsp. salmonicida remains poorly understood, and no single characteristic or phenotype was found only in virulent strains (16,17,30,42) until the recent description of an A. salmonicida subsp. salmonicida type III secretion system (6, 11).In order to better understand the virulence strategies employed by A. salmonicida subsp. salmonicida, we have focused on the initial stages of infection: adherence and invasion. Bacteria use complex intercellular mechanisms and specific and nonspecific adhesins to achieve these aims (34). The most well-studied A. salmonicida subsp. salmonicida adhesin is the surface layer or S-layer, sometimes referred to as the A-layer (additional layer) (33). This layer is a nonspecific but important factor for adherence due to its hydrophobic nature.Pili allow bacteria to attach to solid surfaces, including host tissues, and are considered impor...

Functioning in situ: gene expression in Methylotenera mobilis in its native environment as assessed through transcriptomics

“…This type of pili is important in a number of bacterial processes, such as flagellumindependent 'twitching' motility, DNA uptake, adherence to surfaces, and biofilm formation (Graupner et al, 2000;Mattick, 2002;Tomich et al, 2007;Boyd et al, 2008). It has been speculated before that in M. mobilis, this pilus may serve as a specific phage receptor (Kalyuzhnaya et al, 2008).…”

Genomic and proteomic analyses of the coral pathogen Vibrio coralliilyticus reveal a diverse virulence repertoire

“…In addition, several genes responsible for colonization were found in the P1 genome. The syp genes, a group of genes described in V. fischeri that promote symbiotic colonization and biofilm formation via Sigma54 regulation (Yip et al, 2005), and the flp genes for biogenesis of type IV Flp pilus described in Actinobacillus actinomycetemcomitans (Boyd et al, 2008), may represent important loci for the process of coral colonization. In addition, strain-specific genes found in the P1 genomes revealed the importance of phage genes driving intra-species diversification.…”

An Overview of Virulence-Associated Factors of Gram-Negative Fish Pathogenic Bacteria

“…A tapA mutant constructed by allelic exchange was found to be slightly less pathogenic than wild type when delivered by intraperitoneal injection (Masada et al, 2002). Boyd et al (2008) showed that the Tap pilus also made a moderate contribution to virulence in Atlantic salmon (Salmo salar), while the Flp pilus made little or no contribution. In addition to type IV pili, A. salmonicida has also a type I or Fim pilus system (similar to Pap fimbriae of Escherichia coli) encoded by several genes clustered into an operon.…”

“…salmonicida contains in its genome a complete set of genes for two type IV pilus systems, Tap and Flp (Boyd et al, 2008). In vivo experiments in rainbow trout (Oncorhynchus mykiss) showed that the Tap pilus contributes moderately to virulence.…”