Investigation of the role of type IV Aeromonas pilus (Tap) in the pathogenesis of Aeromonas gastrointestinal infection

Although there is substantial evidence that type IV pili purified from diar rhea-associated Aeromonas species (designated Bfp for bundle forming pilus) are intestinal colonization factors (S. M. Kirov, L. A. O'Donovan, and K. Sanderson, Infect. Immun, 67:5447-5454, 1999), nothing is known regarding t he function of a second family of Aeromonas type IV pili (designated Tap fo r type IV Aeromonas pilus), identified following the cloning of a pilus bio genesis gene cluster tapABCD. Related pilus gene clusters are widely conser ved among gram-negative bacteria, but their significance for virulence has been controversial. To investigate the role of Tap pili in Aeromonas pathog enesis, mutants of Aeromonas strains (a fish isolate of A. hydrophila and a human dysenteric isolate of A. veronii bv. sobria) were prepared hy insert ional inactivation of the tapA gene which encodes the type IV pilus subunit protein, TapA, Exotoxic activities were unaffected by the mutation in tapA . Inactivation of tapA had no effect on the bacterial adherence of these tw o isolates to HEp-2 cells. For the A. veronii bv. sobria isolate, adhesion to Henle 407 intestinal cells and to human intestinal tissue was also unaff ected. There was no significant effect on the duration of colonization or i ncidence of diarrhea when the A. veronii bv. sobria strain was tested in th e removable intestinal tie adult rabbit diarrhea model or on its ability to colonize infant mice. Evidence was obtained that demonstrated that TapA wa s expressed by both Aeromonas species and was present on the cell surface, although if assembled into pili this pilus type appears to be an uncommon o ne under standard bacterial growth conditions. Further studies into factors which may influence Tap expression are required, but the present study sug gests that Tap pili may not be as significant as Bfp pili for Aeromonas int estinal colonization.