Functional characterization of type IV pill expressed on diarrhea-associated isolates of Aeromonas species

Our past work has shown that long, flexible type IV pill (single or in bund les) are the predominant pill expressed on fecal isolates of diarrhea-assoc iated species of Aeromonas (Aeromonas veronii biovar sobria and A. caviae). They represent a family of type IV pill which we have designated Bfp (for bundle-forming pill). Reports from Japan suggest that Bfp are intestinal co lonization factors. This study presents compelling evidence to support this conclusion. Aeromonas bacteria and/or Bfp purified from a strain of A. ver onii biovar sobria were shown to adhere to epithelial and intestinal cell l ines, freshly isolated human enterocytes, and fresh and fixed human and rab bit intestinal tissues, as determined by light and electron microscopy and immunohistochemical detection. Removal of Bfp by mechanical means decreased adhesion to cell lines by up to 80%. Purified Bfp blocked adhesion of the test strain to intestinal cells in a dose-dependent manner. Adhesion was al so blocked by the Fab fraction of anti-Bfp immunoglobulin G. Moreover, ultr astructural studies (ruthenium red staining and transmission and scanning e lectron microscopy) demonstrated for the first time that Aeromonas adhesion to human enterocytes is pilus mediated and suggested that Bfp may also pro mote colonization by forming bacterium-to bacterium linkages. Bfp-positive isolates examined for type TV pilus-mediated twitching motility in agar and slide culture assays developed for Pseudomonas aeruginosa did not, however , exhibit this function.