Virulence markers of mesophilic aeromonads: association of the autoagglutination phenomenon with mouse pathogenicity and the presence of a peripheral cell-associated layer.

Autoagglutination (AA phenotype) of mesophilic aeromonads in broth was found to be a virulence-associated marker. There were two kinds of AA+ strains: those that spontaneously pelleted (SP+), and those that pelleted only after boiling (PAB+). Of 79 strains tested, 24 (30%) were AA+, and 18 of these were recovered from clinical specimens. Most of the AA+ strains (n = 21) were identified as either Aeromonas sobria or Aeromonas hydrophila. Of the well-documented clinical isolates of A. sobria and A. hydrophila available, 5 (46%) of 11 from invasive disease and 4 (14%) of 29 from noninvasive disease were SP- PAB+. The SP- PAB+ phenotype was significantly associated with invasive infections (e.g., bacteremia and peritonitis [chi 2, P less than 0.05]). All seven of the SP- PAB+ A. sobria and A. hydrophila strains tested killed mice within 48 h after intraperitoneal infection with 1 x 10(7) to 3 x 10(7) CFU, whereas only two of four SP+ PAB+ strains tested were lethal. All of the SP- PAB+ A. sobria and A. hydrophila isolates examined shared common O somatic antigens and possessed an external layer peripheral to the cell wall as determined by thin-section electron micrography. The LL1 strain of A. hydrophila used by Dooley et al. (J. S. G. Dooley, R. Lallier, and T. J. Trust, Vet. Immunol. Immunopathol. 12:339-344, 1986) to demonstrate an S membrane protein component in aeromonads virulent for fish also was SP- PAB+ and possessed the peripheral membrane, suggesting an association between these two components. Seven AA- and three SP+ strains tested lacked this layer; furthermore, 22 (71%) of 31 such isolates did not kill mice. The AA phenotype was a stable characteristic upon long-term passage of isolates in vitro. Study of SP+ and PAB+ aeromonads by surface charge and hydrophobicity analyses indicated that neither property correlated with either virulence or the presence of an external layer.