Surface properties of autoagglutinating mesophilic aeromonads.

The surface characteristics of 24 autoagglutinating (AA+) mesophilic aeromonads were investigated. One group of 16 was found to be highly related serologically by their reactive pattern against O antisera generated against three reference strains. Subsequent characterization of 11 of these isolates (group 1) indicated that they had the following properties in common: precipitation after boiling (PAB+), membership of serogroup O:11 (typing scheme of Sakazaki and Shimada), resistance to lysis by bacteriophage Aeh1, and possession of a surface layer (S layer) as determined by transmission electron microscopy. Strains not exhibiting the same serologic reactivity pattern belonged to diverse serogroups (other than O:11), were generally susceptible to lysis by Aeh1, and were S layer negative by transmission electron microscopy (group 2). Analysis of selected isolates representing both groups indicated that group 2 strains were usually more hydrophobic than group 1 isolates in several different assays; both groups, however, possessed high surface charge as determined by binding to DEAE-cellulose. Group 1 isolates were more virulent than group 2 strains tested as determined by lower 50% lethal doses for mice. On the basis of the results of the kinetics of autoagglutination in broth, relative surface hydrophobicity, uptake of Congo red, agglutination of yeast cells, and electrophoretic protein profiles of whole-cell extracts, the surface layer associated with O:11 mesophilic aeromonads appears to be distinct from that of Aeromonas salmonicida. The results suggest that a new pathogenic group of mesophilic aeromonads linked through a common AA phenotype, serogroup, and S layer cause serious infections in both humans and animals (fish).