Electron microscopic examination of factors influencing the expression of filamentous surface structures on clinical and environmental isolates of Aeromonas veronii biotype sobria

Strains of Aeromonas veronii biotype sobria isolated from clinical and environmental sources were examined for their expression of surface structures under a variety of culture conditions. When grown on solid media at 37 C, more than 95% of bacteria from the majority of strains isolated from human diarrheal feces and chicken carcasses were non-piliated or expressed only a few pill of long, flexible morphology per cell. Strains isolated from water or other foods were much more likely to express pill. Heavily piliated strains (all sources) possessed pill of several morphological types, including long, flexible pill of varying widths and rigid pill of varying lengths. Expression of pill was favored by growth at temperatures ca, 20 C and below and growth in liquid medium. Most fecal strains expressed some pill under these conditions. In addition, other surface structures (fibrillar aggregates, fibrillar networks, bundle-forming pill) were seen on some strains from most sources. These were also seen most frequently when bacteria were grown in liquid media at temperatures ca. 20 C and below. Pill expression was not dramatically influenced by growth under anaerobic conditions, or in iron-depleted media, or by combinations of the above conditions. The role of the above surface structures in Aeromonas pathogenicity remains to be elucidated.