The incidence and properties of Aeromonas species found in milk were examined to evaluate the potential of milk as a vehicle for the transmission of Aeromonas gastroenteritis. Aeromonads are common in raw milk (60%, 43 of 72 samples, positive). Pasteurization is effective at removing this contamination. Nevertheless, around 4% (seven of 183) pasteurized milk samples contained potentially significant strains, apparently introduced by subsequent handling of the milk. Some of these strains were indistinguishable from diarrhea-associated strains and were able to produce exotoxins at 37°C and adhere to epithelial cells. Adhesive ability and piliation increased when these strains were grown at low temperature. Such strains, although mesophilic, could reach high numbers in refrigerated milk without detectable spoilage of the milk. They pose the risk of colonization and in vivo toxin production. Further studies are required, but ingestion of preformed toxins produced in stored pasteurized milk may be of less concern, as psychrotrophic aeromonads, with the ability to produce large amounts of exotoxins in milk, appear to be uncommon and exotoxin production in milk was significantly lower than in bacteriological medium.
SummaryAdhesion to HEp-2 cells has been shown to correlate with enteropathogenicity forAeromonasspecies. Such adhesion is thought to reflect the ability of strains to adhere to human intestinal enterocytes, although HEp-2 cells are not of intestinal origin. In this study strains ofAeromonas veroniibiotype sobria isolated from various sources were investigated in parallel assays for their ability to adhere to HEp-2 cells and to an intestinal cell line (Caco-2). Quantitative assays showed identical adhesion values were obtained with both cell lines. Adhesion was best when bacteria were grown at 22 °C compared with 37 °C and 7 °C. Some environmental isolates showed greater adhesion when grown at 7 °C than when grown at 37 °C. Filamentous structures on these strains are also optimally expressed under the above conditions (reported elsewhere). Mechanical shearing or trypsin treatment to remove surface structures from several adhesive strains grown at 22 °C decreased adhesion to cell lines by 50–80% providing further indirect evidence that filamentous adhesins may play a role in cell adhesion for thisAeromonasspecies.
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 pili of long, flexible morphology per cell. Strains isolated from water or other foods were much more likely to express pili. Heavily piliated strains (all sources) possessed pili of several morphological types, including long, flexible pili of varying widths and rigid pili of varying lengths. Expression of pili was favored by growth at temperatures ca. 20 C and below and growth in liquid medium. Most fecal strains expressed some pili under these conditions. In addition, other surface structures (fibrillar aggregates, fibrillar networks, bundle-forming pili) 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. Pili 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.
A total of 182 Aeromonas hydrophila strains isolated from environmental (food and water) and clinical (stool and other sources) samples taken in mainland Australia, Tasmania and New Zealand were assigned to one of three DNA/DNA hybridization groups (HGs) on the basis of biochemical characteristics, and tested with regard to their ability to produce virulence factors. Strains from HG2 were rarely isolated; strains from HG1 were most commonly isolated from clinical sources; and strains from HG3 formed the majority of environmental strains. There was no correlation of HG to geographic source. Strains from HG2 infrequently produced virulence factors. Strains from HG1 were more likely to produce virulence factors if they came from a clinical source. Overall, strains from mainland Australia produced virulence factors more frequently than those from Tasmania or New Zealand. Strains from HG1 may be of more clinical significance than strains from the other two HGs.
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