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.
2Present address: Department of Health, Division of Analytical Laboratories, Lidcombe, N.S.W. 2141 Australia.