Milk as a Potential Source of Aeromonas Gastrointestinal Infection

Abstract

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). Pasteuriza-tion 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 refriger-ated 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. Aeromonas species have been strongly implicated in many countries as causative agents of gastroenteritis, particu-larly in children less than two years of age, the elderly, and immunocompromised patients, during the summer months. The epidemiology and pathogenesis of Aeromonas gastroen-teritis are not well-understood, but Aeromonas species are widespread in water supplies, and hence, these have been considered as the possible source of most infections (3,15,28). In recent years, however, there has been increasing interest in Aeromonas spp. as possible causes of foodborne gastroenteritis (7,28). Concern has arisen because, as with water, Aeromonas strains identical to diarrhea-associated strains are readily isolated from a wide variety of retail foods (7,16,20,23). Moreover, many of these strains are able to grow at refrigeration temperatures (1,24). This raises the possibility that high numbers of organisms, and/or preformed enterotoxins, may be ingested if foods are kept for some time under refrigeration before consumption. Such effects may be exacerbated if stored foods suffer temperature abuse. As both the bacterium and its cytotoxic enterotoxin are destroyed by heating (56°C, 10 min) (5,11,24), foods that are consumed