Aeromonas

(endorsed 1996)

Guideline

No guideline value has been set for Aeromonas in drinking water. The absence of Escherichia coli (or alternatively thermotolerant coliforms) does not indicate the absence of Aeromonas.

General description

Aeromonas spp. are Gram-negative, rod-shaped, nonsporing bacteria which are presently classified in the family Vibrionaceae, although they also bear many similarities to the Enterobacteriaceae. They are isolated from certain patients with diarrhoea and may cause septicaemia.

The genus Aeromonas is divided into two groups. The group of psychrophilic nonmotile aeromonads consists of one species A. salmonicida, an obligate fish pathogen that will not be considered further here. The group of mesophilic motile aeromonads consists of three biochemically distinguishable groups: A. hydrophila, A. sobria and A. caviae. Each of these three species consists of at least three different DNA-hybridisation groups.

Aeromonas is a normal inhabitant of fresh water, and occurs in water, soil and food, particularly meat, fish and milk.

Australian significance

Aeromonas spp. have been isolated from several drinking waters in Australia but the relationship between the isolates and clinical disease is not clear.

Treatment of drinking water

Free available chlorine residuals of 0.2–0.5 mg/L are generally sufficient to control Aeromonas in distribution systems.

Method of identification and detection

The numbers of Aeromonas in drinking water can be quantified using membrane filtration and anaerobic incubation (Cunliffe and Adcock 1989).

Health considerations

Mesophilic aeromonads have long been known to be pathogenic for cold-blooded animals such as fish and amphibians. In humans, three types of infections are described: systemic infections, usually in people who are seriously immunocompromised; wound infections (mainly surface contact); and diarrhoea (Jana et al. 1988). They have given rise to serious cases of septicaemia, often in people with underlying disease; and they have been linked with gastroenteritis in children (Gracey et al. 1982), although no causative role has been established, and their significance as an enteropathogenic organism is not clear. In animal test models, such as the suckling mouse test and the rabbit ileal loop test, pure cultures of Aeromonas have been found to cause marked fluid accumulation. This can partially be ascribed to the production of extracellular cytotoxins; however, despite the strong toxin production by Aeromonas strains in vitro, it has not been possible to induce diarrhoea in test animals or human volunteers.

It is assumed that Aeromonas strains are only poorly able to colonise the gastrointestinal tract. Little information is available on adhesion factors of Aeromonas or their interaction with receptors in the gastrointestinal tract.

Epidemiological investigations on the significance of Aeromonas as an enteropathogenic organism have been contradictory. In some studies the occurrence of Aeromonas in faeces of patients with diarrhoea was higher than in control groups, whereas other studies showed no difference. Sometimes the bacterium was even found more often in control groups. Aeromonads have sometimes been associated with acute self-limiting gastroenteritis.

Derivation of guideline

No specific guideline value can be established for Aeromonas because of difficulties in determining the pathogenicity of an isolate and its relevance to human health. Further work in the area is currently under way in Australia. Water must be tested directly for Aeromonas if their presence is suspected.

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NOTE: Important general information is contained in PART II, Chapter 5

References

Cunliffe DA and Adcock P (1989). Isolation of Aeromonas spp. from water by using anaerobic incubation. Applied and Environmental Microbiology, 55, 2138–2140.

Gracey M, Burke V and Robinson J (1982). Aeromonas-associated gastroenteritis. Lancet, ii, 1304–1306.

Jana JM and Duffy PS (1988). Mesophilic aeromonads in human disease: current taxonomy, laboratory identification, and infectious disease spectrum. Reviews of Infectious Diseases, 10, 980.