sub-species, and the Hydrophila–Punctata group containing a number of
motile species, includingA. hydrophila A. sobria, andA. caviae.
A. salmonicida is not a human pathogen but causes diseases of
freshwater fish which can be an important economic problem in fish
farming. Members of the Hydrophila group can cause extra-intestinal
infections, commonly in the immunosuppressed or as a result of swim-
ming accidents where the skin is punctured. The first report of gastro-
enteritis due toAeromonascame from Jamaica in 1958, but evidence of
its ability to cause gastroenteritis in otherwise healthy individuals is
patchy.
Epidemiological investigations in several countries have reported
higher rates of isolation of aeromonads from patients with diarrhoea
than from control groups, although this does not necessarily indicate
a causal relationship. In one study, the incidence of A. hydrophilain
American travellers to Thailand with diarrhoea was significantly high-
er than in unaffected individuals. Interestingly, isolation rates in the
Thai population were similar for both groups of patients suggesting
that Aeromonas may be a cause of ‘travellers diarrhoea’ in these
regions.
Good supporting evidence from sources other than epidemiological
studies has proved difficult to obtain. In a feeding trial involving 50
volunteers with doses as high as 5 1010 , only two cases of diarrhoea
resulted, although a laboratory accident has been reported where ap-
proximately 10^9 cells were ingested by a worker mouth pipetting who
later suffered acute diarrhoea.
7.1.2 The Organism and its Characteristics
Aeromonads are Gram-negative, catalase-positive, oxidase-positive rods
which ferment glucose. They are generally motile by a single polar
flagellum.
A. hydrophilais neither salt (o5%) nor acid (min. pHE6.0) tolerant
and grows optimally at around 28 1 C. Its most significant feature with
regard to any threat it may pose in foods is its ability to grow down to
chill temperatures, reportedly as low as 0.1 1 C in some strains. Its
principal reservoir is the aquatic environment such as freshwater lakes
and streams and wastewater systems. The numbers present will depend
on factors such as the nutrient level and temperature but can be as high
as 10^8 cfu ml^1 in a relatively nutrient rich environment such as sewage.
Although it is not resistant to chlorine, it is found in potable water,
where it can multiply on the low level of nutrients available in piped
water systems. It has also been isolated from a wide range of fresh foods
and is a transient component of the gut flora of humans and other
animals.
Chapter 7 183