Overview of Waterborne Pathogens - Waterborne Pathogens: Detection Methods and Applications - page 21

2.2.2. Escherichia coli
Large numbers of Escherichia coli bacteria exist naturally in the human
intestine ( Fig. 2.6 ). However, a limited number of enteropathogenic strains
can cause acute diarrhea. These strains have been classified according to
virulence, and the include enterohemorrhagic E. coli (EHEC), enterotoxi-
genic E. coli (ETEC), enteropathogenic E. coli (EPEC), enteroinvasive E .
coli (EIEC), enteroaggregative E. coli (EAEC) and diffusely adherent E. coli
(DAEC).
E. coli O157:H7 (an EHEC) is a well-known example of one of these
strains. Ingestion of this bacteria can lead to symptoms ranging from mild
to highly bloody diarrhea, and in around 2-7% of cases it can lead to hemo-
lytic uraemic syndrome. This latter disease, which is more likely to occur in
children under 5 years old, is potentially fatal. EHEC is the main zoonotic
source of pathogenic E. coli , known to infect cattle and sheep and to a lesser
extent goats, chickens and pigs. Although infection with the EPEC strains
is far less common than, for example, Campylobacter infection, the symptoms
can be much more severe. Additionally, the infective dose is low at fewer
than 100 organisms.
In 2000, an outbreak in Walkerton, Canada caused 7 deaths and 2300
illnesses. More recently, the largest ever verocytotoxigenic E. coli outbreak
occurred in Europe in 2011, with 3929 illnesses and 47 deaths. This out-
break was linked to the consumption of sprouts from contaminated seeds, 20
which may have become contaminated through the use of poor quality
water for irrigation purposes.
2.2.3. Salmonella
Salmonella is a motile, Gram negative bacteria that does not ferment lactose.
Most produce hydrogen sulfide or gas from carbohydrate fermentation. It
is now thought that there are two main species, namely enterica and bong-
ori , with 2000 subspecies. Some species show host-specificity, particularly
Salmonella typhi , which infects humans. Animal reservoirs of other species
include poultry, cows, sheep, pigs and birds.
Various types of disease result from the different species, including gas-
troenteritis, septicemia and typhoid fever. Salmonella enterica generally results
in self-limiting diarrhea with an onset 6-72h after the consumption of
contaminated food or water (or contact with infected animals/people)
and typically lasting 3-5days. The non-typhoidal subspecies rarely cause
waterborne outbreaks. Infection with typhoid species is the most common