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Lesser Known Bacteria Campylobacter jejuni is one of the world's major
food poisoning bacteria. British scientists have recently determined
its entire genetic make up, which should lead to an understanding of
the mechanism of Campylobacter virulence as well as strategies to control
it.
Campylobacter jejuni has been taken seriously as a food pathogen only
since the 1970's, but it is probably responsible for maybe twice as
many cases of reported enteritis than the better known Salmonella. Originally
thought to be a harmless organism living within some animals, one of
its mysteries is how it lives in the gut of birds without causing disease
but becomes an invasive pathogen in humans. It has a very low infective
dose and over the last 20 years the incidence of Campylobacter food
poisoning has soared in developed countries. Healthy humans do not carry
the organism nor is it passed from infected humans to other people.
The main symptom of infection from contaminated food or water is diarrhoea,
but others such as fever, nausea, headache and abdominal pains can also
occur. Illness typically starts 2-5 days after ingestion of the bacteria
and the effects can be very debilitating, lasting for up to 10 days.
Determination of the entire genetic sequence of Campylobacter was recently
completed and scientists in three UK centres are now hard at work to
find out which genes do what. One avenue of investigation will explore
the activity of C.jejuni genes and the amounts and types of different
proteins produced by them when the organism faces different environmental
challenges. It will show, for example, how C.jejuni responds to changes
in temperature, to low levels of nutrients and to different levels of
acidity and bile salts. The results will help to explain its ability
to survive in conditions as different as those in water, raw meat and
the human gut, as well as to suggest possible strategies for preventing
its growth in foods. Those genes thought to be crucial to the virulence
of C.jejuni will then be studied in greater detail, for example by seeing
how their function is altered by the introduction of defined changes
to the gene (mutations).
This information can be derived using so-called microarrays or DNA chips.
These are specially treated microscope slides onto which are printed
the complete 1700 gene sequences of Campylobacter. When extracts from
Campylocbacter cells are passed over the slides, those genes active
in the cells at that time will be highlighted and can be identified.
This is an inexpensive and efficient way of comparing gene activity
in cells growing under different conditions.
Interesting data is already emerging. For example, there are no immediately
recognisable genes that correspond to those that produce some of the
key virulence factors in other pathogens. On the other hand the organism
seems to have several copies of a gene coding for an enzyme associated
with changes to the outer surface of the bacterium. Over a third of
the genes appear to have no known counterparts anywhere in nature, suggesting
a unique Campylobacter infection strategy which would not have been
found without the help of DNA-testing.  |
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