Campylobacter jejuni: A lesser known bacteria
01 July 1999
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.