In a study that could benefit medical and food-safety research, scientists have used comparative genomics tools to find clues about why some strains of the bacterium Campylobacter – which each year cause more than 400 million cases of gastrointestinal disease – are more virulent than others.
The study, which appears in the January 2005 issue of PLoS Biology, compares the complete genome sequences of two strains of Campylobacter jejuni – the species most often associated with human illness – and supplements that analysis by contrasting those with the mostly-finished sequences of three other Campylobacters, including one species that may be an emerging pathogen in Africa.
In their analysis, the researchers found a set of genes that may be closely associated with the virulence of some Campylobacter strains as human pathogens. They also found sequence variations among the four Campylobacter isolates, including major structural differences related to the insertion of new stretches of DNA in the genome sequences. Those "insertions" and other gene variations may help scientists understand why there are major differences in the biology of various Campylobacter strains.
"The comparative genome sequences give scientists some new ideas to better control and detect these organisms," says TIGR’s Nelson, the study’s senior author. She and Fouts also say the study has helped scientists better understand the evolutionary relationships among Campylobacter species. In addition, the phage and megaplasmids discovered by the genome analysis may yield clues to the intra- and inter-species lateral transfer of DNA among Campylobacter strains.
Campylobacter are the leading cause of bacterial gastrointestinal illness in the United States, where about 15 out of every 100,000 people are diagnosed with campylobacteriosis every year. Many other cases go unreported due to the sporadic nature of the disease. The illness lasts for a week to 10 days, with symptoms that include diarrhea, cramps, abdominal pain and fever. Infrequently, the infection can be more serious or even fatal when victims develop Guillain-Barré syndrome, which involves damage to the nerves that link the spinal cord and brain to the rest of the body.
Campylobacteriosis is usually caused by C. jejuni, a microbe normally found in cattle, swine and birds, where it causes no problems. But the illness can also be caused by C. coli (also found in cattle, swine and birds), C. upsaliensis (found in cats and dogs), and C. lari (present in seabirds in particular). People are often exposed to the disease-causing bacteria when they eat contaminated food – in many cases, undercooked or poorly handled poultry.
While C. jejuni colonizes the gastrointestinal tracts of many animals, it appears to be especially adapted to the enteric tracts of birds, including chickens and turkeys. That is why poultry is considered to be a source of human campylobacteriosis. The disease can also be transmitted via human contact with contaminated water, livestock or household pets.
USDA’s Mandrell says the new Campylobacter sequence data has allowed the PSMRU group in California to develop more comprehensive detection methods, including microarrays, for analyzing human and environmental isolates of the bacteria. The goal is to be able to "fingerprint" strains, an important aspect of determining their source, fitness and epidemiology.
By identifying similar housekeeping genes among the non-jejuni species sequence data, the group has expanded its original fingerprinting method and initiated a study at several ARS locations to characterize differences among C. coli strains isolated from different animal and clinical sources.
Robert Koenig | EurekAlert!
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