MedWire News: A third of Crohn's disease (CD) patients are infected with Mycobacterium avium subspecies paratuberculosis (MAP), report researchers who believe the bacterium may be a causal factor.

"The association between MAP and CD has been widely researched, but is controversial," says the team.

To investigate further, Robert Bentley (Christchurch School of Medicine & Health Sciences, New Zealand) and colleagues analyzed blood samples from 361 CD patients and 200 blood donor controls.

The samples were screened for DNA specific to MAP using polymerase chain reaction analysis - which can detect 20 fg purified MAP DNA.

Bentley et al detected MAP-specific IS900 DNA in 33.8% of the CD patients, compared with 21.5% of controls - giving a significant odds ratio (OR) of 1.86.

They did not, however, find any evidence of an association between MAP-specific DNA and mutations in the NOD2 gene, which are reportedly independent risk factors for CD.

NOD2 is involved in bacterial sensing through peptides in their cell wall. Other researchers have proposed that common mutations in NOD2 impair detection of bacteria and, thereby, raise CD susceptibility.

Bentley and colleagues say in the American Journal of Gastroenterology: "The over-representation of MAP DNA in CD suggests either a role or a probable role for MAP in the etiology of CD."

They point out, however, that "it will be important to determine whether there is evidence of other bacteria in the peripheral blood of CD patients compared with controls."

They explain: "This would suggest nonspecific bacterial translocation through damaged intestinal tissue with increased permeability rather than an etiological role for MAP."

The team also calls for further work on the "detection of MAP in peripheral blood by conventional microbiological techniques and molecular techniques to indicate mycobacterial viability."

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