Approximately 4% of the Helicobacter pylori genome, significantly more than that of any other known bacterial species, is composed of genes encoding outer membrane proteins (OMPs). The best described H pylori OMP is the adhesin BabA, which is thought to mediate host/bacterial interactions and has been linked to microbial pathogenicity.^1–4 Dissection of OMP function helps our understanding of the complex gastric biology of infection and may offer a possibility of identifying those with an increased risk of disease development.

In an interesting and comprehensive study, Yamaoka et al (Gut 2005;55:775–81) analysed expression of several outer membrane proteins (BabA, BabB, SabA, OipA) in a large number of H pylori strains and described their association with gastroduodenal diseases. One important finding was the strong linkage of OipA expression with the cag pathogenicity island (cagPAI), a major H pylori virulence factor. The authors showed that in 94% of Columbian and US H pylori strains, CagA and OipA phenotypes were identical.

OipA expression is regulated by slipped strand mispairing based on the number of CT dinucleotide repeats in the 5′ region of the gene. We sequenced this genetic region of H pylori strains from 53 German patients with chronic gastritis and further determined the presence of the cagPAI marker gene cagA, as described previously.^3,5 Similarly to Yamaoka et al, we found linked expression of CagA and OipA in our European strain population, with 83% concordance (fig 1A). The reason for this strong association is not understood. However, as there is no obvious common genetic mechanism controlling expression of the two genes, linkage of cagA and oipA seems to be based on selection in the host. Indeed, it has been demonstrated that H pylori is capable of regulating OMP expression in vivo by phase variation …