Emergence of recombinant strains of Helicobacter pylori during human infection

Genetic recombination can be important evolutionarily in speeding the adapt ation of organisms to new environments and in purging deleterious mutations . Here, we describe polymerase chain reaction (PCR), hybridization and DNA sequence-based evidence of six such exchanges between two strains of Helico bacter pylori during natural mixed infection of a patient in Lithuania. One parent strain contained the 37 kb long, virulence-associated cag pathogeni city island (PAI), and the other strain lacked this PAI. Most H. pylori fro m the patient had descended from the cag(+) parent, but had become cag(-) d uring infection. This had resulted from transfer of DNA containing the 'emp ty site' allele from the cag(-) strain and homologous recombination, not fr om excision of the cag PAI without DNA transfer. Other cases of recombinati on involved genes for an outer membrane protein (omp5 and omp29; also calle d HP0227 and HP1342) and a putative phosphoenolpyruvate synthase (ppsA; HP0 121). Replacement of a short patch of DNA sequence (36-124 bp) was also see n. As the chance of forming any given recombinant is small, the abundance o f recombinants in this patient suggests selection for particular recombinan t genotypes during years of chronic infection. We suggest that genetic exch ange among unrelated H. pylori strains, as documented here, is important be cause of the diversity of this gastric pathogen and its human hosts. Certai n H. pylori recombinants may grow better in a given host than either parent . The vigour of growth, in turn, could impact on the severity of disease th at infection can elicit.