Evidence for a conjugation-like mechanism of DNA transfer in Helicobacter pylori

Abstract

Many strains of Helicobacter pylori are naturally competent for transformation in vitro. Since there is a high degree of genetic variation among H. pylori strains, we sought to determine whether mechanisms of DNA exchange other than transformation exist in these organisms. Studies were done with H. pylori cells that each were resistant to two different antibiotics; the procedure used involved mating of cells on plates or in broth, in the absence or presence of DNase. In each experiment, such matings produced progeny with the markers of both parents. Examination of the full resistance profile and random arbitrarily primed DNA PCR (RAPD-PCR) profiles of the progeny indicated that DNA transfer was bidirectional. DNase treatment reduced but did not eliminate transfer; only the presence of both DNase and a membrane separating the cells did so. For progeny derived from matings in the presence of DNase, antibiotic resistance and RAPD profiles indicated that transfer was unidirectional. DNase-treated cell-free supernatants also did not transform, ruling out transduction. These experiments indicate that both a DNase-sensitive mechanism (transformation) and a DNase-resistant conjugation-like mechanism involving cell-to-cell contact may contribute to DNA transfer between H. pylori cells.

FIG. 1

RAPD-PCR of DNA isolated from Strep^r Spec^r progeny resulting from coculture of HPK1 (Spec^r Rif^r) (strain A) and HPK5 (Strep^r pHP1 Kan^r) (strain B). (A) Progeny obtained in the absence of DNase I, as analyzed with primer 9355 (1). Some of the progeny show the profile of each of the parental strains, consistent with bidirectional DNA transfer. (B) Progeny obtained in the presence of 285 μg of DNase I per ml, as analyzed with primer d11344 (1). All progeny have the same profile as HPK1 (Spec^r Rif^r), consistent with unidirectional DNA transfer from HPK5 to HPK1.