Variation of the natural transformation frequency of Campylobacter jejuni in liquid shake culture

Natural transformation, a mechanism that generates genetic diversity in Campylobacter jejuni, was studied in a novel liquid shake culturing system that allowed an approximately 10 000-fold increase in cell density. C. jejuni transformation frequency was analysed in this system under 10%, 5.0% and 0.7% CO2 atmospheres. At 5.0% and 10% CO2 concentrations, when purified isogenic chromosomal DNA was used to assess competence, transformation frequency ranged from 10(-3) to 10(-4) at low cell concentrations and declined as cell density increased. Transformation frequency under a 0.7% CO2 atmosphere was more stable, maintaining 10(-3) levels at high cell densities, and was 10(-3) to 100-fold higher than that under a 10% CO2 atmosphere. Three of four C. jejuni strains tested under a 5.0% CO2 atmosphere were naturally competent for isogenic DNA; competent strains demonstrated a lack of barriers to intraspecies genetic exchange by taking up and incorporating chromosomal DNA from multiple C. jejuni donors. C. jejuni showed a preference for its own DNA at the species level, and co-cultivation demonstrated that DNA transfer via natural transformation occurred between isogenic populations during short periods of exposure in liquid medium when cell density and presumably DNA concentrations were low. Transformation frequency during co-cultivation of isogenic populations was also influenced by CO2 concentration. Under a 0.7% CO2 atmosphere, co-cultivation transformation frequency increased approximately 500-fold in a linear fashion with regard to cell density, and was 1000- to 10 000-fold higher during late-exponential-phase growth when compared to cultures grown under a 10% CO2 atmosphere.