Fate of free DNA and transformation of the oral bacterium Streptococcus gordonii DL1 by plasmid DNA in human saliva

Competitive PCR was used to monitor the survival of a 520-bp DNA target seq uence from a recombinant plasmid, pVACMC1, after admixture of the plasmid w ith freshly sampled human saliva. The fraction of the target remaining ampl ifiable ranged from 40 to 65% after 10 min of exposure to saliva samples fr om five subjects and from 6 to 25% after 60 min of exposure. pVACMC1 plasmi d DNA that had been exposed to degradation by fresh saliva was capable of t ransforming naturally competent Streptococcus gordonii DL1 to erythromycin resistance, although transforming activity decreased rapidly, with a half-l ife of approximately 50 s, S, gordonii DL1 transformants were obtained in t he presence of filter-sterilized saliva and a 1-mu g/ml final concentration of pVACMC1 DNA, Addition of filter-sterilized saliva instead of heat-inact ivated horse serum to S. gordonii DL1 cells induced competence, although wi th slightly lower efficiency. These findings indicate that DNA released fro m bacteria or food sources within the mouth has the potential to transform naturally competent oral bacteria. However, further investigations are need ed to establish whether transformation of oral bacteria can occur at signif icant frequencies in vivo.