Comparison of Streptococcal DNA amplified from Bite Marks and Teeth

Species of the genus Streptococcus constitute the numerically dominant bacteria on the teeth of almost all humans. Streptococci are genotypically diverse and isolates cultured from bite marks may be matched to those from the teeth of the biter; potentially providing evidence from bite mark investigations in which human DNA cannot be recovered. Objective: To investigate the feasibility of applying PCR and denaturing gradient gel electrophoresis (DGGE) as a non-culturing approach to compare streptococcal DNA from bite marks and teeth. Methods: Swabbed samples were taken from self-inflicted bite marks (three hours old), unbitten skin sites and the mandibular incisors of 24 participants. Extracted DNA was amplified by PCR using streptococcus-specific primers and the products separated by DGGE. PCR products from various laboratory strains provided reference markers. Relative migration distances of amplicons were measured manually. Following exclusion of skin amplicons, the number of DNA fragments amplified from bite marks that matched those from the teeth of the same and different individuals were determined. Results: The mean percentage match of amplicons from bite marks with those from the tooth samples of the same individual (19 comparisons) was 82.8% (95% confidence interval, 58.2% - 100.0%). The mean percentage match between bite marks and tooth samples of different individuals (345 comparisons) was 36.2% (95% confidence interval, 2.9% - 69.5%). A significant difference was found between the two groups (P < 0.0001; Student t-test). Conclusions: The results indicate a high likelihood of matching bacterial DNA amplified directly from a bite mark with bacterial DNA from the teeth responsible for the bite mark. Although some bite marks displayed a number of amplicons matching heterologous teeth, refinement of the method, including additional PCR primers and computer-assisted analysis, should enhance the specificity of this approach.

Supported by the NZ Dental Research Foundation Board and Colgate Travel award.