Bacteria Put the Bite on Assailants

By Stephen Luntz

New techniques to study the bacteria left behind after biting could prove a boon for forensics, the Otago School of Dentistry claims.

Bite marks are common in cases of sexual assault, and even in fights in which Queensberry Rules do not apply. In cases where the identity of the assailant is in dispute, attempts have been made to match the shape of the marks to the teeth of suspects.

However, Dr Geoffrey Tompkins says this process has had little success in court. “Bite marks have been used as evidence using morphometric analysis, but there is no scientific evidence that people have unique dentition and there has always been some subjective element in matching bite marks to teeth.” Tompkins adds that defence lawyers have become aware of this, and produce forensic dentists to cancel out any identification on this basis.

However, biting also leaves behind traces of bacteria. Our mouths play host to more than a dozen species of Streptococcus bacteria, and many varieties of the most common species exist. The strains present vary from mouth to mouth, and prove remarkably stable over time. “It is very difficult to implant a new strain, even when done deliberately with probiotic bacteria. You need to suppress others first,” Tompkins says. “Kissing wouldn’t have much effect.”

The obstacle to using bacteria as a way of identifying the biter is that oral bacteria have a short life outside the mouth, so attempts to collect and breed them for testing require a victim to get very quick treatment.

However, dentistry student Ms Lilian Hsu has now proven it is possible to collect the DNA from dead streptococci from bite sites and amplify them to the point where the mix of strains present can be identified.

Astonishingly, Hsu did her work during a 10-week summer project despite having no background in molecular biology. Tompkins says that Hsu’s matching technique risks false positives, but serves as a proof of principle.

Meanwhile Otago PhD student Darnell Kennedy is getting close to publishing what Tompkins describes as “a much more accurate means of testing”.