Could bacterial DNA be crucial to forensic bite mark analysis?


Bites inevitably transmit bacteria from assailant to victim. In 2010, Dr Paul Spradbery demonstrated that some bacterial genes exhibit polymorphism between hosts and could, therefore, be useful in determining which individual has (and which has not) caused a traumatic wound.


While having insufficient discriminatory power to identify an assailant, this clever forensic technique has proven capability to exonerate defendants that might otherwise be wrongfully convicted.

Author Comments

Dr Paul S Spradbery, BSc (Hons), BDS
Dr Paul S Spradbery, BSc (Hons), BDS
Intertek Life Sciences
London | United Kingdom
It is pleasing that, since establishing proof of principle, the technique has been validated and used in criminal courts of law. Further application is possible by identifying and utilizing other - perhaps more efficient - genetic markers from the same bacterial species.Dr Paul S Spradbery, BSc (Hons), BDS


Molecular identification and genotyping of Streptococcus mutans from saliva samples of children from Medellín, Colombia
Saliva as a Forensic Tool

Restriction fragment length polymorphisms of mutans streptococci in forensic odontological analysis

Spradbery P.S.

Bioscience Horizons (Oxford University Press)

Bioscience Horizons, 3(2), 166-178

Humans bite each other with alarming frequency. Such injuries can result in forensic investigation and, ultimately, criminal prosecution. Bite mark analysis techniques include odontometric measurement, ABO serotyping and DNA profiling, none of which is infallible. Research into additional techniques would be advantageous. Biting involves, also, bacterial transfer from teeth to skin and vice versa. The principal species found on anterior teeth is Streptococcus mutans, which is universal among humans. The aim of this research was to establish, in the context of forensic odontology, whether chromosomal DNA profiles of this ubiquitous oral bacterium would vary significantly among a sample of Caucasian individuals (P = 0.05). Hence, if sufficient discriminatory power were present, the technique could be deemed useful to forensic investigation. Oral fluid was recovered from the lower incisors of 10 adults. Samples were cultured selectively using mitis salivarius bacitracin agar and microbiological tests carried out for the purpose of differential identification. These included visual assessment of colony morphology, Gram staining and microscopic analysis, followed by chemical testing for the enzyme catalase. Chromosomal DNA was extracted from subcultured cells, resolved by agarose gel electrophoresis and viewed using ultraviolet transillumination. The presence of DNA was confirmed. Subsequently, the 16S ribosomal RNA gene was amplified by polymerase chain reaction using specific 27 forward and 1492 reverse primers. Amplicons were resolved and viewed as previously. Amplified products were digested by incubation with restriction endonuclease HaeIII, resolved by polyacrylamide gel electrophoresis and viewed. Linear regression analysis of gel profiles was used to calculate restriction fragment lengths. A Kruskal–Wallis (analysis of variance) test was performed on ranked data (H = 8.161, df = 9, P = 0.518). Consequently, the null hypothesis (no inter-subject variation) was accepted. However, all but two profiles were proved to be unique. Proof of principle was provided regarding the application of oral bacterial genotyping to forensic bite mark cases.
January 2010
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