The teeming life that takes root in a dead body after the final breath has been a forensic tool in constant refinement. From the bugs that devour the edges of a corpse to the bacteria that break down the body from within, the natural processes of the dead can tell investigators about the time of death, no matter how unnatural the death may have been.

The mouth microbiome in death, the oral mélange of bacteria, is the focus of a new investigation in the journal Molecular Oral Microbiology.

The natural progression of the post-mortem changes in the mouth can provide a blueprint for up to 12 days of breakdown, according to the researchers from the Universitat de Girona in Spain, and the Forensic Anthropology Center at the University of Tennessee in Knoxville.

“As the decomposition process advances, bacterial communities change according to the newly set environmental conditions,” they write. “Bacterial communities are clearly arranged in a temporal axis according to oxygen availability.”

The latest study involved three bodies: an elderly man with fixed crowns; an octogenarian female without teeth; and a young woman with a full set of natural teeth.

The bodies were kept at room temperature, with normal humidity. Throughout the decomposition process, their mouths were swabbed. The resulting genetic palette was analyzed with next-generation DNA sequencing.

An overall bacterial narrative emerged, the scientists found.

The Firmicutes and Actinobacteria, which are most prevalent in the living, were still dominant in the mouths of the dead from days 1 to 5, they report. But they two phyla declined gradually until day 6.

Around the bloat stage of days 5 to 7, Tenericutes appeared—likely having spread from the gastrointestinal tract up to the mouth.

Firmicutes reasserted their dominance from days 6 to 12—but appeared during this time of advanced decay in the different spore-forming species of Clostridiales and Bacillaceae, the researchers add. Dry remains were best characterized by these Bacilli and Clostridia, whose spores allow fast colonization of the changing conditions.

“Oxygen availability seems to be a major driver of bacterial changes during the stages of decomposition,” they write. “This variable strongly determines the ability and efficiency of organic matter degradation, which is basically performed by opportunistic Firmicutes, already present in the oral cavity. Then, immediately after oxygen depletion and onset of bloat stage, anaerobic bacteria take the leading role.”

The work could be another forensic clue to further the understanding of post-mortem interval, said Dawnie Steadman, one of the authors, of the University of Tennessee - Knoxville.

"I think that any time we can add important information into a body of existing work, such as gut microbiome studies, it sheds more light on the forensic relevance of bacteria as a whole to estimate the postmortem interval," she told Forensic Magazine. "It is a very exciting area of research." 

Other microbiome work at the University of Tennessee’s “body farm” has focused on the gut, as previously featured in Forensic Magazine.

Another study in the journal Scientific Reports last year looked at genetic sequencing of a cross-section of organs in 27 bodies, and found there were distinctions that could be forensically applied to criminal investigations.