How to Identify a Suspect… By His Microbes?

There are many established ways to identify a criminal suspect after the fact—DNA or fingerprints, for example (but not bite marks). But in 2010, a theory was introduced that a person could be identified by the bacteria they carried on their skin. The idea is that each person’s specific mix of microbes (called their ‘microbiome’ in scientific circles) is so individual that it could uniquely identify them.

Let’s back up a step because some of you are probably thinking Yeeeccccchhhhhhhh! I’m carrying WHAT on me? Actually, the bacteria that we always carry on us are an important part of our biological stability and are genuinely good for us. While the microbes that live on us make up only approximately three to five pounds of our total body mass, because they are so much smaller than our somatic cells, they actually outnumber our cells by approximately 10:1. In fact, there are enough bacteria that live on us that if we could collect them all, they’d fill a large soup can.

We’re born essentially microbe free, but breastfeeding is the first important transfer of good gut bacteria to newborns. By the time we are adults, it’s estimated that well over 500 species of bacteria live in our gut. These are symbiotic bacteria, meaning they live off us, but we reap the benefits of them in return. We provide a food source for them, but they help us digest our food and better obtain nutrients from our meals. Gut bacteria also help maintain our mucosal immune system making us healthier. Just look to your nearest grocery story or pharmacy for probiotics or probiotic yogurt sold to supplement your natural gut flora. This is exactly why it’s become a significant industry in recent years.

But what about the bacteria that live on your skin? The skin is the body’s largest organ and is our first line of defense against pathogenic invaders. But we’re not alone in the fight—a diverse panorama of bacterial flora live on our skin. Some bacteria help protect us by producing antimicrobials to kill pathogenic bacteria, while others work with our cells, actively using substances we produce to kill invaders. Because the skin is a varied environment with a multitude of cells, glands, follicles, natural moisture levels, and layers of epidermis, we are host to a wide range of bacteria that each live in their own preferred environments. It is these bacteria that give us a unique microbial fingerprint.

This fingerprint is analyzed by identifying the DNA sequences of the bacteria an individual carries. Most individuals host a similar complement of types of bacteria, but it is the relative amounts of these hundreds of strains that constitutes a fingerprint. So each time you touch an object and leave an oil and amino acid slurry fingerprint, you also leave a microbial fingerprint behind. The more commonly used the item, the stronger the microbial fingerprint, with cell phones, keyboards, and shoes bearing the strongest signal. A theory exists that since we leave a bacterial trail behind as we move through the environment, it might be possible to trace a suspect by the trail left by his shoes. It’s even been suggested that a forensic trail left at a crime scene might remain intact for up to two weeks, a very significant period of time in any investigation.

Much study must be done before an individual’s microbes could be used as a forensic tool during a criminal investigation. Previous experiments have been done on groups between two and one hundred and twenty individuals, so the power of this technique can only lie in a specificity that could eliminate millions of individuals to match the power of DNA profiling or fingerprinting. But it’s an intriguing possibility and one that can only expand as studies on the human microbiome, a relatively new field, expands into new and exciting territory.

Photo credit: Wikimedia Commons

Forensic Case Files: Medieval Cemetery Discovered Under a Parisian Supermarket

Recently, a surprising discovery was made in the basement of one of Paris’ Monoprix supermarkets. During some renovation work to lower the level of the basement (two full floors underground) to create additional storage space, a number of human remains were discovered beneath the floor, previously undisturbed since medieval times. Inrap, the French Institut National de Recherches Archéologiques Préventives, was called in to examine the find and then to do a full excavation before any renovation work would continue.

But how did so many human remains come to be found under an ordinary supermarket? In the end the answer was a simple one of land usage: the supermarket was built on the site of the cemetery of Hôpital de la Trinité. Trinité was established in the 12^th century, but was destroyed at the end of the 18^th century, allowing the land to be repurposed following the hospital’s demolition.

As the excavation space was cleared, eight separate graves spread over three parallel rows were discovered. Seven of the graves held two to five bodies, and one grave held over one hundred and fifty. Each grave was layered in five or six levels and part of the excavation disappears under the existing building wall, out of reach of archeologists.

The remains comprise a diverse group of male and female, children and adults of all ages. The skeletons are laid very closely together, arranged in alternating head to toe pattern to maximize the number buried in a small space. From this arrangement, Inrap archeologists propose that a ‘mass mortality crisis’ occurred, requiring a rapid and common burial. There are no injuries on any of the bones to explain any kind of violent death, implying that disease is responsible. The Bubonic plague epidemic of the late 1340s is one of the suggested causes; large-scale famine is another. Scientists are conducting DNA tests on the remains as well as carbon dating to obtain a more complete picture of catastrophe.

Inrap archeologists excavated the more than 1,000 square-foot site for two and a half months to remove all the remains. They hope to continue their study of the dead following the excavation with the hope of learning more about past burial practices―how the dead were added to the grave, the spatial and chronological organization, and sanitary criteria during an epidemic.

Much of Paris has been occupied since the Middle Ages, but due to dense population and building within the city, it is only when construction or renovation work reveals the city’s hidden dead that archeologists can shed light on Paris’ distant past. As a result, the Monoprix site is a wonderful opportunity for French archeologists, having only had twelve previous sites with which to study early funereal practices. Archeologists feel that work on this site could shed much light on not only how the Medieval French lived, but also how they died.

Photo credit: Denis Gliksman, Inrap

Forensics Under the Microscope: Bite Mark Analysis

In our continuing series on forensics run amok, we’re going to look at the trials and tribulations of bite mark analysis as a forensic tool.

What is bite mark analysis? It’s the comparison of a suspect’s dentition (teeth) to a bite mark found at a crime scene in human flesh or another solid substrate. This type of analysis tends to be associated with rape, murder, child abuse, and animal-related infractions because of the nature of the crimes involved. The underlying assumption of bite mark analysis is that human dentition is unique and human skin (or other material) can be imprinted in a way to reflect that uniqueness. The analysis depends on imperfections and deviations such as tooth alignment, chips and broken teeth, tooth wear, dental work, braces, and ethnically-correlated tooth shape to make each set of teeth unique.

How is bite mark analysis done? The first thing any bite mark analyst must do is ensure that the bite is human, and not left by an animal. Measurements are taken of the wound to mark orientation, depth and size of the bite, and then photographs are shot to preserve the bite before any degradation or alteration (decomposition in deceased victims, or wound healing in live victims). Plaster casts can be made of the impression for later comparison with a suspect’s dentition. Sometimes the skin containing the bite mark is excised from deceased victims and fixed in formaldehyde for long term storage and/or later analysis.

How is a match made? When a suspect is apprehended, a plaster cast is made of their teeth for comparison with the bite mark. A forensic dentist will attempt to compare the suspect’s dentition to the bite mark or a photo of the bite mark.

Why is bite mark analysis not a reliable science? The first thing to note here is that many of the scientists doing bite mark analysis are excellent forensic dentists truly trying to assist in case investigations. But the biggest strike against this kind of pattern matching is that there is no scientific proof that the guiding principal―that each person’s dentition is truly unique―is correct. No large population studies have been conducted to answer this question, as opposed to fingerprints or DNA profiling, where the results are truly individual. Therefore, the guiding principle of the technique is based upon an assumption. In actuality, the analysis is the subjective opinion of the technician, based purely on his or her own experience. To make matters worse, the shape of each bite depends on multiple circumstantial factors that can heavily influence the analysis―the force applied during the bite, the substrate, if the subject or substrate is in motion, and the angle of the bite.

What are the problematic technical issues of the analysis itself? There are many technical issues which can influence analysis, such as:

• Tooth wear patterns that change over time—in older cases, current dentition might not reflect dentition when the crime was committed.
• Unlike fingerprints or DNA, there is no dental database for reference.
• Bite marks change significantly over time and decomposition following skin slippage/desiccation.
• Sometimes bodies are exhumed months after burial, producing an imprint that is not identical to the fresh bite. Several of these cases have resulted in convictions.
• Some analysis is done using photos of the bite. Once such analysis noted a gap in the dentition that matched the suspect. The forensic expert recanted a decade later, saying the gap was a flaw in the photo. Nevertheless, this conviction was not overturned.

What are the judicial ramifications? In 2009, the National Academy of Sciences made the damning statement that the forensic science of bite marks had “no evidence of an existing scientific basis for identifying an individual to the exclusion of all others.” According to the Innocence Project, there have been twenty-four people exonerated by other evidentiary means (usually DNA, often not a tool available at the time of the trial). It is estimated that there are hundreds of other prisoners, including fifteen persons awaiting execution, who could be exonerated if bite mark evidence is overturned.

The science of bite mark analysis remains a controversial one, and current forensic investigators always try to find another way to use bites as evidence, i.e. swabbing for the presence of DNA. It is worth noting that not all forensic dentistry is questionable; using dentition to identify human remains is a well-established and successful method in the eyes of both the scientific and forensic communities.

Photo credit: Surlygirl

Authors for Indies Day!

This past Saturday, May 2^nd, was Authors for Indies Day in Canada and Independent Bookstore Day in the U.S―a day when authors spent time visiting local independent bookstores, not to talk up their own books, but to talk up everyone’s books and to support local bookstore owners. I took part in Author Day at my favourite local bookstore, A Different Drummer Books. Joining me was an impressive roster of local authors: Linwood Barclay, Rebecca Bender, Melodie Campbell, Gillian Chan, Jill Downie, Bonnie Lendrum, Sylvia McNicoll, Janet Turpin Myers, John Lawrence Reynolds, Gisela Tobien Sherman, and Joyce Wayne.

The crowd of authors and readers milling around as we were starting:

Fellow Crime Writers of Canada writers Jill Downie (left) and Melodie Campbell (right) with Jill’s husband, actor Ian Downie. Yes, that is TWO PARTS BLOODY MURDER that Melodie is holding:

Brilliant Canadian thriller writer (and hometown boy), Linwood Barclay:

Our host, the wonderful Ian Eliott, welcoming us to the store and the event:

 

The crowd listening to Ian. It was only when I lowered my phone and looked at my picture that I realized who’d snuck in when I didn’t even know she was coming:

A lovely display of visiting authors’ books. If you look hard, you’ll see I didn’t do a good job of capturing all three of our novels on the far end of the table (we’d sold a bunch so the piles were low, always a good thing…):

Hopefully some of you heard about it and got out to support your wonderful local independent booksellers. Thanks to the national organizers of Authors for Indies Day! It was a roaring success and we’ll all be back to do it again next year!