Tag Archives: DNA

Bacterial DNA Profiling

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 Finding DNA

Earlier this month, researchers at Washington University School of Medicine (St. Louis) and at the European Molecular Biology Laboratory (Heidelberg, Germany) reported a new type of DNA profile: DNA profiles from human gut bacteria. The researchers analyzed microbial DNA in 252 stool samples from 207 individuals, focusing on 101 species of microbes commonly found in the intestine. They found many types of DNA differences, the sort of differences that generate unique DNA profiles. For 43 subjects, the researchers collected two stool samples one month to six months apart. The scientists found little variability in the microbial DNA. In short, they discovered variability in DNA profiles between subjects and consistency in DNA profiles in subjects over time.

“The microbial DNA in the intestine is remarkably stable, like a fingerprint,” said George Weinstock, associate director of The Genome Institute at Washington University. “Even after a year, we could still distinguish individuals by the genetic signature of their microbial DNA.”

It’s not immediately obvious how this discovery could be incorporated into a crime story. But, if you can do it, then consider yourself on the cutting edge.

A more apparent application of bacterial DNA profiling to mystery stories is found in the discoveries of Noah Fierer and his colleagues at the University of Colorado in Boulder. They found that a typical human hand shelters about 150 species of bacteria. The types of bacteria living on skin vary greatly. In a 2008 study, they identified more than 4,700 different bacteria species living on the hands of 51 people. Yet only five species lived on the skin of every participant of the study.

In 2010, the researchers reported that computer users leave DNA traces of bacteria on computer mice and keyboards. These DNA traces more closely match the DNA of bacterial colonies that inhabit the hands of the individual who used the computer, compared with bacterial DNA traces of randomly selected people. The researchers obtained useful samples of bacterial DNA two weeks after a person touched computer equipment.

Hunting Burglars with DNA Tests

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Source: Office of Biological and Environmental Research of the U.S. Department of Energy Office of Science. science.energy.gov/ber.

Forensic DNA analysis is usually cast as an important tool for the investigation of a violent crime. However, investigators also use the technology to solve property thefts.

The September-October 2012 issue of Evidence Technology Magazine features an article by Joseph Blozis on “Using DNA to Fight Property Crime.” He explains how New York City’s Biotracks program demonstrated this new use of DNA testing. The NYPD recovered DNA samples from burglary cases with no suspects and forwarded the samples to private companies for analysis. The Office of the Chief Medical Examiner Biological Laboratory then uploaded DNA profiles to the FBI’s CODIS system. Eventually, the efforts led to the identification of burglars in no-suspect cases and established links between apparently unrelated burglaries.

The successes of the NYPD program and similar programs in other jurisdictions have proven the value of DNA testing to solve property crimes. Finding the funds and staff to run extra DNA tests is another challenge.

Tricks for Obtaining DNA

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In the Law & Order: LA episode, “Ballona Creek,” Detective Tomas Jaruszalski picks up a suspect’s discarded cigarette butt for DNA analysis. This type of surreptitious DNA collection is so common in fiction, that the practice doesn’t even merit a raised eyebrow. In real life, surreptitious DNA collection sparks protests from members of the public. At the same time, law enforcement agencies argue that the practice is justified, because an individual has no privacy interest in an abandoned DNA sample.

Instead of retrieving an abandoned item for DNA, law enforcement officials sometimes trick a suspect into giving up a DNA sample. In a case reported in 2009, police sent a letter about a class action lawsuit to a suspect. Eager to join the case, the suspect signed a form and mailed it in a self-addressed envelope. Investigators extracted DNA from the saliva on the envelope flap and found a match to a semen sample gathered from a 1982 rape-killing.

More recently, ABC News reported a recent success for a case concerning the 1976 killing a 70-year-old woman. During a recent review of the cold case, investigators obtained DNA from the old evidence.

The police had a suspect, now they needed the suspect’s DNA to check against the evidence DNA. So, they used a phony “gum chewing survey” and got their saliva sample from the suspect. The results led to an arrest.

An Expert’s View on DNA Evidence

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NPR recently posted an interesting story entitled “Analysing The Evidence On DNA.” Ira Flatow interviewed Greg Hampikian, Director of the Idaho Innocence Project, about the uses and abuses of DNA evidence. The topics that they discussed include:

  • problems with DNA evidence collection,
  • advantages and disadvantages of the high sensitivity of modern DNA analysis,
  • use of DNA evidence in investigations, and
  • myths about evidence analysis perpetuated by TV shows.

The interview is definitely worth a look (or a listen).