Metals found in the hair of corpses have solved all kinds of mysteries. For instance, high levels of arsenic found in Napoleon's hair suggest the former emperor of France might have been poisoned to death, intentionally or unintentionally.

However, scientists now find that bacteria can sprinkle gold dust onto the hair of corpses, which suggests microbes could deposit arsenic and other poisonous metals on bodies as well, potentially complicating criminal and archaeological investigations.

Hair is one of the best preserved human tissues found over the millennia, and as such, analyzing it can not only shed light on crimes, but also on ancient civilizations. For instance, arsenic poisoning of the Chinchorro people of Chile from contaminated water, as revealed via the hair of 6,800-year-old mummified infants, probably led to high child mortality rates. This in turn may have led grief-stricken parents to make some of the earliest known intentionally created mummies to preserve their dead offspring.

One concern with such analyses, however, is that microorganisms might creep in and deposit metals onto a corpse's hair, tainting it. Microbes naturally found in the soil often concentrate and disperse metals to neutralize the deadly effects such metals might have on the germs.

"People have been convicted and reprieved based on the reliability or unreliability of hair analysis, so one wants to learn as much about it as possible," said researcher Otto Appenzeller at the New Mexico Health Enhancement and Marathon Clinics Research Foundation in Albuquerque.

Appenzeller and his colleagues incubated samples of Appenzeller's hair for up to six months in soil from an Australian gold mine. In some experiments, this soil contained the bacterium Cupriavidus metallidurans, which thrives in environments loaded with heavy metals and can help form grains of gold.

The researchers found gold levels did not increase in the hair to any statistically significant degree when it was incubated without the microbes in naturally gold-laden soils. However, gold levels rose dramatically in the hair if the bacterium was there.

"Instead of just anecdotal stories that bacteria might contaminate hair with metals, we now have experimental proof," Appenzeller said.

The fact that bacteria can deposit gold in hair suggests they can leave other metals there as well.

"There are more than 100 different bacteria in the soil that are resistant to arsenic, and really the only way they can do that is by depositing it into biofilms, substances they secrete that can sequester the poison within," Appenzeller said.

When investigating the hair of corpses in the future, the researchers suggest analyzing them for signs of germs, such as microscope scans to look for biofilms or genetic probes that can detect bacterial genes. One might then be able to remove such contamination with bleaches that strip off the outer cuticles of hairs, and then analyze the hair, which should reflect what the person's metabolism was truly like, as well as what poisons might have killed them.

Appenzeller and his colleagues detailed their findings online Feb. 19 in the journal PLoS ONE.