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Stains at crime scenes can sometimes be hard to identify, but the unique combination of bacteria they contain may help.

Large splatters of blood at a crime scene can be self-evident, but investigators sometimes need to work out if tiny stains are significant. 鈥淚f you see something that looks like a trace, you want to know if it鈥檚 important,鈥� says Natasha Arora, of the Zurich Institute of Forensic Medicine in Switzerland.

Luminol spray can reveal blood spots, but it isn鈥檛 a definitive test, as it can cross-react with other substances, including faeces and household bleach.

Now Arora and her colleagues have found that the microbes in small traces of body fluids can persist in a room for at least thirty days. This could lead to new ways to tell if crime scene stains are blood, faeces, or vaginal fluid, for instance, says Arora

Previous work has shown that different parts of the body have distinctive communities of bacteria, viruses and fungi. So Arora鈥檚 team swabbed different body fluids, to see if their microbial mix would still be distinguishable after being exposed to air for a month. 鈥淚f you go to a crime scene and you find traces, they鈥檙e not generally completely fresh,鈥� says Arora.

The team took multiple samples of blood, menstrual blood, semen, vaginal fluid, saliva, and skin, and placed the swabs on a high shelf in a frequently used room in their lab. To identify the bacteria present, they analysed the genetical material of the swabs at the beginning and end of the month.

The ordinary blood sample yielded little usable data, probably because blood doesn鈥檛 normally contain many bacteria. The team also couldn鈥檛 tell the difference between between vaginal fluid and menstrual blood on the basis of microbes. But the other swabs could generally be distinguished, whether the sample was fresh or a month old.

鈥淚鈥檓 not surprised that some bacteria are there, but I am that the specific community that contains the signature are there after thirty days,鈥� says Arora.