Getting to know the real living dead

Someday you too will join the ranks of the living dead. A dead body, you see, teems with life. Waves of bacteria set up shop alongside insects and other decomposers. A few researchers have started to call this community the necrobiome, and by studying it they are piecing together the ecology of death.

Until recently, we knew surprisingly little about which microbes are part of the necrobiome. One research team broke new ground by reporting in September that the microbial community living on dead mice changed in a consistent pattern and could help establish time of death (SN: 11/2/13). Now another group has identified microbes growing inside human cadavers.

For the new study, published October 30 in PLOS ONE, researchers in Texas studied two human cadavers during one important stage of decomposition: bloat. Bloat is what it sounds like. During decay, bacteria growing inside the body produce gases such as hydrogen sulfide (which stinks) and methane (which does not, despite many body-function jokes to the contrary). These gases inflate the cadaver and eventually force fluids out in a purge, or rupture event. The timing of bloat can vary somewhat, but it’s an important stage of decomposition that all forensic scientists and medical examiners recognize.

Two cadavers may not sound like much, but keep in mind that very few people donate their bodies to forensic science. Jennifer Pechal, a forensic scientist at the University of Dayton in Ohio who is conducting her own studies of microbes on human cadavers, says this work is still in its infancy. “Right now we just need to know what’s there,” she says.

Research on microbiomes, which represent all the bacteria in living people, has become sophisticated in recent years thanks to quick and inexpensive genetic sequencing, and these methods are now allowing forensic scientists to get a much better view of the necrobiome. Older methods could detect only the 1 percent or so of bacteria that will grow in a laboratory.

The cadavers in the new study decomposed under natural outdoor conditions at the Southeast Texas Applied Forensic Science facility, one of a handful of U.S. forensic study sites, or body farms. Embriette Hyde of the Baylor College of Medicine in Houston and colleagues cataloged the bacteria growing inside the cadavers before and at the end of the bloat stage. They sampled the mouth and rectum, areas you would expect to be rich in bacteria, and at the end of the experiment they also sampled the intestines, colon and inside the body cavity. (They couldn’t sample internally at the beginning of the experiment because that would open the body cavity and change the environment for bacterial growth.)

Next the team identified the bacteria in these samples by looking for characteristic genetic markers. At the end of the bloat period, anaerobic bacteria, which prefer oxygen-free areas, such as Clostridia were dominant. That makes sense because anaerobic bacteria are known for making bloat gases. A greater variety of bacteria turned up as samples moved from the mouth down the gastrointestinal tract. Mouth scrapings of both bodies showed a shift during bloat toward Firmicutes, which include Clostridia.

But overall, the types of microbes seen on the two cadavers differed, and it’s difficult to put together much of a pattern based on these first two cadavers, sampled at just one or two time points. The researchers write that the data are “best not viewed as points of comparison” but as a start to the much larger dataset that is needed.

Pechal has been studying changes in the microbial community of pig carcasses and human cadavers, mainly looking at what grows on outer surfaces. “Hopefully we’ll start to get a picture,” she says, of what kinds of samples could yield forensically useful information such as time of death. “Do you use the outside, do you use the inside, do you use a combination of both?” People’s internal flora can vary from person to person, but external bacteria are subject to weather and other environmental conditions. “Right now,” Pechal says, “we’re just in this unknown, uncharted, let’s-just-survey-everything territory.”