by Elizabeth Preston
If you’re unfortunate enough to be bitten by another person, and if that bite becomes infected, it’s likely you’ll find Prevotella in the wound. These bacteria live on our teeth, among other parts of the body. What may be more noteworthy, though, is their role after we bite, chew and swallow our food. Prevotella may be a key to understanding the human gut microbiome.
Prevotella bacteria are anaerobes, thriving in airless crevices of our bodies. The rod-shaped bugs sometimes seize opportunities to create harmful infections, such as around the teeth or in the gums or lungs. It’s a genus well known to dentists.
Farther south in the landscape of a human body is a more populous ecosystem: the gut. Prevotella bacteria are among the life forms that sometimes inhabit it. Our own view into this environment is still distant and obscured. But Prevotella, like a cactus or polar bear glimpsed from afar, may provide a hint as to what type of ecosystem a particular person’s gut contains.
To begin to get a clearer view of how the gut microbiome works, scientists have sampled the bugs living in different groups of people and compared them. There’s no easy way to do a bacterial census inside a person’s body, though. Scientists have to study gut microbes by collecting human feces and sequencing the bacterial genes they find traveling inside it.
One such study compared the intestinal bacteria of several American kids in affluent West-Coast suburbs to those of Bangladeshi kids in an urban slum. In the guts of the Bangladeshi children, researchers found greater biodiversity, including large populations of Prevotella bacteria. The American children—whose diets included more animal protein than Bangladeshis’ did—lacked Prevotella and carried more bacteria from the genus Bacteroides.
A similar study looked at gut microbes in children from Italy and from a small village in Burkina Faso. In the guts of the mostly vegetarian Africans, Prevotella bacteria were abundant. European kids were missing Prevotella; instead, they had beefed-up communities of Bacteroides and the phylum Firmicutes.
Other studies have looked at larger, broader samples of adults and concluded that these differences aren’t trivial: they may represent categories that define us and are critical to our health.
Researchers call these categories “enterotypes.” Studies have found two or three enterotypes: one defined by an abundance of Prevotella, another by Bacteroides, and perhaps a third that’s characterized by the genus Ruminococcus.
Gary Wu, a gastroenterologist at the University of Pennsylvania’s Perelman School of Medicine who has studied enterotypes, says the subject is controversial. Some research groups, looking at their censuses of bacteria in human feces, see people falling into discrete categories. Others see people sitting along gradients of bacterial populations. Wu says much of this interpretation depends on the mathematical tools that researchers use; different people looking at one dataset might find multiple ways to describe it.
One thing seems clear, though. Prevotella and Bacteroides don’t tend to share their turf. “For whatever reason, bugs that belong to these two genera don’t seem to coexist very well together in the same environment,” Wu says. “That being the gut.”
Wu’s own research found the Prevotella enterotype in people with more plant-based diets, and Bacteroides in people who ate more animal proteins. This would agree with the suggestion of the studies comparing Bangladeshi or rural African kids to kids with a Western diet. While food may affect what kind of bacterial community exists in a person’s gut, though, Wu thinks other factors such as age and location could be important too. “These are very early days,” he says.
Prevotella itself isn’t telling researchers much. Wu says little is known about how the genus functions inside humans. For now, it’s only a kind of marker: its presence, like the glimpsed cactus, signals that a person’s digestive tract holds one type of ecosystem and not another.
As scientists get their bearings in the gut microbiome, new findings may help us understand human health. Are certain bacterial ecosystems, for example, tied to disease? Are others tied to a lack of disease? If so, in which direction do cause and effect travel? Most importantly for doctors, can we modify our gut communities to improve health? It would be only fitting for humans, having arrived at a new frontier, to begin altering the landscape to suit our needs.
About the Author
Elizabeth Preston is the editor of Muse, a magazine about science and ideas for kids, and writer of Inkfish, a blog about science and cephalopods for everyone. Her writing has also appeared in National Geographic and at ScienceNOW. Say hello on Twitter at @InkfishEP.