The communities of bacteria that live in most mammals guts are mostly passed down from mothers to their offspring, rather than traded among neighbors or picked up from the environment, suggests a new study in mice. But a few exceptions may suggest something interesting about the evolution of some human pathogens.
Bacteria living on and in your body outnumber your actual human cells several many times over. In your digestive tract, those invisible ecosystems play important roles not only in digestion but also in the immune system and the endocrine system, influencing the hormones that help regulate bodily functions. Those bacteria have to come from somewhere, whether theyre inherited in a process called vertical transmission in the womb or during childbirth or whether we pick them up from the environment or through contact with other people.
Fortunately for science, other animal species also carry whole ecosystems around in their digestive tracts, and mice reproduce much faster than humans (and are much more amenable to being kept in a carefully controlled laboratory environment for several generations). University of California Berkeley biologist Andrew Moeller and his colleagues spent three years breeding 17 separate lineages of mice in their laboratory (think of each lineage as a family tree with no branches). Some descended from mice caught in Arizona, while others traced their roots back to Alberta, Canada.
The mice didnt interact or share cages with non-relatives, but Moeller and his colleagues say there were plenty of chances for bacteria to pass between them through the laboratory environment or on the hands of their human handlers. Even so, three years and 10 generations later, Moeller and his colleagues could still tell the difference between descendants of Arizona mice and Canadian mice just by looking at their gut microbes.
That doesnt mean there was no change at all. Moeller and his colleagues sampled gut contents from mice in each generation and sequenced genetic material from the sample. That added up to 212 tiny samples of partially digested mouse food in three years, if anyones counting. The biologists compared each sample of genetic sequences with samples from mice from other lineages in order to see what percent of bacterial DNA sequences turned up in both samples. Although the researchers could still tell the mouse lineages apart based on their gut microbes, they noticed that later generations of mice had more DNA sequences in common with mice they werent related to than their ancestors had.
That could just have been because all the gut-microbe communities were evolving in response to the same environment: the laboratory, which could favor the same bacteria in different mice and could shape the communities into something more similar over time. But it could also be a sign that at least some gut bacteria are contagious.
The researchers sorted the DNA sequences from the gut samples by genus, then they took a closer look at which ones seemed to be spreading between mouse lineages. Most of the bacteria tended to stick to the same mouse lineage, transmitted from mothers to offspring—like the genus Bacteroidia and the genus Clostridia. But a small minority of others, most notably the genus Bacillus, tended to spread between lineages, either through the environment or through contact with human handlers.
The bacteria that spread horizontally between mice tended to be related to species that cause sickness in humans. Moeller and his colleagues combined their results with epidemiological data about foodborne and hospital-acquired bacterial infections in the US. They found that the numbers of infections and hospitalizations a genus caused in humans correlated with how it spread in mice. That doesn't mean that horizontally transmitted gut bacteria are necessarily dangerous, but it says something interesting about how they've adapted to spread to new hosts.
“Bacterial pathogens in humans belong to genera that appear to be adapted for transmission through the indoor environment,” Moeller and his colleagues wrote. The bacteria that succeed as pathogens tend to be generalists, capable of surviving either in a host or in the hospitable environment outside a hosts body. And oxygen might be part of the secret to that success.
Moeller and his colleagues noticed that obligate anaerobes, bacteria that can only survive in an oxygen-free environment, tended to pass from mouse mothers to their offspring but not between other mice through the environment. Aerobic bacteria, those that need oxygen to live, were much more prone to horizontal transmission and much less dependent on vertical. That suggests that the ability to tolerate oxygen makes a difference in whether bacteria can linger in the environment in hopes of being picked up by a new host.
The results likely also apply to humans and other mammals, so Moeller's mice may shed some light on our own internal ecology.