• What is already known on this topic
    The gut microbiota affect our digestive, immune, and neuroendocrine systems, but how these bacteria are transmitted between individuals is not well-understood.

  • What this research adds
    In mice, gut bacteria are transmitted from mothers to offspring and stay more or less the same over generations. While vertical inheritance is the dominant mode of transmission, some bacterial pathogens are transmitted in a horizontal fashion between non-kin individuals.

  • Conclusions
    The finding suggests that pathogens evolved to be transmitted between individuals rather than being inherited from parents.

Commensal gut microbes are inherited from parents and change very little over generations, whereas pathogens tend to be transmitted between non-kin individuals. That’s according to a study in mice published in
Science and led by Andrew Moeller of the University of California, Berkeley.

The bacteria that reside in the gut are known to influence our digestive, immune, and neuroendocrine functions, but how these microbes are transmitted between people is still unclear. For example, because of the difficulty of monitoring the gut microbiota for several generations, it’s not easy to understand whether gut bacteria are transmitted vertically (from parents to offspring) or horizontally (between non-kin individuals).

To address this question, the researchers captured two populations of wild mice in Arizona and Canada, bred them separately for 5 to 11 generations in the laboratory, and tracked the rodents’ gut microbiota for 3 years.

Inbred lines were housed in the same room but in separate cages, so mice came into direct contact only with their mothers and siblings.

Vertical inheritance is the main mode of gut bacterial transmission

The researchers collected samples of gut content from the mice’s cecum and sequenced 16S ribosomal DNA to characterize the gut microbiota composition.

To assess the relative contributions of vertical and horizontal transmission to the gut microbiota composition, the team calculated the binary Sorensen-Dice coefficient for every pairwise stool sample comparison.

The main mode of gut bacterial transmission was vertical inheritance (from mothers to offspring), and the microbiota composition between different mouse lineages remained distinct for 10 generations.

Gut bacteria can be transmitted horizontally

Although the Arizona and Canada mouse lineages kept distinct gut microbes for several generations, their microbiota became more similar over time.

To test whether the two lineages exchanged gut bacteria, the team compared the microbiota of laboratory-bred Arizona mice with the that of the wild mice from Canada, and vice versa. The results showed that these gut microbiotas diverged initially and then converged over time, suggesting that gut bacteria were transmitted in a horizontal fashion — through the mouse facility or animal handlers.

Transmission mode is associated with bacterial lifestyle

To assess how specific bacteria were transmitted between mice, the researchers calculated for each bacterial genus a binary Sorensen-Dice dissimilarity (BSDD) between mice from the same lineage and from different lineages. The ratio of these BSDD indicated how bacterial genera were transmitted between mice, with a ratio >1 indicating that the bacteria were vertically inherited and a ratio <1 indicating that they were horizontally transmitted.

The results show that Bacilli were mainly transmitted horizontally, whereas Bacteroidia were vertically inherited. Aerobic bacteria often showed ratios indicative of horizontal transmission, while obligate anaerobes didn’t. That’s likely because obligate anaerobes aren’t able to thrive outside the host. Spore-forming bacteria didn’t show any specific transmission mode.

Horizontally transmitted bacteria are most likely pathogenic

Next, to test whether transmission modes are linked to bacterial virulence, the researchers collected data on infections caused by gut bacteria in the United States. Horizontally transmitted bacteria in mice were significantly associated with foodborne and hospital–associated infections as well as hospitalizations from these infections in people.

This suggests that horizontally transmitted bacteria are more likely to exhibit virulence than vertically inherited bacteria.

This long-term, multigenerational mouse study offers new insights into the transmission modes of gut bacteria, suggesting that pathogens evolved to be transmitted between individuals rather than being inherited from parents.