Where Did Your Bacteria Come From?
Transmission patterns of bacteria that colonize the human gut and mouth
Background
We typically associate bacteria with sickness, but we are colonized by many species of bacteria that are essential for our health. Humans have commensal relationships with many of these bacteria, which means that both organisms benefit. Notably, in the large intestine bacteria help us absorb additional nutrients from food and prevent overgrowth of harmful bacteria.
The species and strains (diverse groups within a species) of bacteria that colonize humans can vary between individuals and can change over a person’s lifetime. But how do these bacteria colonize us? Do they come from mothers during gestation, person-to-person transmission or both? Do a person’s bacterial microbiomes in the gut and mouth change over time? This study was designed to answer those questions.
Summary
The researchers were interested in studying strain-sharing between individuals from the same and different populations. That is, they wanted to see how similar the bacteria were in two different people (they always compared pairs). They studied oral bacteria using saliva samples from subjects and gut bacteria using stool samples.
The researchers used metagenomic analyses to compare colonies of bacteria in different subjects. A metagenomic analysis is a test that enables scientists to analyze many species of bacteria simultaneously. A specific sequence of DNA is chosen and then examined in all harvested bacteria. The sequence will vary because there will be many different species and strains of bacteria present in a stool or saliva sample. Viewing the DNA allows the scientists to categorize the bacteria into species and strains and analyses from different subjects can be compared.
The researchers looked at numerous variables in this study to find out more about bacterial transmission. They were particularly interested in rates of strain-sharing between mothers and children. It is assumed that children have a high rate of strain-sharing with their mothers due to gestation and breastfeeding. The researchers did find higher rates of strain-sharing of gut bacteria between mothers and children than between non-related individuals. Additionally, children delivered vaginally had higher rates of strain-sharing with mothers than children born by C-section. They also found that rates of strain-sharing between mothers and children dropped after the child was 1–3 years old.
They were also interested in strain-sharing rates of gut bacteria between subjects that lived together (genetically related or not). They found that subjects who lived together had more similar gut biomes than subjects that lived in the same geographical area, but not in the same house. This shows that gut bacteria do not always have to be transmitted gestationally. Also, older people tend to have more diverse gut biomes, presumably because they have been exposed to more people.
When the researchers examined strain-sharing of oral bacteria between pairs they observed cohabiting partners sharing more strains than mothers and their children. This is most likely due to intimacy between partners and the fact that oral bacteria can be easily transmitted through saliva.
Finally, the researchers detected certain phenotypic patterns in the species of gut bacteria that were commonly shared between pairs. Most of the transmitted bacteria were gram-negative, likely because these bacteria are more resistant to sanitization. Also, most shared bacteria were aerotolerant (can survive in an environment with oxygen) and could form spores. This would give an advantage because spore-forming bacteria can survive long periods of time in harsh conditions. These patterns were not detected in commonly transmitted oral bacteria.
Overall, the data given in the paper suggest that bacteria that colonize our intestinal tract mostly come from our mother during gestation and early years of life. The gut biome can change throughout life due to bacteria transmitted between people living together. Oral bacteria are mostly transmitted between people living together, whether they are related or not. Also, the oral environment changes more frequently than the gut.
