March 01, 2023

Study reveals exchange of microbiome bacteria could increase risk of disease

The results suggest that some diseases that are considered non-communicable, such as cancer, could have a component that makes them transmissible by microorganisms in the digestive system.

Here’s an unsettling fact: there are more bacterial cells in a person’s body (38 trillion) than human cells (30 trillion). One of the world’s gurus of microbiology, Frederic Bushman, goes as far as to suggest that we stop seeing the human being as an individual organism and think of it as a coral reef inhabited by billions of other beings instead.

Now, a macro-study has revealed that these tiny tenants jump from one person to another in vast amounts: two cohabitants share 12% of the strains in their intestines and up to 32% of those in their mouths, including bacteria associated with problems such as cancer, obesity, diabetes, and cardiovascular diseases, according to the main author of the research, Spanish microbiologist Mireia Vallès. Even some diseases that used to be considered non-communicable have a contagious component. “It is a paradigm shift,” says Vallès.

The new study analyzed stool and saliva samples from approximately 5,000 people from 20 countries on five continents. The results confirm that social interactions determine the composition of the so-called microbiome, even in the gut. A mother shares 34% of her intestine’s bacterial strains with her young children. Two people who live together, 12%. Two twins who live in different houses, 8%, and two independent adults from the same city, also 8%. Inherited microorganisms fade away after childbirth: it is a coexistence that shapes the microbiome. “The percentage that an adult shares with their mother is equal to the percentage they share with people they live with or with work colleagues,” says Vallès, from the University of Trento, in Italy.

Three out of every four deaths in the world are caused by non-communicable diseases such as cancer, heart attacks, strokes, diabetes, and asthma. More than 40 million people die every year from these causes, according to the World Health Organization. Three years ago, Canadian microbiologist Brett Finlay suggested a provocative hypothesis: “Are noncommunicable diseases communicable?” he asked in Science magazine. Finlay argued that factors such as junk food, smoking, and alcohol consumption cause an imbalance in the microbiome that can influence noncommunicable diseases, or their risk factors, such as obesity. According to the scientist, that altered microbiome could be transmitted from person to person, potentially contributing to the spread of disease.

800 species of bacteria

The new study points in the same direction. “Our results reinforce the hypothesis that several diseases and conditions that are currently considered non-communicable should be reevaluated,” state the authors of the paper, published in the scientific journal Nature. This is the largest investigation into the transmission of the human microbiome conducted to date.

The scientists looked at more than 800 species of bacteria, identifying each person’s ultra-specific strains. The study confirms that the oral microbiome is transmitted differently than the one in the depths of the digestive tract. “By the oral route, the vehicle is saliva, but we still don’t know the specific mechanism of the intestinal one. It could be due to a lack of hygiene or a fecal-oral transmission that later reaches the intestine, but it is not clear,” acknowledges Vallès.

Researchers from 10 countries participated in the macro-study, including agronomist María Carmen Collado from the Institute of Agrochemistry and Food Technology of the Spanish National Research Council in Paterna, Spain. Collado is a specialist in what is known as the vertical transmission of the microbiome (from mother to children). “Now we see that horizontal transmission, from person to person, is very important, much more than was initially thought,” she says.

The microbiome is in the spotlight of the scientific community these days. Three months ago, a team from Yale University announced that a common intestinal microbe was suspected of playing an important role in the development of colorectal cancer, the second deadliest in the world. Some strains of this bacterium, called Morganella morganii, produce molecules that are toxic to human DNA and which, when injected into mice, cause tumors. Another common bacterium in the stomach, Helicobacter pylori, is associated with an increased risk of gastric cancer.

Vallès’s team detected some microbes that are transmitted more frequently than others. Many have not even been named yet, despite their apparent importance. “What surprises us is that there are some bacteria that we know very little about, that have never been cultured, and that are at the top of the list [of shared microbes],” she warns. For the study, her group used metagenomics, the large-scale analysis of all the genetic material present in stool and saliva samples.

Finlay, a pioneer of the hypothesis, applauds the new research (in which he did not participate), describing it as a fascinating study that draws from his own proposal: that noncommunicable diseases are communicable through the microbiome. The results of the study open the door to a reconsideration of the current policies against noncommunicable diseases, which today account for the largest share of morbidity and mortality worldwide.