Most of us don’t think of ourselves as bags of microbes. But according to UBC microbiologist Brett Finlay, a person carries 10 times as many microbial cells as human cells. And there are 100 times as many genes encoded in microbes on people’s bodies as there are human genes in the body.

In total, he says in an interview in his office at the Michael Smith Laboratories at UBC’s Point Grey campus, we could each be carrying a kilogram of bugs as part of our body mass. Finlay suggests that this is the equivalent of an organ. And in a single gram of human feces, there are more microbes than the entire population of the world.

“Here’s another one—there’s 100 trillion bacteria for every time you take a dump,” Finlay adds with a laugh.

But it’s no laughing matter how these organisms interact with the body to either prevent or promote disease. Researchers in his lab have discovered that thin mice get fat when injected with a fat mouse’s feces. And if you put thin mice feces in a fat mouse, it loses weight.

“You’re not changing the mouse,” Finlay, a UBC Peter Wall distinguished professor, says. “You’re changing the fecal content. So there’s a realization that these bugs can impact, for example, the weight of an organism.”

That prompts a joke about what might happen if a Hollywood star’s feces was transferred into an obese person.

Earlier this year, the New England Journal of Medicine published a paper showing tremendous success in treating recurrent Clostridium difficile (commonly known as C. difficile) infection by transferring a healthy person’s feces into patients. It was deemed far more effective than the use of the antibiotic vancomycin. “The data is just overwhelming,” Finlay declares.

It used to be extremely difficult to study microbes in the gut because they survive by interacting with one another and die when exposed to oxygen. However, Finlay says, in the past five years DNA sequencing techniques have enabled researchers to make tremendous gains.

Finlay’s experiments have demonstrated that the use of antibiotics in mice to change their microbiota can have an impact on asthma. (The word microbiota refers to living organisms in a specific location.) Fecal transfers between mice have also dramatically affected the level of infectious diarrhea.

Finlay also says that children born via cesarean section have a 20 percent greater chance of developing asthma. That’s because the infant isn’t exposed to vaginal microbiota during birth that could ward off this condition.

Taking antibiotics in the first year of life can change an infant’s microbiota, he says, killing microbes and making the child more susceptible to asthma and other diseases in the future. “And we know that kids living on farms—where it’s dirty and they get more bugs—have less asthma than nice city slickers,” Finlay adds.

At this point, Finlay brings up a graph on his office computer showing that over the past 50 years, there’s been a sharp drop in developed countries in the incidence of hepatitis A, rheumatic fever, mumps, measles, and tuberculosis. But over the same period, there’s been a significant increase in Type 1 diabetes, Crohn’s disease, multiple sclerosis, and asthma in developed countries.

“The question is, how come?” he asks. “We haven’t changed genetically in 50 years.”

That’s where the hygiene hypothesis comes into play. The hygiene hypothesis suggests that if people aren’t exposed to certain microbes early in life or the bacteria in their gut are killed by antibiotics, they have a greater chance of developing Crohn’s disease, multiple sclerosis, Type 1 diabetes, asthma, and possibly even autism. That’s because changes in microbiota have been associated with inflammation of the gut, effectiveness of the immune system, and the release of molecules affecting the development of the brain.

“Can you link IQ [intelligence quotient] to bugs?” Finlay asks. “That’s one of the crazy ideas out there.…If you believe that microbes are involved in brain development, you can see how that can be linked.”

He then tells a surprising story about a woman whose son had autism. She persuaded a physician to prescribe vancomycin to change the child’s microbiota, and the autism disappeared. But it returned when he went off the drug. The experiment was resumed, and the autism went away again. And a fecal transfer had a lasting effect so that more than a decade later, the young man works in a microbiology lab. Finlay wouldn’t identify him, saying he’s only been granted permission to tell the story.

“When you see smoking guns like that, I’m sure a subset of cases of autism are due to microbiota,” he says.

However, Finlay emphasizes that researchers still haven’t figured out which microbes are responsible for increasing or reducing the likelihood of autoimmune diseases or neurological disorders. That will likely be the subject of intensive investigation in the coming years.

“The field is really rich with associations,” he says. “What it’s not rich with is causations.”

Brett Finlay will give the Peter Wall Institute for Advanced Studies spring 2013 Wall Exchange lecture on Tuesday (May 21) at the Vogue Theatre. Free tickets can be reserved at the PWIAS website.