We’ve been hearing a lot lately about the human microbiome — the bacteria, fungi, and viruses that live in various parts of our bodies. This is not occurring because we suddenly have become more infested with germs. Rather, it’s because we now have a better way to study these bacteria and viruses.
In the past, researchers would take a piece of tissue, pus, or blood and grow it on a petri dish with certain nutrients for a defined amount of time, and then look to see what grew. (The same thing is done when a doctor cultures your throat to see if, for example, you have strep.) Now, scientists can study all of the bugs and viruses in our bodies by mapping out all the DNA and RNA in a tissue or blood sample. Next they subtract all the human DNA and RNA from the list. What’s left are the non-human viruses and bacteria harbored in our bodies. Much to everyone’s surprise, there is way more bacteria in our bodies than we had thought. Microorganisms make up about one to three percent of the body. In a 200-pound adult, that’s between two and six pounds of bacteria!
This discovery has led us to new research in the breast — yes, the breast has its own microbiome! It has also pushed scientists to investigate how the microbiome interacts with and affects the immune system as well as drugs that we use to treat cancer. The November 27, 2015, issue of Science includes two great examples.
Both of the studies were done in mice. The first compared melanoma tumor growth in mice that had different patterns of bacteria in the colon. When the mice were kept separate, a difference was seen in tumor growth. (The mice with one of the bacteria patterns had their tumors grow more quickly.) But when they put the mice together in the same cage, exposing them to one another’s fecal bacteria, or transplanted some feces from one mouse into another, the differences disappeared.
The researchers also found that one particular bacteria called bifidobacterium was associated with antitumor effects! In fact, when they gave the bacteria to the mice by mouth, it was just as effective as the new immunotherapy drugs called PDL-1 checkpoint inhibitors. And when they gave the immunotherapy drug with the bacteria, it was even more effective. This suggests that we may want to rethink our strategies about avoiding germs, or at least some of them!
The second study found an immunotherapy did not work in mice that were germ free or had been treated with antibiotics. However, once the mice were given a bacteria called B fragilis, either by mouth or through a human fecal transplant, the immunotherapy worked great!
While this work was in mice, it certainly suggests that in humans our bacteria likely affect how well drugs work. It also suggests there may be ways to introduce certain bacteria that could make cancer therapies more effective. It’s an exciting new frontier. Stay tuned!