University of Chicago researchers think the garden of microbes that live on human skin and in the digestive system help determine whether a transplanted organ will be accepted or rejected by the body. Their research was published June 20 in The Journal of Clinical Investigation.

In the study, mice treated with antibiotics before a skin graft survived roughly twice as long as mice who didn't receive the medicine. Mice who were raised in a sterile environment also survived longer following a skin graft.

Researchers also noted that if germ-free mice were covered with microbes from untreated mice, skin grafts were rejected at a faster rate. However, if they received microbes that were left over following antibiotic treatment of conventional mice, the skin grafts they received behaved similarly to those received by the sterile mice.

"The species that form the community of microbes colonizing the mice — and supposedly humans, too — have different effects," said Dr. Maria-Luisa Alegre, professor of medicine at the University of Chicago and co-senior author of the study. "One community of bacteria from the normal mice is capable of inducing accelerated rejection of a transplant, but another community of bacteria, those that are left after antibiotics, doesn't have that capacity."

In humans as well as lab animals, survival rates for skin, lung and intestinal transplants are much worse than those of internal organs like the kidney, liver and heart. The research team wanted to determine whether the difference in outcomes could be attributed to the external microbiota that live on and in these organs.

During the study, mice were treated with antibiotics for 10 days prior to the skin transplants. Grafts in treated mice survived 53 days while the control group's grafts lasted 27 days. Grafts between mice raised in a germ-free environment also survived longer.

To compare, mice from germ-free conditions were given fecal microbiota from conventional mice that weren't given antibiotics. These mice rejected the grafts quicker. However, germ-free mice that received fecal microbiota from conventional mice that were treated with antibiotics rejected the skin grafts at a slower rate.

Researchers say this shows the composition of the microbiota had distinct effects on the transplants' success. The amount of bacteria present on the mice's skin was the same but there were fewer species between the untreated and treated mice.

"In terms of the total number of bacteria that are present, it's the same before and after antibiotics. But instead of 1,000 species, let's say you have just 500 after treatment," Alegre said in a University of Chicago press release. "So it's not the bacterial load that makes the difference, it's the composition of the bacterial community."

Alegre said the information shows there needs to be a better understanding of how microbes influence the body's immune response following a transplanted organ. For example, the development of narrow-spectrum antibiotics that target the bacteria that cause a rejection response or using probiotics with bacteria that suppress immune response could help generate more successful human transplants.

First, though, science will need to determine which types of bacteria are responsible for which types of response, Alegre said.

"We have evolved to cohabit effectively with our microbes, and they are very beneficial," she said. "We need them. They make vitamins we need. They digest foods we can't digest. They help maintain our health by poising our immune system for fighting infections. So we have to be careful with anything that's going to alter that balance."