The Critters Change After You're Cut

Bacteria after circumcision changes in ways that might reduce HIV risk

/ Author:  / Reviewed by: Robert Carlson, M.D

(RxWiki News) One of the decisions parents of boys must make is whether to circumcise their sons. But uncircumcised men can also decide to become circumcised if they wish.

The reason a grown man might decide to get circumcised could be to reduce his risk of HIV, HPV, herpes or possibly other sexually transmitted diseases. Past studies have shown that circumcision can reduce the risk of HIV by 50 to 60 percent.

A recent study aimed to find out possible reasons why circumcision might reduce a man's risk of HIV.

The researchers found that it might relate to changes in the bacteria present on a man's penis before and after circumcision.

"Discuss circumcision risks and benefits with your doctor."

The study, led by Cindy M. Liu, MD, of the Division of Pathogen Genomics at the Translational Genomics Research Institute in Flagstaff, Arizona, aimed to find out more about the effect of circumcision on the bacteria of men's penises.

The researchers compared the penile "microbiome" of 156 HIV-negative and uncircumcised men, aged 15 to 49, living in Uganda.

The penile microbiome is the composition of different types of bacteria living on the tip of a man's penis. All parts of the human body contain a variety of bacteria, some of which is helpful, some of which can be harmful and much of which has no effect on humans.

After using a cotton swab to collect and study the bacteria present for each man, 79 of the men were circumcised. The other 77 men were scheduled to be circumcised two years later.

This study was part of a larger trial looking at the effectiveness of circumcision for reducing HIV transmission. Circumcision has been shown to reduce HIV and the transmission of several other sexually transmitted infections, such as herpes and HPV (human papillomavirus), in both men and in their partners.

At the start of this trial, the men who were not going to be circumcised right away had an average 140,000 bacteria under their foreskin, compared to an average 200,000 bacteria under the foreskins of the men about to be circumcised. These numbers are considered close to each other for a measurement of bacteria.

One year after the 79 men were circumcised, they had 38,000 bacteria on their penis, compared to 57,000 on the uncircumcised men.

It's not clear why the number of bacteria decreased in both groups, but the difference here was enough to be considered significant by the researchers. There was about a 5 percent likelihood that the difference between the two groups was due to chance.

However, the more significant difference is the way the microbiomes changed in each group. There were 15 different types of bacteria that decreased in the circumcised men, and 12 of them were "anaerobic," which means they grow better without oxygen.

Three other anaerobic bacteria did not decrease on the circumcised men, and seven types of aerobic bacteria (which require oxygen to grow) increased on these men.

Five of the aerobic bacteria that increased on the circumcised men also increased on the uncircumcised men. It's possible that environmental or behavioral changes influenced this increase in both groups.

"From an ecological perspective, it's like rolling back a rock and seeing the ecosystem change," said co-author Lance Price in a prepared statement. "You remove the foreskin and you're increasing the amount of oxygen, decreasing the moisture - we're changing the ecosystem."

Overall, after making adjustments for the time that passed and the changes in both groups, the researchers determined that the change in microbiome in the circumcised men was a drop overall in the types and amount of bacteria. They said this decrease includes bacteria that may be responsible for inflammation.

"We're used to thinking about how disrupting the gut microbiome can make someone more susceptible to an infection," Price said in the statement. "Now we think maybe this disturbance [in the penile microbiome] could be a good thing – could have a positive effect."

It is thought that certain types of bacteria that cause inflammation might make HIV transmission more likely in circumcised men. It's possible that having fewer of these bacteria after circumcision is part of the reason men's HIV risk drops after circumcision.

However, the researchers noted that other changes related to the physical change during circumcision or behavior changes related to being in the trial might play a part in reduced HIV risk as well.

This trial involved only adult men who started out as uncircumcised. There is not information in the study to determine whether these sorts of changes exist between infants who were circumcised or not.

The study was published April 16 in the journal mBio. The research was funded by the National Institutes of Health, the Northern Arizona University Technology and Research Initiative Fund, the Cowden Endowment in Microbiology at Northern Arizona University and the Doris Duke Charitable Foundation Clinician Scientist Development Award.

Reviewed by: 
Review Date: 
April 14, 2013
Last Updated:
January 20, 2014