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Scientists may have finally discovered the reason why circumcision drastically lowers a man's chances catching HIV and other sexually transmitted diseases like genital warts and herpes.

According to a new study, the removal of the foreskin protects men against STDs because it deprives harmful bacteria of a place to live and multiply.

The research, published April 16 in the journal mBio, revealed that the surgical procedure significantly alters the microbiome of the penis, thereby cutting the risk of catching HIV or other harmful STDs by at least 50 percent.

Study author Dr. Lance Price and his team at George Washington University wanted to look at the effects of adult male circumcision on the types of bacteria that live under the foreskin before and after the circumcision.

Using swab samples from a large circumcision trial in Uganda, Price and his team compared samples from uncircumcised men with samples from circumcised men that were taken both before the procedure and one year after the procedure.

Price and his team found that the total bacterial load in that area of the penis dropped significantly in participants a year after having undergone the procedure.

Specifically, the prevalence of anaerobic bacteria, which thrive in locations with limited oxygen, declined, while numbers of some aerobic bacteria rose slightly one year post-procedure.

"The change in the communities is really characterized by the loss of anaerobes. It's dramatic," Price said in a journal release.

"From an ecological perspective, it's like rolling back a rock and seeing the ecosystem change. You remove the foreskin and you're increasing the amount of oxygen, decreasing the moisture - we're changing the ecosystem," he explained.

Price said the findings indicate that circumcision had a "dramatic and significant change in penis microbiome".  While the microbiota of both circumcised and uncircumcised men were similar at the beginning of the study, a year after their operation the decline in the bacterial load in circumcised men was significantly greater than uncircumcised men.  Researchers explain nearly all the bacterial groups that declined were strict anaerobes or facultative anaerobes, leaving a reduced biodiversity overall.

"From a public health perspective the findings are really interesting because some of these organisms that are decreasing could cause inflammation," Price explained.

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

Researchers say the next step is to find out exactly how the penis' living conditions affects HIV transmission by studying possible links between changes in the microbiome and cytokine responses, signaling mechanisms that can trigger the immune system.

Researchers say the latest findings have implications beyond circumcision.  For example, understanding changes in the microbiome following surgical operations could eventually lead to interventions that don't require surgery.

"The work that we're doing, by potentially revealing the underlying biological mechanisms, could reveal alternatives to circumcision that would have the same biological impact," Price explained.  "In other words, if we find that it's a group of anaerobes that are increasing the risk for HIV, we can find alternative ways to bring down those anaerobes," and protect HIV infection in all sexually active men, Price concludes.