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Penis bacteria linked to HIV acquisition

, by Emily Newman

bacteriaThree large clinical trials—involving thousands of men in Africa—have shown that male circumcision reduces risk of HIV acquisition. The protection afforded by circumcision is high, with efficacy estimates ranging between 50% and 60%. Some researchers estimate that circumcision helps prevent HIV infection for the male partner simply because it decreases the surface area of skin potentially exposed to HIV infection during sex. Other researchers have a different theory, and it involves the bacteria found on our skin.

At the Conference at Retroviruses and Opportunistic Infections (CROI) held last month, Lance Price, from George Washington University presented compelling research supporting a theory linking the penile microbiome (the colonies of bacteria found on the penis) to HIV risk.

“It’s likely that the bacteria living on our genitalia can affect our HIV risk. Bacteria, interacting with our immune system, can make us more or less susceptible to HIV,” he explained.

The theory is as follows: Men that are uncircumcised tend to have more colonies of bacteria that produce inflammation. Men who are circumcised are less likely to have these types of anaerobic bacteria colonizing the penis. Inflammation, caused by anaerobic bacteria, causes CD4 T-cells to be recruited to the site of inflammation, which then become vulnerable to HIV infection when the virus is present.

Lance showed research comparing the microbiomes between men enrolled in a male circumcision study in Uganda. The researchers compared the types and abundance of penile bacteria among 77 uncircumcised men and 79 circumcised men at baseline between the two groups, and then a year later (one year after people in the circumcision group were circumcised).

In people who were circumcised, the researchers saw a shift in the types of bacteria present—specifically, a significant decrease in total bacterial load, microbiota diversity and microbiota composition. Specifically, the prevalence and absolute abundance of 12 anaerobic bacteria, including anaerobic bacteria like Prevotella and Porphyromonas, were reduced in circumcised men. Circumcised men also had an increase in normal skin bacteria.

Plot of microbiome communities

Non multi-dimensional scaling plot of microbiome data from two groups at baseline and one year later: the Control group (uncircumcised) and the Intervention group (circumcised after baseline). In the plot on the bottom, you can see how the microbial communities change dramatically after circumcision. (Slide: Lance Price)















“It’s pretty intuitive that if you change the environment, the organisms that live there are going to change as well. Here, we removed the foreskin, we decrease anaerobic micro environments, and we make it hard for the anaerobes to live,” said Lance.

Lance then explained a case-controlled study of anaerobic bacteria, cytokines (signaling proteins involved in inflammation), and HIV seroconversion. This study, he explained, aimed to determine if there was any association between the types of penile bacteria present, markers of inflammation, and HIV seroconversion. Samples were taken from participants in the two-year male circumcision study conducted in Uganda. Participants were people who seroconverted during the study, each matched to two controls also enrolled in the study.

There were eight penile anaerobes that were significantly associated with the concentration of the inflammatory marker IL-8 (including Prevotella, Dialister, and Porphyromonas). In addition, the odds of HIV seroconversion increased, by 30% to 60%, in the presence of some of these anaerobes, particularly Prevotella and Dialister.

Lance explained that this state, called “anaerobic dysbiosis,” which causes an increase in HIV susceptibility, is largely resolved in response to circumcision. Other factors, like increased washing, may also affect the presence and type of bacteria, he explained during the question and answer portion of the talk.


Abundance of penile anaerobes, IL-8, and the risk of HIV acquisition, Rakai, Uganda. CROI, 2017. Watch the webcast.


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