Circumcision Alters Penile Microbiome, Possibly Protective

Ricki Lewis, PhD

April 16, 2013

Changes to the bacterial communities that reside on the human penis after circumcision may protect against certain viral infections, including HIV, according to a study published online April 16 in mBio.

Male circumcision is associated with a lower risk of contracting and transmitting certain viral infections (HIV, herpes simplex virus 2, and human papillomavirus) and may also protect against certain bacterial infections. The mechanism may be elimination of the moist, oxygen-poor environment of the prepuce, which in turn reduces the burden of bacteria that elicit inflammation, which may draw HIV target cells to the region.

Cindy M. Liu, MD, from the Division of Pathogen Genomics, Translational Genomics Research Institute, and the Center for Microbial Genetics and Genomics, Northern Arizona University, Flagstaff, as well as the Department of Pathology, School of Medicine, Johns Hopkins University, Baltimore, Maryland, and colleagues compared the coronal sulcus microbiota compositions of 77 uncircumcised men with those of 79 circumcised men in Rakai, Uganda. The men were similar in sexually transmitted infection history, symptoms, and sexual practices and were randomly assigned either to receive circumcision or not. Microbiota were similar in the 2 groups before the intervention.

The researchers assessed copies of the ribosomal 16S RNA gene to estimate coronal sulcus bacterial load. The overall bacterial load declined in both groups, going from 2.0 × 105 to 3.8 × 104 copies at the 1-year mark among the circumcised men,, and from 1.4 × 105 to 5.7 × 104 copies in the control group.

The researchers used high-throughput parallel DNA sequencing to distinguish bacterial species. Fifteen bacterial taxa significantly decreased in prevalence during the year in the circumcised men (P < .05), including 12 that are strict anaerobes. Two subspecies of Atopobium remained.

Seven taxa of bacteria became more prevalent among the circumcised men, 5 of which also increased in prevalence in the control group. The aerobe Kocuria and the facultative anaerobe Facklamia increased in prevalence only among the circumcised men.

The researchers conclude that after adjusting for changes over time in the control group, circumcision decreased the total bacterial load and microbiota biodiversity.

Reduction in anaerobic species diversity after circumcision may explain empiric observations that circumcised men are less likely to transmit HIV to their partners, the investigators hypothesize. The mechanism might be a decrease in the types of bacterial antigens that stimulate Langerhans cells, preventing them from presenting HIV particles to CD4+ T cells, and thereby protecting against HIV infection.

A limitation of the study is that it catalogues changes in the microbiota but does not identify mechanisms of infectious disease resistance or susceptibility. The study also does not distinguish between effects on the penile microbiota attributable to altered anatomy versus behavioral changes among trial participants.

The researchers have disclosed no relevant financial relationships.

mBIO. Published online April 16, 2013. Full text

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