Do Certain Vaginal Bacteria Hike Women's Risk of Acquiring HIV?

African study found association between seven types of bacteria, higher risk of HIV infection

Certain types of vaginal bacteria were associated with an increased risk of HIV infection among women, a nested case-control study of African women found.

Out of 20 taxa examined via bacterium-specific real-time polymerase chain reaction testing, seven showed significant links between the quantity of these bacteria and the risk of HIV infection, reported R. Scott McClelland, MD, director of the University of Washington (UW) School of Medicine Global Health Clinical Elective Program and associate director of the UW Center for AIDS Research International Core in Seattle, and colleagues, writing in The Lancet Infectious Diseases.

A total of 56% of new HIV infections in Africa in 2015 were in women, the researchers noted. Bacterial vaginosis, an infection resulting from overgrowth of bacteria in the vagina, may be contributing to HIV transmission and the "disproportionate burden" of HIV infection in these women, but the association between specific bacteria that underlie the association between bacterial vaginosis and HIV infection is still "poorly understood," the team explained.

But recent studies have taken advantage of advances in molecular biology, including a recent study that found that young women with "high-diversity vaginal bacterial communities" had a fourfold risk of HIV acquisition compared with women with low-diversity Lactobacillus crispatus-dominated communities.

The researchers performed a nested case-control study of women who acquired HIV infection versus those who did not. Five cohorts of African women included risk groups such as female sex workers, pregnant and postpartum women, and women in serodiscordant relationships. Vaginal swab samples were collected from 87 women with HIV infection and 262 controls. A gene-sequencing technique was used to identify the vaginal bacteria that may be associated with HIV infection, and from this, a subset of 55 cases and 55 controls were selected.

Not surprisingly, the team found, the Shannon Diversity Index, a measurement of bacterial diversity, was significantly higher in cases than in controls (median 1.3 versus 0.7, respectively, P=0.03).

Of the 20 bacteria identified for further analysis, seven had concentration-dependent associations with increased risk of HIV infection:

• Parvimonas species types 1 and 2
• Gemella asaccharolytica
• Mycoplasma hominis
• Leptotrichia/Sneathia
• Eggerthella species type 1
• Megasphaera

"When we see a dose-response effect, it increases our confidence that this is real," the study's senior author, David Fredricks, MD, director of the Infectious Diseases Fellowship Training Program at Fred Hutchinson Cancer Research Center in Seattle, said in a statement. "No matter how we sliced and diced it, we found the same result. That tells us this signal is likely to be true."

Of the seven bacteria, Parvimonas type 1 and Gemella asaccharolytica were found to be the strongest predictors of HIV infection, the team reported.

In an accompanying editorial, JoAnn Passmore, PhD, of the University of Cape Town Medical School in South Africa, and Heather B. Jaspan, MD, of UW, noted that both Parvimonas type 1 and Gemella asaccharolytica are common constituents of bacterial vaginosis, but prior research found an association between the presence of these bacteria and genital inflammation in heterosexual South African women.

"This study ... suggests that both the presence but also the absolute quantity of these particular organisms might be important drivers of inflammation and HIV risk in sub-Saharan African women," the editorial stated. "It remains to be determined whether bacterial vaginosis-associated species such as Parvimonas and Gemella are also important contributors to women-to-men or mother-to-child HIV transmission, or both."

McClelland and colleagues also noted "strong correlations" between many of the 20 bacteria evaluated, which suggested "they might be frequently found together, establishing high-risk bacterial communities." The team also found these results to be consistent across the three risk groups of women: female sex workers, pregnant and postpartum women, and women in serodiscordant relationships.


Because bacterial vaginosis is "an extremely heterogeneous condition," defining bacteria associated with HIV risk in women may lead to "additional specific targets and inform future strategies for HIV prevention research," the researchers added.

Limitations to the data, they said, include the observational nature of the study, meaning that no causal relationship can be inferred, and neither did the study explore how certain bacteria might increase the risk of HIV; in addition, there was a potential for residual confounding.

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