Scientists have identified seven bacterial species whose presence in high concentrations may significantly increase the risk of HIV infection in women.
The findings, published online in the journal Lancet Infectious Diseases, add strength to a growing body of evidence that the make-up of bacterial communities in the vagina -- the vaginal microbiome -- may increase or decrease HIV risk for women, depending on which bacteria are there.
The results are based on data collected for years by AIDS researchers in six African nations.
The research could lead to a better understanding of how biological conditions may promote infection by the virus that can lead to acquired immune deficiency syndrome, or AIDS, and perhaps offer targets for future prevention research, said senior author of the study David Fredricks, a physician-scientist at Fred Hutchinson Cancer Research Center in Seattle, Washington.
These clues are particularly important in sub-Saharan Africa, where women account for 56 per cent of new HIV infections.
Of seven bacterial species linked to higher risk, the most conspicuous of them is Parvimonas Type 1, an otherwise common bug not considered particularly worrisome.
But the researchers found that women carrying high concentrations of that bug had much higher odds -- 4.6 to one -- of acquiring HIV than those who did not.
The study also showed that the odds of HIV infection increased as concentrations of that bacterium increased.
Biologists call this a "dose-dependent response" -- the more bugs, the more risk of HIV infection. Three other bacterial species had a similar dose response.
"When we see dose-response effect, it increases our confidence that this is real," Fredricks said.
Gathering the information required a collaboration involving five of the largest studies of HIV risk carried out in Africa.
It involved intensive molecular testing of swab samples from 87 women who had acquired HIV infection and of 262 who did not, and sophisticated analysis comparing the vaginal microbial communities in each of those groups.
The results were consistent throughout Africa, and among three distinctly different groups of women at high risk of HIV infection: Pregnant women, sex workers and women who were initially HIV negative but had HIV-positive male partners.
"What this means is that these results are generalisable to many women, at least in sub-Saharan Africa," said Fredricks.