The vagina is home to millions of microorganisms; some are beneficial and make up the majority of species in healthy vaginas, and others, such as bacteria associated with bacterial vaginosis, can be pathogenic. There is a correlation in women between having an abnormal vaginal microbiota and an increased risk of HIV-1 genital shedding. In contrast, subsets of beneficial vaginal lactobacilli that produce the antimicrobial compound hydrogen peroxide (also commonly found in drug stores) have been associated with a reduced risk of HIV shedding. VIDD associate member David Fredricks, VIDD Staff Scientist Jennifer Balkus, and colleagues set out to improve our understanding of methods used to detect vaginal hydrogen peroxide-producing Lactobacillus species among HIV-infected women.
To tackle this question, the authors assayed cervicovaginal lavage (CVL) samples collected from 57 HIV-1 infected US women between the years 2002 and 2007 at the University of Washington and University of Rochester, New York. Two common, but very different, laboratory techniques were used to detect Lactobacillus species: quantitative PCR (qPCR) for the presence or absence of bacterial DNA, and culture to identify which bacterial species grow. Each of these techniques has unique pros and cons; qPCR is highly sensitive for specific bacterial species yet yields only genotypic data that cannot measure hydrogen peroxide production, while culturing allows for a multitude of molecular and biochemical assays, such as hydrogen peroxide levels, but can often miss hard to grow organisms. As detected by culture, Lactobacillus bacteria were present in 82% of samples, of which 76% produced hydrogen peroxide. Two commonly found Lactobacillus species, L. crispatus and/or L. jensenii, were detected in 57% of samples by qPCR and had a concordance rate with hydrogen peroxide production of 75%. To determine which other Lactobacillus species were producing hydrogen peroxide (i.e., the other 25%), the authors performed additional broad-range PCR assays on the samples that were negative for L. crispatus and L. jensenii but had very high levels of hydrogen peroxide producing bacteria. From these fine-tuned studies, L. gasseri was found to be the dominant species (81%) that was likely responsible for the high hydrogen peroxide levels in the absence of the other, more common Lactobacillus species. This paper suggests that future studies of vaginal ecosystems use both qPCR and culture to define the role of these bacteria in protection from mucosal infections.
Balkus JE, Mitchell C, Agnew K, Liu C, Fiedler T, Cohn SE, Luque A, Coombs R, Fredricks DN, Hitti J. Detection of hydrogen peroxide-producing Lactobacillus species in the vagina: a comparison of culture and quantitative PCR among HIV-1 seropositive women. BMC Infect Dis. 2012 Aug 13;12:188.