Women whose vaginal lactobacilli are hydrogen peroxide (H2O2) producers are at less risk of reproductive tract infections
than women whose lactobacilli are non-producers. This epidemiological correlation, together with in vitro observations
of H2O2 producers inactivating pathogens, has made H2O2 production a prime criterion for selecting lactobacilli strains to be
developed as vaginal probiotics.
However, epidemiological correlation does not imply causation, and the in vitro observations of pathogen inactivation were
made under aerobic conditions, in the absence of antioxidants, and disregarded activation of lactobacilli themselves. Whereas in
1) Lactobacilli do not produce H2O2 under the hypoxic (low oxygen) conditions that usually prevail in the vagina. 2) The
high antioxidant capacity of cervicovaginal fluid (CVF) means that little H2O2 accumulates even under aerobic conditions. The
low concentration of H2O2 that does accumulate in CVF does not inactivate pathogens. 3) When enough exogenous H2O2 is
supplied to inactivate pathogens, vaginal lactobacilli are also inactivated.
We conclude that H2O2 production by vaginal lactobacilli is implausible as a mechanism of protection against reproductive
tract infections, and therefore an unsound basis for probiotic selection. We hypothesize, instead, that H2O2 production correlates
with another characteristic that does provide in vivo protection. Alternatively, H2O2 producers may be more sensitive to the
presence of some pathogens; in this case, H2O2 producers would correlate with the absence of these pathogens, without implying
that H2O2 producers protect against those pathogens.
Deirdre Elizabeth O?Hanlon is from the Republic of Ireland. She received her B.S. in biochemistry from Wells College, Aurora NY, and her Ph.D. in
biology from Johns Hopkins University, Baltimore MD in the United States. Her doctoral research focused on the different parameters for pathogen
inactivation by vaginal lactobacilli in vivo versus in vitro. She is currently continuing her research in the university?s Department of Biophysics, looking
at probiotic development and other means of supplementing women?s mucosal and microbiotal protection against reproductive tract infections. She
is the author or co-author of four peer reviewed articles publishing her findings.
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