The concept of pH as a measure of hydrogen ion concentration, and thus the acidity or alkalinity of a solution, was introduced by the Danish biochemist, Sorensen, in 1899. The term pH may be derived from the Latin, pondus hydrogeni; the French potential hydrogene; or the British potential of hydrogen. Examples of pH values of various solutions are shown in Table 1 .

Vaginal pH has been an enduring topic of interest in the areas of vaginal physiology and microbiology. Two historical factors combined to create this focus. First, the early beginnings of scientific research on vaginal physiology were in the antiseptic era of the early 20th century, so it was natural to presume that some antiseptic substance regulated the vaginal microflora. Second, the new availability of pH paper evoked an interest in pH testing of clinical specimens such as vaginal secretions. Scizly and Friedenthal, as described by Szabadvary, were the first to introduce colorimetric determination of pH with test papers in 1903. Early researchers presumed lactic acid produced by Lactobacilli w as the likely antiseptic. It was theorized that the mildly acidic vaginal pH of around 4.5 provided this regulatory action.

At the same time, vaginal flora was being studied through microscopic examination, and the early work of Doderlein led to the belief that vaginal flora associated with asymptomatic reproductive-age women was composed of a single microbial entity, which became eponymically known as Doderlein’s bacillus. A corollary of the microscopic observations of Doderlein was that a pure vaginal culture of large rods (with the morphology of Lactobacillus ) indicated a healthy vagina and one that contained a mixed flora or in which Lactobacillus morphotypes were absent was unhealthy.

This observation was prophetic because in recent decades a morphologic scoring system (Nugent scoring) based on Lactobacilli morphotypes for vaginal flora has emerged, and its correlation with vaginal pH and abnormal flora has been widely reported. In essence, the Reinheitsgrad of Doderlein became the Nugent score of the contemporary practitioner.

The science of microbiology at its early stage had not evolved to the level at which the full extent of normal vaginal colonization with other acid-producing microbes was appreciated. Because the earliest view of vaginal flora was based simply on the microscopic observation of vaginal secretions, asymptomatic women were deemed to be populated with a single organism, based on Gram stain smears. The concept that vaginal health was due to a pure culture of Lactobacillus was intellectually appealing and logical and became firmly entrenched in the minds of physicians.

Microbiology research, particularly when the ability to grow bacteria on artificial media and isolate different species became possible, subsequently identified multiple species coexisting as components of healthy vaginal flora. Anaerobic culture technology provided the next innovation and revealed that previously unknown anaerobic organisms were also present in the healthy vagina.

The past decade has witnessed another expansion in vaginal flora microbiology as gene amplification methods to detect microbial DNA have identified difficult-to-culture organisms as components of the flora. Whereas information about the physiology (including substrates consumed and end products released into spent culture medium) can be studied for cultivable organisms, the contributions of the as-yet-uncultivated members of the bacterial flora remain to be determined.

Although lactic acid is the primary acid in normal vaginal secretions, other organic acids such as acetic, mydriatic, and linoleic acid are also normally found in vaginal fluid. Some healthy women actually lack vaginal Lactobacilli , but their vaginal pH is in the normal (moderately acidic) range, and other lactic acid–producing bacteria, such as Atopobium, Megasphaera , and/or Leptotrichia species, are present. However, because evidence of the tendency to fixate on easy answers, the historically interesting, yet simplistic view of vaginal antimicrobial regulation via lactate produced by Lactobacillus remains popular to the present time.

Another discovery that suggested an easy answer to understanding vaginal flora regulation was the finding that some strains of Lactobacillus produce hydrogen peroxide. Hydrogen peroxide became a new focus as a possible mechanism of a Lactobacillus -mediated antiseptic control of the vaginal flora. Unfortunately for this theory, hydrogen peroxide can be produced only when adequate oxygen is present.

The vagina is a low oxygen environment, as reflected by the known abundance of anaerobic bacteria in normal vaginal fluid. Hydrogen peroxide may be detected in a laboratory culture of Lactobacillus , but the vaginal microenvironment ostensibly would mitigate against significant accumulation of H ₂ O ₂ . This is only compounded by the presence of catalase, a peroxide-degrading enzyme elaborated by several bacterial species typically present in the vagina and by peroxide degrading systems in host epithelial cells. Thus, a significant amount of hydrogen peroxide is not likely to be produced in the vagina and, if it were to be produced in significant quantity, would not be expected to accumulate in the normal vaginal environment. A recent study has cast strong doubt on hydrogen peroxide having any in vivo physiological function in the vagina.

It is often forgotten that a substantial body of work was done in the early decades of the 20th century in an attempt to elucidate the connection between vaginal pH and vaginal microbiology. A series of studies by Weinstein and coworkers explored vaginal pH in monkeys, adult humans, infants, and postmenopausal women. They discovered that the presence of estrogen, and not Lactobacillus , was related to the acidic vaginal pH. In addition, the cervical canal has a nearly neutral pH and yet has a flora that is characterized by Lactobacillus , much as in the lower portions of the vagina.