The vaginal flora of a normal, asymptomatic, reproductive-aged woman includes multiple aerobic, facultative anaerobic, and obligate anaerobic species. Of these, anaerobes predominate and outnumber aerobic species approximately 10 to 1 (Bartlett, 1977). These bacteria exist with the host in a symbiotic relationship, which is alterable depending on the microenvironment.
Certain bacterial species normally found in vaginal flora have access to the upper reproductive tract. The female upper reproductive tract is not sterile, and the presence of these bacteria does not indicate active infection (Hemsell, 1989; Spence, 1982). Together, these findings illustrate the potential for infection following gynecologic surgery and the need for antimicrobial prophylaxis.
Typically, the vaginal pH ranges between 4 and 4.5. This is due in part to gram-positive aerobic Lactobacillus species producing lactic acid, fatty acids, and other organic acids. Other bacteria also can add organic acids from protein catabolism, and anaerobic bacteria donate by amino acid fermentation.
Glycogen, which is present in healthy vaginal mucosa, provides nutrients for many vaginal ecosystem species and is metabolized to lactic acid (Boskey, 2001). Glycogen content within vaginal epithelial cells normally diminishes after menopause and is low in childhood. As a result, postmenopausal women not receiving estrogen replacement and young girls have a lower prevalence of Lactobacillus species and less acid production compared with that of reproductive-aged women. This leads to a rise in vaginal pH. For menopausal women, hormone replacement therapy restores vaginal lactobacilli populations, which protect against vaginal pathogens (Dahn, 2008).
Changing other elements of the vaginal ecology may alter the prevalence of various species and may lead to infection. With the menstrual cycle, transient changes in flora are observed. These are predominantly during the first days of the cycle and are presumed to be associated with hormonal changes (Keane, 1997). Menstrual fluid can serve as a nutrient source for several bacterial species, resulting in their overgrowth. The role of this in the development of upper reproductive tract infection following menstruation is unclear, but an association may be present. For example, women symptomatic with acute gonococcal upper reproductive tract infection classically are menstruating or have just completed their menses. Last, treatment with broad-spectrum antibiotics may result in symptoms attributed to inflammation from Candida albicans or other Candida species by eradicating other balancing species in the flora.
This common, complex, and poorly understood clinical syndrome reflects vaginal flora in which anaerobic species are overrepresented. These include Gardnerella, Prevotella, Mobiluncus, and Bacteroides species; Atopobium vaginae; and BV-associated bacteria, provisionally named BVAB1, BVAB2, and BVAB3. These latter three are newly recognized bacteria found in women with BV (Fredricks, 2005). BV is also associated with a significant reduction of normal Lactobacillus species.