GARDNERELLA VAGINALIS Educational Commentary Is

EDUCATIONAL COMMENTARY – GARDNERELLA VAGINALIS

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Learning Outcomes Upon completion of this exercise, the participant will be able to: • Discuss the association of Gardnerella vaginalis with bacterial vaginosis. • Discuss the laboratory’s role in the diagnosis of bacterial vaginosis.

The organism identified in sample GC-11 of this test event is Gardnerella vaginalis, a catalase-negative, oxidase-negative, non-spore-forming bacillus. On Gram stain, it appears as pleomorphic gram-variable coccobacilli. It is normal vaginal flora in many women, and it is also found in the distal urethra of many men. G. vaginalis is occasionally associated with urinary tract infections, and, rarely, with bacteremia; however, it is best known for its association with bacterial vaginosis.

Bacterial vaginosis, or BV, is the most common vaginal infection in women of childbearing age. Although it does not cause complications in most cases, BV is associated with significant health risks. Women who have BV are at increased risk for pelvic inflammatory disease, ectopic pregnancy, and infertility. Pregnant women who have BV are more likely to have babies born prematurely or with low birth weight. BV is associated with an increased susceptibility to infection with HIV and other sexually transmitted diseases, such as chlamydia and gonorrhea. BV is also associated with an increased risk that an HIV-infected woman will pass HIV to her partner.

Despite research spanning nearly five decades, the cause of BV is still not fully understood. In 1955, Gardner and Dukes published results suggesting BV was caused by the bacterium now known as Gardnerella vaginalis, and for many years the condition was referred to as Gardnerella vaginalis vaginitis. Studies done since 1955 have shown that G. vaginalis is present as normal flora in many women, so clearly it is not the sole cause of BV.

Researchers now believe BV results from a disruption of normal vaginal flora that involves a loss of lactobacilli and a corresponding overgrowth of G. vaginalis, Mycoplasma hominis, and anaerobes such as Prevotella spp., Bacteroides ureolyticus, Fusobacterium nucleatum, and Mobiluncus. Because G. vaginalis is present in almost 100% of women with BV, it likely has a role in the development of the condition. Researchers now suspect a more important bacterium is Mobiluncus, an anaerobic Gram-negative bacillus usually present in the rectum. Mobiluncus is commonly found in the vaginal secretions of patients with BV, but it is almost never found in patients without BV.

Because G. vaginalis is present as normal flora in up to 58% of asymptomatic women, culture is of little value in the diagnosis of BV. In fact, studies have shown that the predictive value of a positive culture for G. vaginalis is less than 50%. For this reason, culture is no longer recommended as a diagnostic tool for BV. Instead, clinicians now diagnose BV when at least three of the following four criteria are present: • Thin, homogeneous, noninflammatory vaginal discharge • “Fishy” odor when 10% potassium hydroxide is added to vaginal secretions (positive “whiff test”) • Vaginal pH greater than 4.5 • Presence of clue cells in vaginal fluid

Of these, the criterion most specific for BV is the presence of clue cells. Clue cells are vaginal epithelial cells that are covered with bacteria, including G. vaginalis, Mobiluncus, and others. On a wet mount, clue cells appear stippled, glittery, or granular with fuzzy, indistinct edges. To diagnose BV, more than 20% of the vaginal epithelial cells should be clue cells.

In 1991, Nugent et al published a standardized method for scoring Gram stains for BV. With Nugent’s method, a Gram-stained smear of vaginal secretions is examined for the presence of three morphotypes: 1) large, parallel-sided Gram-positive rods (lactobacillus-like); 2) curved Gram-negative rods (Mobiluncus-like); and 3) a combination of tiny, Gram-variable coccobacilli and rounded, pleomorphic Gram-negative rods with vacuoles (Gardnerella/Bacteroides-like). Based on the average number of organisms seen per field, each morphotype is assigned a score ranging from 0 to 4. The three scores are then added together. A total score of 7-10 is diagnostic for BV. A table that describes the scoring system can be found in reference 2, p 377 (Forbes et al).

Since 1991, studies have clearly established the value of a Gram stain examination of vaginal fluid in the diagnosis of BV. Nugent’s standardized method readily shows the loss of lactobacilli and concurrent overgrowth of G. vaginalis and anaerobes that characterize BV. A Gram stain also confirms the presence of clue cells seen on a wet mount. Finally, Gram stains are easily prepared; they can be quickly screened; and they provide a permanent record.