10.1 Genital Flora and Vaginitis Pathogens

The M(ASCP) analytic bacteriology domain (official content guideline revised 2025-09-25) tests genital-tract work under II.H: specimen sources, indigenous flora colony and Gram-stain morphology, methods for vaginitis pathogens, and—covered in the next section—culture or molecular detection of Neisseria gonorrhoeae, Chlamydia trachomatis, Streptococcus agalactiae, and Mycoplasma spp. This section is about what belongs in the lower genital tract, how those organisms look on Gram stain and on plates, and how the laboratory diagnoses the three common vaginitis syndromes without treating a routine aerobic vaginal culture as if it were a sterile-site workup.

Specimen sources: vaginal, cervical, urethral, and endocervical

The source on the requisition determines both the setup and the interpretation. Vaginal mucosa is squamous epithelium. A vaginal swab is the correct specimen for bacterial vaginosis (BV), candidiasis, and Trichomonas vaginalis. Many NAATs are validated on clinician-collected or self-collected vaginal swabs. Endocervical mucosa is columnar epithelium—the tissue N. gonorrhoeae and C. trachomatis actually infect. An endocervical swab (or a NAAT-validated vaginal swab) is the female genital source for those pathogens. A loosely labeled cervical swab is not automatically an endocervical collection: gonococcal culture requires sampling the endocervical canal, not the ectocervical squamous surface used for a Pap smear.

Urethral swabs are the classic culture source for gonorrhea in symptomatic men. First-void urine is the preferred non-invasive NAAT specimen in men. Intraurethral sampling is reserved for culture when susceptibility testing is needed. Transport must match the test. NAAT collection kits are manufacturer-specific; do not substitute a dry swab or an Amies tube unless the package insert allows it. Specimens destined for gonococcal culture go onto enriched selective media immediately or travel in charcoal transport that is not refrigerated. Wet mounts for trichomonads must be read while the fluid is still warm. Motility collapses as the specimen cools, and a nonmotile trophozoite is easy to miss or to misread as a white cell.

Indigenous organisms: colony and Gram-stain morphology

A healthy vaginal Gram stain is dominated by large Gram-positive rods consistent with Lactobacillus spp. Lactobacilli ferment glycogen to lactic acid and often produce hydrogen peroxide, holding vaginal pH in the 3.8–4.5 range. On sheep-blood agar they form small gray-white colonies that are nonhemolytic or alpha-hemolytic and catalase-negative. Loss of this morphotype—not the mere presence of Gardnerella—is the central Gram-stain event in BV.

Other colonizers appear in smaller numbers and must be recognized as flora rather than as reportable pathogens:

• Diphtheroids (Corynebacterium spp.): club-shaped Gram-positive rods in palisades or Chinese-letter arrangements; small catalase-positive colonies.
• Coagulase-negative staphylococci: Gram-positive cocci in clusters; white, catalase-positive, coagulase-negative colonies (Staphylococcus epidermidis and related species).
• Viridans-group streptococci: Gram-positive cocci in chains; small alpha-hemolytic, catalase-negative colonies.
• Anaerobes (Prevotella, Bacteroides, Mobiluncus): Gram-negative or curved rods that will not be recovered on a routine aerobic plate.
• Gardnerella vaginalis: small Gram-variable coccobacilli; pinpoint catalase-negative colonies that are beta-hemolytic on human-blood (V) agar, not on sheep blood.

Enterococci, genital mycoplasmas, and scant yeast can also be present without disease. Exam items often show a Gram stain or a plate and ask whether the organism is expected flora or a pathogen requiring workup. In a vaginal specimen, lactobacilli, diphtheroids, coagulase-negative staphylococci, and viridans streptococci are expected.

Do not set a routine vaginal culture expecting a single pathogen

This is one of the highest-yield genital-tract traps on M(ASCP). Vaginitis is a syndrome—BV, candidiasis, trichomoniasis, mixed infection, or noninfectious irritation—not a hunt for one aerobic isolate. Plating a vaginal swab to blood and MacConkey agar and reporting Escherichia coli, heavy growth or Gardnerella vaginalis isolated does not diagnose vaginitis. G. vaginalis is a member of a polymicrobial biofilm with anaerobes and Atopobium. Do not culture it as a pathogen in isolation, and do not issue a report that equates any quantity of G. vaginalis with BV.

What the laboratory should do is match the method to the syndrome: vaginal Gram stain with Nugent (or Hay–Ison) scoring and/or Amsel criteria for BV; KOH or yeast culture for Candida; wet mount, antigen, or NAAT for T. vaginalis. Yeast culture is appropriate when microscopy is negative and the clinician still suspects candidiasis, especially non-albicans species. Orders for gonorrhea, chlamydia, and prenatal GBS are separate tests with selective media or molecular methods. They are not a by-product of a routine vaginal aerobic culture.

Trichomonas vaginalis

T. vaginalis is a sexually transmitted flagellated protozoan. A saline wet mount of fresh vaginal fluid shows a pear-shaped trophozoite with an undulating membrane and jerky, tumbling motility. Wet-mount sensitivity is only about 50–70%, so a negative wet mount does not exclude trichomoniasis. Historical culture in Diamond's medium is more sensitive than wet mount but slow and now uncommon in routine laboratories. Rapid antigen tests improve on microscopy. NAAT is the most sensitive widely available method and is preferred when the assay is validated for the specimen type. T. vaginalis does not grow on bacterial media and stains poorly on Gram stain; do not expect a Gram-stained smear to make this diagnosis. Vaginal pH is usually greater than 4.5, and a whiff test may be positive, so trichomoniasis can overlap clinically with BV. The laboratory distinction is motile trophozoites or a positive NAAT, not clue cells.

Candida spp.

Vaginal candidiasis is overgrowth of yeast that already colonize the mucosa, not a classic bacterial sexually transmitted infection. Candida albicans accounts for most cases; C. glabrata and other non-albicans species cause a minority and may fail azole therapy. Ten percent KOH dissolves epithelial cells and reveals budding yeast and pseudohyphae. Gram stain shows Gram-positive yeast. Culture on Sabouraud dextrose agar or chromogenic agar is used when microscopy is negative, infection is recurrent, or species identification is needed. Germ-tube production or MALDI-TOF MS identifies C. albicans. Unlike BV and trichomoniasis, candidiasis usually leaves vaginal pH normal (less than 4.5)—a useful discriminator on the exam and at the bench.

Bacterial vaginosis: Nugent score, clue cells, and Amsel criteria

BV is a dysbiosis: lactobacilli fall, pH rises above 4.5, and a biofilm of G. vaginalis, Mobiluncus, and anaerobes expands. It is not infection with a single cultivable bacterium.

Amsel clinical criteria require three of four findings: homogeneous thin gray-white discharge; vaginal pH greater than 4.5; a positive amine (whiff) test after 10% KOH; and clue cells on wet mount.

Clue cells are squamous epithelial cells so densely coated with coccobacilli that the cell border is blurred. They support BV. They are not trichomonads, not yeast, and not a reason to work up G. vaginalis from culture.

The Nugent score on a Gram-stained vaginal smear is the laboratory reference method. Large Gram-positive Lactobacillus morphotypes are scored 0–4 inversely (abundant lactobacilli contribute 0 points). Small Gram-variable Gardnerella/Bacteroides morphotypes are scored 0–4. Curved Gram-negative Mobiluncus morphotypes are scored 0–2. Totals of 0–3 are normal flora, 4–6 are intermediate, and 7–10 are BV. Sialidase rapid tests can support the diagnosis in clinics that do not Gram-stain. None of these methods requires isolating G. vaginalis as a stand-alone pathogen.

When a requisition says vaginal culture, clarify the clinical question. If the question is vaginitis, set up microscopy and the appropriate molecular or yeast tests. If the question is gonorrhea, chlamydia, or GBS, those are different orders. Do not convert a vaginal swab into a sterile-site workup of mixed flora.