Identifying Bacteria and Epithelial Cells on Wet Mount
On a saline wet mount, bacteria appear as small morphotypes (including coccobacilli, tiny rods, and mobile curved rods), while epithelial cells are large, flat cells with distinct borders and nuclei—when bacteria heavily coat epithelial cells obscuring their borders, these become "clue cells" which are diagnostic of bacterial vaginosis 1.
Key Microscopic Features to Identify
Epithelial Cells (Vaginal Epithelial Cells - VECs)
- Normal appearance: Large, flat, polygonal cells with well-defined borders and visible nuclei
- Size: Significantly larger than bacteria (easily visible at 400x magnification)
- Clue cells: Epithelial cells with borders obscured by adherent bacteria, creating a stippled or granular appearance—this is the single most reliable indicator of bacterial vaginosis with 98.2% sensitivity and 94.3% specificity 2
Bacterial Morphotypes
- Small bacterial morphotypes: Include coccobacilli, tiny rods, and mobile curved rods (characteristic of bacterial vaginosis)
- Lactobacilli: Larger rod-shaped bacteria, typically predominant in normal vaginal flora
- Motile trichomonads: Pear-shaped, flagellated organisms with jerky motility (when present, diagnostic of trichomoniasis)
Technical Considerations for Optimal Visualization
Critical Timing
- Specimens must be examined within 30 minutes (optimal) to 2 hours of collection when submitted in saline 1
- Motile organisms (trichomonads) require live organisms and rapid examination
- Delayed examination significantly reduces sensitivity
Microscopy Technique
- Phase-contrast microscopy is strongly recommended over standard bright-field microscopy to improve performance and reproducibility 3
- Use 400x magnification for detailed examination
- Scan multiple fields to assess the overall bacterial morphotype pattern
Diagnostic Scoring Approach
A bacterial morphotype scoring (BMS) system weights small bacterial morphotypes against lactobacilli 4, 5:
- Score 0-1: Normal (lactobacilli predominant)
- Score 2-4: Intermediate phase, grade I
- Score 5-6: Intermediate phase, grade II
- Score 7-8: Bacterial vaginosis (small bacterial morphotypes predominant)
This wet mount BMS system has superior accuracy (likelihood ratio 15.4-20.3) compared to Gram stain (likelihood ratio 7.6-10.9) for bacterial vaginosis diagnosis 4.
Important Clinical Caveats
Limitations of Wet Mount
- Sensitivity for trichomoniasis is only 40-80%, requiring live organisms to visualize movement 1
- Proficiency in microscopic examination is essential as infections may be mixed or present atypically 1
- Wide variation in sensitivity (40-70%) exists for both trichomonads and yeast compared to NAAT and culture 1
When to Use Alternative Testing
- Consider culture or NAAT when wet mount is negative but clinical suspicion remains high 1
- For recurrent cases, culture is preferred over wet mount alone
- NAAT has become the preferred method for trichomoniasis due to superior sensitivity
Bacterial Preference Pattern
In bacterial vaginosis, bacteria preferentially colonize vital epithelial cells rather than dead or dying cells, which explains why not all epithelial cells become clue cells even in established infection 6. Look for the presence of ANY clue cells rather than expecting all cells to be affected.