Gram Stain Characteristics of Neisseria meningitidis
Neisseria meningitidis appears as Gram-negative diplococci in Gram stain preparations, with approximately 75% of cases showing positive Gram stain results when properly performed. 1
Microscopic Appearance
- Gram-negative (pink/red colored)
- Diplococcal arrangement (pairs of cocci)
- Typically kidney or coffee bean-shaped
- Often seen inside polymorphonuclear leukocytes (intracellular)
Diagnostic Yield of Gram Stain for N. meningitidis
- Sensitivity of 75% for N. meningitidis meningitis 1
- Higher sensitivity than for other pathogens like Listeria monocytogenes (25-35%)
- Lower sensitivity than Streptococcus pneumoniae (90%) and Haemophilus influenzae (86%) 1
- Specificity of approximately 97% for bacterial meningitis overall 1
Factors Affecting Gram Stain Sensitivity
- CSF bacterial concentration significantly impacts detection:
- 10³ CFU/mL: 25% positive rate
- 10³-10⁵ CFU/mL: 60% positive rate
10⁵ CFU/mL: 97% positive rate 1
- Prior antibiotic treatment reduces sensitivity by approximately 20% 1
- Cytospin techniques can increase sensitivity up to 100-fold 1
Clinical Implications
- Gram stain is recommended for all patients with suspected meningitis (A-III recommendation) 1
- Rapid test that provides results within minutes
- Can guide initial antimicrobial therapy before culture results are available
- Particularly valuable when patients have received prior antibiotics, as culture sensitivity decreases more than Gram stain sensitivity 1
Limitations and Pitfalls
- False-positive results may occur due to:
- Observer misinterpretation
- Reagent contamination
- Contamination from skin fragments during lumbar puncture 1
- May be difficult to distinguish from other Neisseria species or Moraxella catarrhalis 1
- Negative Gram stain does not rule out N. meningitidis infection
- Supplementary testing with PCR is recommended, especially in cases with prior antibiotic treatment 1
Complementary Diagnostic Methods
- CSF culture remains the gold standard but takes 24-48 hours
- PCR has higher sensitivity (95.7%) compared to culture (81.3%) 2
- PCR is particularly valuable when antibiotics have been administered 1
- Latex agglutination has limited incremental value over Gram stain and culture 1
The accurate identification of N. meningitidis through Gram stain is crucial for rapid diagnosis and appropriate management of potentially fatal meningococcal disease, which can lead to serious complications including septicemia, Waterhouse-Friderichsen syndrome, and multiple organ failure if not promptly treated 3.