What are the Laboratory diagnosis of neisseria meningitidis?

1. Gram Stain: In a Gram-stained smear of centrifuged deposit of specimen (CSF or sterile body fluid), N. meningitidis appear as Gram-negative, coffee-bean-shaped diplococci occurring intracellularly or extracellularly in PMN leukocytes.

2. Culture: Isolation of N. meningitidis (from blood, CSF, or other normally sterile sites) remains the gold standard as it also provides isolates for strain differentiation and susceptibility testing.

Since meningococci are fastidious, samples from sterile body sites are inoculated on either blood or chocolate agar. The chocolate agar base can be enriched with antibiotics such as vancomycin, colistin, nystatin, and trimethoprim for selective isolation of N. meningitidis. For culture from non-sterile sites such as the nasopharynx, a selective media such as Modified New York City or Modified Thayer Martin medium is required. Culture plates should be incubated for a minimum of 48 hours with a source of 5% CO2. On Blood agar, young colonies of N. meningitidis are round, smooth, moist, glistening, and convex, with a clearly defined edge whereas actively growing colonies are grey and unpigmented. Older cultures (> 24 hours) become more opaquely grey and sometimes cause the underlying agar to turn dark.

3. Serological tests: Several serological tests, such as enzyme immunoassay, latex agglutination, and rapid diagnostic tests, are available to detect antibodies against capsular antigens of Neisseria meningitidis. Serological tests help in the retrospective diagnosis of disease. Antibodies are also seen when vaccination is successful and in cases of chronic meningococcemia.

4. Molecular diagnosis: PCR-based diagnosis provides confirmation of meningococcal disease from blood, CSF, or other normally sterile sites with a validity comparable to that of culture-based diagnosis.