Morphology and Physiology
Under the microscope, meningococci appear as paired cocci (diplococci) resembling kidney beans or coffee beans, with their concave sides facing each other. They measure approximately 1 µm in diameter. They do not retain crystal violet and are therefore gram-negative.
The bacterial surface features pili and a microcapsule that can transform into a pronounced polysaccharide capsule.
Neisseria meningitidis requires specific growth conditions: they are obligate aerobes and capnophiles (requiring elevated carbon dioxide levels). Growth media must be enriched with amino acids, the optimal temperature is 37 °C, and the optimal pH is slightly alkaline (7.2–7.6). Biochemically, the pathogen is relatively inactive, fermenting only glucose and maltose with acid production, which helps distinguish it from other Neisseria species.
Pathogenicity Factors
The virulence of Neisseria meningitidis depends on several key structures:
- Endotoxin (Lipooligosaccharide - LOS): The primary virulence factor. When the bacteria undergo lysis, LOS is released into the bloodstream, triggering vascular endothelial damage, hemorrhages, and septic shock.
- Capsule: Protects the bacterium against phagocytosis (antiphagocytic effect).
- Pili and Outer Membrane Proteins: Mediate adhesion to mucosal epithelial cells and subsequent invasion.
- IgA Protease: Cleaves secretory IgA antibodies, helping the bacterium evade mucosal immune defenses.
Epidemiology and Pathogenesis
Humans are the sole reservoir and source of infection (patients or asymptomatic carriers). Transmission occurs via the airborne droplet route, with the bacteria landing on the nasopharyngeal mucosa.
The clinical outcome depends heavily on host immunity:
- Healthy Carriage: The most common scenario, where bacteria colonize the mucosa asymptomatically (up to 80% during epidemics).
- Localized Infection: Meningococcal nasopharyngitis (inflammation of the nasopharynx).
- Generalized Infection: Bacteria breach local mucosal barriers, enter the bloodstream (meningococcemia), disseminate systemically, and cross the blood-brain barrier to cause purulent meningitis.
Diagnosis and Prevention
Diagnostic specimens include cerebrospinal fluid (CSF), nasopharyngeal swabs, and blood. Diagnostic methods include:
- Bacterioscopy: Microscopic examination of CSF for intra- and extracellular gram-negative diplococci.
- Culture: Inoculation on specialized media followed by biochemical differentiation from non-pathogenic Neisseria species.
- PCR and Direct Immunofluorescence (DFA): Used as rapid diagnostic assays.
- Serology: Performed from the second week of the illness onward.
Prevention involves patient isolation, adequate ventilation, and minimizing overcrowding. In epidemic settings, polysaccharide conjugate or plain vaccines are used (targeting serogroups A, C, Y, W-135, noting that standard ones do not cover group B).