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Neisseria meningitidis (Meningococcus)
The genus Neisseria is included in the family Neisseriaceae (Figure 7.6). It contains two important pathogens Neisseria meningitidis and Neisseria gonorrhoeae, both the species are strict human pathogens. N. meningitides causes meningococcal meningitis (formerly known as cerebrospinal fever).
The word Meningitis is derived from Greek word ‘meninx’ means membrane and ‘it is’ means inflammation. It is an inflammation of meanings of brain or spinal cord. Bacterial meningitis is a much more severe disease than viral meningitis.
Morphology
They are Gram negative diplococci (0.6µm-0.8µm in size) arranged typically in pairs, with adjacent sides flattened. They are non – motile, capsulated (Fresh isolates). Cocci are generally intracellular when isolated from lesions (Figure 7.7).
Cultural Characteristic
They are strict aerobes, but growth is facilitated by 5-10% CO<sub>2</sub> and high humidity. The optimum temperature is 35°C-36°C and optimum pH is 7.4-7.6. They are fastidious pathogens, growth occurs on media enriched with blood or serum. They grow on the following media and show the characteristic colony morphology (Table 7.4).
Table 7. 4: Colony morphology of Neisseria Meningitides on media
|
Name of the Media |
Colony Morphology |
| Chocolate agar | Colonies are large, colorless to grey opaque colonies. |
| Mueller Hinton agar | Colonies are small, round, convex grey, translucent with entire edges. The colonies are butyrous in consistency and easily emulsified. |
Pathogenesis
N. meningitidis is the causative agent of meningococcal meningitis, also known as pyogenic or septic meningitis. Infection is most common in children and young adults. Meningococci are strict human pathogens. Human nasopharynx is the reservoir of N.meningitidis. The pathogenesis is dicussed in the
flowchart 7.2.
Source of infection – Airborne droplets
Route of entry – Nasopharynx
Site of infection – Meninges
Incubation period – 3 days
Flowchart 7.2: Pathogenesis of Neisseria Meningitides
Laboratory Diagnosis
Specimens:
CSF, blood, nasopharyngeal scrapings from petechiae lesions are the specimens collected from pyogenic meningitis patients.
Direct Microscopy:
CSF is centrifuged, and smear is prepared from the deposit for gram staining. Meningococci are Gram negative diplococci, present mainly inside polymorphs and many pus cells are also seen.
Culture:
The centrifuged deposit of CSF is inoculated on chocolate agar. The plate is incubated at 36°C under 5-10% CO2 for 18-24 hours. After incubation period, meningococcusis identified by gram staining, colony morphology and biochemical reactions. N. meningitides is catalase and oxidase positive (Figure 7.8).
Treatment and Prophylaxis
Penicillin – G is the drug of choice. In penicillin allergic cases, chloramphenicol is recommended.
- Monovalent and polyvalent vaccines (capsular polysaccharide) induce good immunity in older children and adults.
- Conjugate vaccines are used for children below the age of 2 years.