Neisseria meningitidis also referred to as meningococci are fastidious, aerobic Gram-negative diplococci with adjacent sides flattened (lens-shape/half-moon-shaped). Though it is a commensal of nasopharynx for immunocompetent individuals, it may cause sepsis and meningococcal meningitis in susceptible individuals.
Incidence rates of meningococcal-meningitis are generally highest in children less than five years of age and in adolescents.
N. meningitidis can be either encapsulated or not. Only encapsulated strains have the ability to invade the immunocompetent hosts.
Based on the antigenic nature of capsular polysaccharides, N. meningitidis can be typed into 13 serogroups (A-D, X-Z, 29E, W135, H-J, and L) of which five serogroups (A, B, C, Y, and W135) are associated with majority cases of invasive disease.
Pathogenesis and clinical manifestations:
The bacteria are transmitted from person-to-person through droplets of respiratory or throat secretions from carriers. Smoking, close and prolonged contact (such as kissing, sneezing or coughing) or living in close quarters with a carrier, facilitates the spread of the disease.
A number of virulence factors such as capsule, lipooligosaccharides, fimbriae, IgA1 protease, Por A, Por B proteins, etc. contribute to the pathogenesis of N.meningitidis. Find details here: Virulence factors produced by Neisseria meningitidis and their roles in pathogenesis
Once having colonized a new susceptible individual, in some cases, the organisms may breach the nasopharyngeal mucosal barrier and enter the bloodstream to cause invasive disease.
The two main clinical manifestations of invasive meningococcal disease are meningitis (75% – 80%) and septicemia (15% – 20%). Occasionally, meningococci cause other infections including pneumonia, pericarditis, conjunctivitis, endophthalmitis, septic arthritis, pelvic infection or a chronic low-grade septicaemia.
- Meningococcal meningitis: Meningococcal meningitis, is a serious infection of the meninges that affects the brain membrane. Liberation of endotoxin (by the bacteria) into the subarachnoid space provokes a marked cytokine-mediated inflammation of the meninges. Early symptoms are fever, malaise, nausea, shivers, tachycardia, and mild headache. As the illness progresses, headache may become more pronounced, accompanied by photophobia, confusion, and vomiting (due to raised intracranial pressure), followed by coma if untreated. In newborns and babies, symptoms such as being slow or inactive, irritable, vomiting, feeding poorly, or presence of a bulging in the soft spot of the skull (anterior fontanelle) may be an indication rather than the classic symptoms.
- Septicemia: A less common but even more severe (often fatal) form of meningococcal disease is meningococcal septicaemia, which is characterized by a haemorrhagic rash and rapid circulatory collapse. Most patients become progressively ill within 24 to 48 hours, though in a few cases the disease progresses so rapidly that the patient becomes moribund or dies within a few hours of the onset of infection.
Laboratory diagnosis of Neisseria meningitidis:
Cerebrospinal fluid (CSF), blood and skin scrapings from petechial rashes from cases, and nasopharyngeal swabs from carriers. Specimens should be collected in a sterile containers and transported immediately without any delay.
CSF should never be refrigerated as suspected agents of meningitis (pneumococci, meningococci, and Haemophilus influenzae) are delicate and may die on refrigeration).
In 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.
Isolation of N. meningitidis (from blood, CSF or other normally sterile site) remains the gold standard as it also provides isolates for strain differentiation and susceptibility testing.
Since meningococci are fastidious, sample from sterile body sites are inoculated on either blood or chocolate agar. The chocolate agar base can further 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 are 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.
Biochemical tests for the Identification of Nesisseria meningitidis:
- Catalase Positive
- Oxidase positive.
- Produce acid from glucose and maltose but not from lactose or sucrose.
- Nitrate Reduction Test: negative
- Produce gamma-glutamyl aminopeptidase
- Resistant to colistin: meningococci are colistin-resistant and grow on selective media containing VCN inhibitor
- DNAse reaction: negative
- Superoxol Test (reaction with 30% hydrogen peroxide): may show weak to a strong reaction.
- Pigmentation: produce pink-brown pigments.
Various serogroups of N.meningitidis are differentiated by slide agglutination test using monovalent antisera.
Several serological tests such as enzyme immunoassay, latex agglutination, and rapid diagnostic tests are available for the detection 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 also in cases of chronic meningococcemia.
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.
References and Further Readings
- Nadine G. Rouphael N G and Stephens D S “Neisseria meningitidis: Biology, Microbiology, and Epidemiology” Methods Mol Biol. 2012; 799: 1–20.
- Keith A.V. Cartwright “Neisseria meningitidis”
- WHO manual “Laboratory Methods For diagnosis of Meningitis caused by N.meningitidis, S.pneumoniae and Haemophilus influenzae, 2nd edition. WHO_IVB_11.09_eng.pdf