Neisseria meningitidis is a Gram-negative encapsulated diplococcus that colonises the human nasopharynx and can cause invasive meningococcal disease, including meningitis and septicemia.
Microbiology and virulence
eisseria meningitidis appears microscopically as kidney bean-shaped pairs of cells that are oxidase-positive and catalase-positive. It grows on nutrient-rich media under increased carbon dioxide but is less fastidious than its close relative Neisseria gonorrhoeae. The organism possesses a polysaccharide capsule, the primary virulence factor, which defines serogroups such as A, B, C, W, X and Y. Capsule switching through horizontal gene transfer contributes to immune evasion and epidemiologic diversity. Type IV pili and outer membrane proteins Opa and Opc facilitate adhesion to epithelial cells in the nasopharynx. Like other Neisseria, it produces IgA protease and iron acquisition systems. During invasive disease the bacteria enter the bloodstream, where lipooligosaccharide endotoxin triggers a strong inflammatory response. Rapid proliferation and release of endotoxin lead to vascular injury, disseminated intravascular coagulation and shock. Some strains possess genetic islands encoding additional toxins and resistance determinants. People with complement deficiencies or defects in the terminal complement pathway are particularly susceptible because membrane attack complex formation is essential for killing these bacteria.
Disease, epidemiology and prevention
Most carriers of N. meningitidis remain asymptomatic, but in some individuals the bacteria penetrate mucosal barriers and cause meningitis or fulminant septicemia. Symptoms of meningitis include sudden fever, headache, neck stiffness, photophobia and altered mental status; septicemia can present with petechial rash, hypotension and multi-organ failure. Infants, adolescents, military recruits and university students living in close quarters are at higher risk of disease, and outbreaks often occur in crowded settings. Rapid diagnosis is critical; Gram staining and culture of cerebrospinal fluid or blood can reveal Gram-negative diplococci, while polymerase chain reaction assays detect bacterial DNA. Empirical treatment with intravenous cefotaxime or ceftriaxone should begin immediately, followed by targeted therapy based on susceptibility testing. Close contacts of cases receive prophylactic antibiotics such as rifampicin or ciprofloxacin to eradicate carriage. Vaccination is the cornerstone of prevention: conjugate vaccines targeting serogroups A, C, W and Y are widely used, and protein-based vaccines against serogroup B are available. Maintenance of high vaccine coverage and surveillance of circulating strains are key to controlling meningococcal disease.
Neisseria meningitidis combines the ability to colonise the upper airway with the potential to cause rapid, life-threatening invasive disease. Continued vigilance, prompt treatment and effective vaccination programs are essential for limiting its impact.
Related Terms: Meningitis, Capsule, Gram-negative, Diplococcus, Vaccine