Neisseria
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Direct answer
The genus Neisseria holds two human pathogens — Neisseria meningitidis, the meningococcus of pyogenic meningitis and fulminant septicaemia, and Neisseria gonorrhoeae, the gonococcus of gonorrhoea and ophthalmia neonatorum. Both are gram-negative, kidney-shaped diplococci, oxidase and catalase positive, needing enriched media: the meningococcus grows on blood and chocolate agar, the gonococcus on chocolate or selective Thayer–Martin medium with carbon dioxide. Ceftriaxone is the drug of choice for both, and conjugate vaccines exist only for the meningococcus.
What you must remember
- Shared features: gram-negative, non-motile, kidney-shaped cocci in pairs with flattened adjacent sides, oxidase and catalase positive, fastidious and fragile, so specimens are plated promptly.
- Meningococcus: capsulated with polysaccharide capsules of groups A, B, C, W, Y and X, group A causing epidemics; it colonises the nasopharynx and invades to cause meningococcaemia with petechiae and meningitis.
- Meningococcal disease: purulent meningitis with fever, neck stiffness and petechiae; fulminant cases add disseminated intravascular coagulation with the Waterhouse–Friderichsen syndrome of adrenal haemorrhage; cerebrospinal fluid shows neutrophils, high protein, low glucose and gram-negative diplococci within cells.
- Gonococcus: non-capsulate but rich in pili and outer-membrane proteins with marked antigenic variation, which defeats immunity and vaccine efforts; it causes urethritis with purulent discharge, pelvic inflammatory disease, neonatal ophthalmia and disseminated infection.
- Culture: meningococci grow on blood and chocolate agar with carbon dioxide; gonococci need chocolate agar or Thayer–Martin medium, which contains vancomycin, colistin and nystatin to suppress genital contaminants.
- Diagnosis: cerebrospinal fluid microscopy, latex agglutination and blood culture for meningococcus; gram stain of urethral discharge showing intracellular diplococci is useful in men, and nucleic acid amplification is the most sensitive for gonorrhoea.
- Treatment and prevention: ceftriaxone for both; meningococcal conjugate vaccines against groups A, C, W and Y, and chemoprophylaxis with rifampicin, ciprofloxacin or ceftriaxone for close contacts; gonorrhoea control needs partner treatment.
Common confusion
The culture requirements are blurred: the meningococcus grows on ordinary blood agar, the gonococcus does not and needs chocolate or Thayer–Martin medium — a favourite discriminator. The oxidase test identifies the genus, not the species. The gonococcus's antigenic variation explains both repeated reinfection and the absence of any vaccine, whereas the meningococcus has effective vaccines except for group B.
Exam-focused takeaway
In theory, describe the genus, then each organism — capsule and groups for the meningococcus, pili and antigenic variation for the gonococcus — with syndromes, specimens and diagnosis; add cerebrospinal fluid findings and chemoprophylaxis. In viva, expect the cerebrospinal fluid picture, the contents of Thayer–Martin medium and why no gonococcal vaccine exists. At the practical, gram-negative diplococci within neutrophils is a spot diagnosis.
Frequently asked questions
How are the two pathogenic Neisseria differentiated?
By sugar fermentation — the meningococcus ferments glucose and maltose, the gonococcus glucose only — and by culture, the meningococcus growing on blood agar while the gonococcus needs enriched media.
What is the Waterhouse–Friderichsen syndrome?
Fulminant meningococcaemia with adrenal haemorrhage, disseminated intravascular coagulation and shock, needing immediate antibiotics and intensive care.
Which cerebrospinal fluid findings suggest meningococcal meningitis?
Turbid fluid under raised pressure with neutrophils, high protein, low glucose and gram-negative diplococci within neutrophils.
Why is there no gonococcal vaccine?
Extensive antigenic variation of pili and outer-membrane proteins prevents protective immunity, so reinfection is common.
What is Thayer–Martin medium?
A selective chocolate agar with vancomycin, colistin and nystatin that suppresses genital commensals while allowing the gonococcus to grow, incubated with carbon dioxide.
How is gonorrhoea treated?
With a single intramuscular dose of ceftriaxone, since penicillin resistance is widespread; partners are treated, and complications need longer courses.