Gram-negative cocci !
- Bacteria
Coffee-bean diplococci: meningococcus, gonococcus and Moraxella. They cause meningitis, gonorrhea, ear and sinus infections.
Updated 2026-11-01
What they are
Round bacteria that sit in pairs and stain red on Gram stain. Neisseria species live only in humans and grow best on chocolate agar or selective media such as Thayer-Martin. Moraxella catarrhalis colonizes the airway and nearly always makes beta-lactamase.
What they share
- Gram-negative, oxidase-positive diplococci
- Aerobic; meningococcus and gonococcus need enriched media (chocolate agar, Thayer-Martin)
- Lipooligosaccharide (LOS) in the outer membrane acts as endotoxin
- No animal reservoir: spread from person to person
Microorganisms in this group
- Neisseria meningitidisA throat bacterium that can cause meningitis and a bloodstream infection with skin spots that worsens within hours.Exam priority: high
- Neisseria gonorrhoeaeA sexually transmitted bacterium that causes gonorrhea; in women it often gives no symptoms and can lead to infertility.Exam priority: high
- Moraxella catarrhalisA nose and throat bacterium that causes ear infections in children, sinusitis and COPD flares.For completeness
Test yourself
Think of the answer, then tap the question to see it.
Which classic biochemical test separates N. meningitidis from N. gonorrhoeae?
Sugar fermentation: meningococcus ferments glucose and maltose; gonococcus ferments glucose only.
Why is there a meningococcal vaccine but no gonococcal vaccine?
Meningococcus has a polysaccharide capsule that serves as the vaccine antigen; gonococcus has no capsule and keeps changing its pili (antigenic variation).
Which selective medium is used to grow Neisseria from sites with mixed flora?
Thayer-Martin: chocolate agar with antibiotics that suppress the other flora.
Which Gram-negative coccus causes otitis media and COPD flares and makes beta-lactamase?
Moraxella catarrhalis.
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Chapter 14: Meningococcal Disease (CDC, 2024); Gonorrhoea (Neisseria gonorrhoeae infection) (WHO, 2025); Moraxella catarrhalis Infection (MSD Manual Professional Edition, 2024).
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Representative model, for educational purposes only.