diplococcus

Arrangement on Gram stain is one of the fastest routes to a presumptive identification. Gram-positive diplococci in sputum or CSF indicate S. pneumoniae, which is alpha-haemolytic, optochin-sensitive, bile-soluble, and encapsulated. Gram-negative diplococci indicate Neisseria: N. meningitidis, which ferments both maltose and glucose, is encapsulated, causes meningitis and the fulminant meningococcaemia of Waterhouse-Friderichsen syndrome with adrenal haemorrhage, and is prevented by conjugate vaccines; and N. gonorrhoeae, which ferments glucose only, has no polysaccharide capsule, undergoes pilus antigenic variation preventing lasting immunity, and causes urethritis, cervicitis, pelvic inflammatory disease, septic arthritis, and neonatal ophthalmia. Intracellular gram-negative diplococci within neutrophils on a urethral smear are diagnostic of gonorrhoea. Neisseria are oxidase-positive and grown on Thayer-Martin agar containing vancomycin, colistin, nystatin, and trimethoprim. Moraxella catarrhalis is a gram-negative diplococcus causing otitis media, sinusitis, and COPD exacerbation. Terminal complement deficiency predisposes specifically to Neisseria infection.