Diplococci

Diplococci are bacteria of the cocci (spherical) shape that typically occur in pairs after division along a single plane, producing a characteristic diplococcus arrangement. The cells often resemble coffee beans when viewed under a light microscope.

The term applies to several genera. Notable examples include Gram-negative diplococci such as Neisseria gonorrhoeae and Neisseria meningitidis, and Moraxella catarrhalis. The Gram-positive diplococcus most commonly encountered is Streptococcus pneumoniae, which often appears as lancet-shaped diplococci in clinical specimens.

Clinical relevance: Pathogenic diplococci include N. gonorrhoeae, which causes gonorrhea and can disseminate; N. meningitidis, associated with meningitis and septicemia; Streptococcus pneumoniae, a leading cause of pneumonia, meningitis, sinusitis, and otitis media. Moraxella catarrhalis is linked to respiratory infections, particularly in children.

Laboratory identification: On Gram stain, diplococci appear as paired cocci, with Neisseria and Moraxella typically Gram-negative and Streptococcus pneumoniae Gram-positive. Neisseria and Moraxella often grow on enriched media with carbon dioxide; Streptococcus pneumoniae is alpha-hemolytic on blood agar and typically bile soluble and optochin sensitive.

Ecology and prevention: Humans are the primary reservoir for many diplococci. Many are normal flora of the nasopharynx, while others cause invasive disease. Vaccines exist for certain meningococcal serogroups and for pneumococcus, which helps reduce invasive disease.