A diplococcus is a pair of spherical bacteria that remain attached after cell division. Historically the term described genera such as Diplococcus pneumoniae, now classified as Streptococcus pneumoniae. The arrangement arises when cocci divide in one plane and the daughter cells fail to separate.
Explanation
Cocci are spherical bacteria that reproduce by binary fission. The spatial arrangement of the cells after division helps microbiologists identify genera. Cells that remain joined in pairs are described as diplococci. Chains of cocci are called streptococci and grape‑like clusters are staphylococci; packets of eight are sarcinae. In diplococci the division occurs in a single plane and the two daughter cells remain attached when the cell wall does not separate completely. This arrangement is seen in both Gram‑positive and Gram‑negative bacteria. The best known Gram‑positive diplococcus is Streptococcus pneumoniae, a capsulated organism that causes pneumonia, meningitis and otitis media. It appears as lanceolate pairs in Gram‑stained sputum and is bile‑soluble and optochin‑sensitive. Enterococcus faecalis may form short chains or pairs. Among Gram‑negative organisms, Neisseria gonorrhoeae and N. meningitidis are kidney‑shaped diplococci that reside in neutrophils during infection. Moraxella catarrhalis is another Gram‑negative diplococcus found in the respiratory tract. The diplococcal arrangement is not a taxonomic category; it is a morphological description that provides clues for rapid diagnosis. Cultural characteristics, biochemical tests and nucleic acid detection are required for identification.
Examples and importance
Pairs of cocci are an important clue in clinical microbiology. In cerebrospinal fluid, detection of Gram‑negative diplococci suggests meningococcal meningitis, prompting immediate treatment. Gram‑positive diplococci in blood cultures may indicate invasive pneumococcal disease and guide the choice of antibiotics. Some diplococci inhabit mucosal surfaces harmlessly but can become opportunistic pathogens in susceptible hosts.
Morphological arrangements of bacteria help laboratory workers narrow down possibilities before confirmatory tests. Recognising diplococci helps differentiate Streptococcus pneumoniae and Neisseria species from other spherical bacteria.
Related Terms: Cocci, Streptococcus, Staphylococcus, Gram-negative bacteria, Gram-positive bacteria