Bacterial morphology is a fundamental characteristic used for identification in microbiology. Bacteria generally fall into one of three basic shapes: spherical, rod-shaped, or spiral. Spherical bacteria are known as cocci (from the Greek kokkos, meaning berry). Cocci create distinct grouping patterns determined by the plane of cell division. This article explores the specific term used when these spherical cells organize themselves in pairs.
Defining the Paired Spherical Bacteria
The specific term for spherical bacteria that appear in pairs is diplococci. This distinct arrangement occurs when a coccus divides along a single plane and the two daughter cells remain joined at the division site.
Diplococci appear as two rounded spheres clinging together, though some species exhibit a flattened or slightly elongated appearance. For instance, certain genera display a characteristic “coffee bean” or “kidney bean” shape where the adjacent sides of the two cells are concave. This paired configuration is a reliable feature for initial classification when examining clinical samples.
The Full Spectrum of Cocci Arrangements
The paired arrangement of diplococci is just one of several distinct configurations cocci can form, dictated by the plane of division. When the cells divide repeatedly in a single plane and remain attached end-to-end, they form a chain known as streptococci, resembling a string of beads.
If a coccus divides along random, multiple planes, the cells accumulate in irregular, three-dimensional clusters called staphylococci, which look like a bunch of grapes. The lack of a uniform division plane prevents the formation of an orderly line or pair.
When division occurs in two perpendicular planes, the cocci organize into a square group of four cells, a configuration known as a tetrad. Sarcinae results from cell division alternating in three perpendicular planes, creating a cubical packet of eight cells. These varied patterns reflect the precise genetic control bacteria have over their cellular growth and division processes.
Significant Examples and Their Health Implications
Identifying a diplococcal arrangement is often the first step in identifying bacteria important to human health. Two recognized pathogenic examples belong to the genus Neisseria, which consistently exhibit classic diplococcus morphology. Neisseria gonorrhoeae (cause of gonorrhea) and Neisseria meningitidis (cause of bacterial meningitis) are both Gram-negative diplococci.
These Neisseria species possess the distinctive kidney-bean or coffee-bean shape, often observed within white blood cells. This appearance offers strong presumptive identification in a clinical setting, guiding immediate treatment before full culture results are available. The ability to quickly identify this morphology is particularly important for N. meningitidis, where rapid diagnosis is linked to improved outcomes for severe infections.
Another medically significant diplococcus is Streptococcus pneumoniae (pneumococcus). This bacterium is a major cause of infections like pneumonia, otitis media, and bacterial meningitis. Although technically a Streptococcus (sometimes appearing in short chains), it most often occurs as diplococci in clinical specimens, displaying a characteristic lancet shape. The presence of these Gram-positive, lancet-shaped diplococci in a patient’s sputum or cerebrospinal fluid provides strong evidence of a pneumococcal infection.