Bacteria are microscopic organisms categorized by characteristics like shape. One of the most common forms is spherical bacteria, which can be round or oval. A single spherical bacterium is called a coccus, from the Greek word for “berry.” Along with rod-shaped and spiral-shaped bacteria, cocci are one of the three primary shapes used for bacterial classification.
Arrangements of Spherical Bacteria
The way spherical bacteria group together after dividing is a feature used for their identification. These arrangements result from the planes in which the cells divide and whether they remain attached. When cocci divide and remain in pairs, they are called diplococci.
When cocci divide repeatedly along a single plane, they create chains resembling a string of pearls; this arrangement is known as streptococci. In contrast, divisions along multiple, irregular planes result in grape-like clusters called staphylococci. This randomized division pattern leads to a characteristic three-dimensional clumping.
Other arrangements depend on the geometric planes of cell division. Division in two perpendicular planes produces a flat, square group of four cells known as a tetrad. If division continues across a third perpendicular plane, it forms a cubical packet of eight or more cells called sarcinae.
Notable Spherical Bacteria in Humans
Many spherical bacteria live on and inside the human body. Some are pathogens, such as Staphylococcus aureus, a common resident of the skin and nose that can cause serious infections, including the antibiotic-resistant strain MRSA. Another example is Streptococcus pyogenes, the bacterium responsible for strep throat and scarlet fever.
Not all cocci are harmful; many are a normal part of the human microbiota and do not cause disease. Staphylococcus epidermidis, for example, is one of the most common bacteria on human skin. It coexists peacefully with its host, and its presence can help inhibit the colonization of more dangerous pathogens.
How Shape Influences Function
The spherical shape of cocci is related to their survival. A sphere has the lowest surface-area-to-volume ratio of any shape, meaning less of the cell’s volume is exposed to the environment. This makes cocci less susceptible to drying out and other environmental stresses like certain detergents or antibiotics.
This low surface area contrasts with rod-like bacteria, known as bacilli, which have a larger surface area relative to their volume. While this helps bacilli absorb nutrients, it also makes them more vulnerable to desiccation. The compact shape of cocci provides a protective advantage.