Cocci (or coccus for single-cell) are spherical-shaped bacteria or archaea that may exist as diplococci, tetrads, streptococci, staphylococci, or as sarcinae.

Cocci may be oval, elongated, or flattened on one side. They may remain attached after cell division. These group characteristics mentioned above helps to identify certain cocci.

Division in one plane produces either a diplococcus or streptococcus arrangement.

Diplococci are cocci bacteria arranged in pairs. Examples of these include Streptococcus pneumoniae, Moraxella catarrhalis, and Neisseria gonorrhoeae. Streptococci are a type of bacteria that arrange in a chain form (resembling chains). A majority of these bacterial cells are also ovoid and may form paired chains. They are non-motile and gram-positive organisms. One example of Streptococcus bacteria include Streptococcus pyogenes.

Division in two planes produces a tetrad arrangement.

Tetrad bacteria arrange in packets of four cells. Following division, the cells remain attached and grow in this attachment. Common examples of Tetrad bacteria include Pediococcus and Tetragenococcus.

Staphylococci are cocci arranged in irregular, often grape-like clusters, and their arrangement is the result of division that occurs in two planes. The main characteristics of these organisms are that they are immobile, Gram-positive bacteria. Examples of these include Staphylococcus epidermidis and Staphylococcus aureus.

Division in three planes produces a sarcina arrangement.

Sarcina bacteria occur in groups of eight cells and divide through the perpendicular plane. They are strict anaerobes, and gram-positive bacteria. One example of Sarcina bacterium includes Sarcina aurantiaca.

Key Points

• Diplococci bacteria are cocci bacteria arranged in pairs

• Streptococci are a type of bacteria that arrange in a resembling chain form; The majority of these bacterial cells are ovoid in shape and may form paired chains.

• Tetrad bacteria are arranged in packets of four cells and remain attached, and grow in this attachment.

• Staphylococci are cocci arranged in irregular, often grape-like clusters, and their arrangement is the result of division that occurs in two planes.

• Sarcina bacteria occur in groups of eight cells and divide through the perpendicular plane.

Key Terms

gram-positive bacteria: bacteria that give a positive result in the Gram stain test that indicates a thick peptidoglycan layer

cocci: spherical bacteria

diplococci:  cocci bacteria arranged in pairs

streptococci: bacteria that arrange in a chain form

staphylococci: cocci arranged in irregular arrangement due to the result of division that occurs in two planes.

sarcina: bacteria that occur in groups of eight cells and divide through the perpendicular plane