in microbiology, a spherical-shaped bacterium. Many species of bacteria have characteristic arrangements that are useful in identification. Pairs of cocci are called diplococci; rows or chains of such cells are called streptococci; (Streptococcus) grapelike clusters of cells, staphylococci; (staphylococcus) packets of eight or more cells, sarcinae; and groups of four cells in a square arrangement, tetrads. These characteristic groupings occur as a result of variations in the reproduction process in bacteria. See also staphylococcus; Streptococcus.