Like other cocci, streptococci are round bacteria. The species name is derived from Greek words meaning 'a chain' () of berries ( from ) and pus ()-forming(genes), because streptococcal cells tend to link in chains of round cells (see image) and a number of infections caused by the bacterium produce pus. The main criterion for differentiation between "Staphylococcus" spp. and "Streptococcus" spp. is the catalase test. Staphylococci are catalase positive whereas streptococci are catalase-negative. "S. pyogenes" can be cultured on fresh blood agar plates. Under ideal conditions, it has an incubation period of 1 to 3 days.