Streptococcus bacteria. Colonies of Streptococcus pyogenes, a Gram-positive bacterium, growing on blood agar in a petri dish. Streptococci may be classified according to their haemolytic (blood cell destroying) activities. S. pyogenes is beta-haemolytic, meaning that it lyses red blood cells and decolourizes haemoglobin, causing a colourless zone around a colony on blood agar. Healthy humans carry strains of potentially pathogenic S. pyogenes in their throats and noses, which may give rise to a range of diseases. The most common is an acute sore throat, especially in young children. Scarlet fever is caused by S. pyogenes infection.