Sreptococci are round coccal bacteria that form chains. They are 0.5 to 1 µm in size and belong to the normal bacterial flora that live on humans, but can also cause severe diseases.
Some types of streptococci occur in many forms with different antigens.
84 different strains of Streptococcus pneumoniae are known, that have differently structured polysachharides in their walls. Each of these strains is a different serotype.