Streptococcus bacteria. Transmission electron micrograph of Streptococcus bacteria. Streptococci are Gram positive, non-motile, non- sporing and are recognized by a tendency to form chains. They occur as parasites of man and animals, contributing to the normal, healthy flora of human respiratory, alimentary (gut) and female genital tracts. Some species are pathogenic to humans, including S. pneumoniae and S. pyogenes, although both species also occur as commensals ( deriving nourishment from a host without causing harm or benefit). Magnification: x50000, when printed 10 centimetres wide.