Streptococcus pneumoniae, coloured scanning electron micrograph (SEM). S. pneumoniae are oval, Gram-positive bacteria. They are part of the normal human bacterial flora and are carried by many without causing infection. However, in individuals with compromised immune systems, they can infect the upper respiratory tract, causing pneumonia. They may also infect other parts of the body, causing bacterial blood poisoning (bacteraemia) and meningitis.