Streptococcus pneumoniae (blue), light micrograph. S. pneumoniae are oval, Gram-positive bacteria, that are often found in chains or pairs. They are part of the normal bacterial flora, and as such, are carried by many without causing infection. However, in immune compromised individuals 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. Magnification: x500 when printed at 10 centimetres wide.