Streptococcus pneumonia bacteria. Colored scanning electron micrograph (SEM) of Streptococcus pneumonia bacteria (also known as S. pneumococcus), the cause of pneumonia. These Gram-positive oval-shaped bacteria often occur in the short chains seen here. The cells possess capsules which resist ingestion by the phagocytes of the host's immune system. Although often found living harmlessly in the body, S. pneumonia can cause dangerous opportunistic pneumonia infections, often as a complication of serious disease when the victim's immunity is compromised. Magnification unknown.