Light micrograph of a gram positive bacteria Streptococcus pneumoniae found in the cerebrospinal fluid of a human where it caused bacterial meningitis. This speices is both a normal commensal & a common pathogen of the human respiratory tract. It is the commonest cause of lobar pneumonia, bronchopneumonia & a series of other pneumococcal infections including pleurisy, peritonitis & meningitis. The bacteria are spherical & usually paired. They secrete a polysaccharide capsule around the external surface of the cell, which provides protection against the immune system of the invaded host. Magnification: x250 at 35mm size.