Streptococcus pneumoniae bacteria (pneumococci), computer illustration. These Gram-positive spherical bacteria are usually found in pairs. They colonize the respiratory tract asymptomatically in healthy carriers, but can cause pneumonia and are one of the causative agents of bacterial meningitis in individuals with weaker immune system, particularly in the elderly and young children. They also can cause a range of infections in different locations, such as otitis media, conjunctivitis, endocarditis, sepsis, osteomyelitis and other.