Colonies of pneumococci

Colony appearance & alpha-hemolysis on blood agar

Streptococcus pneumoniae: alpha-hemolysis, colonies with depressions in their centers

Streptococcus pneumoniae cultivated on blood agar aerobically, 5% CO2, overnight. Colonies are surrounded by zones of alpha-hemolysis. Encapsulated pneumococci produce round, glistening colonies. The colonies often develop a dimpled, then a craterlike appearance (autolysis).

Pneumococci are common inhabitants of the respiratory tract and may be isolated from the nasopharynx of 5% to 70% of healthy adults. Rates of asymptomatic carriage vary with age, environment, and the presence of upper respiratory infections. Only 5%-10% of adults without children are carriers. In schools and orphanages, 27%-58% of students and residents may be carriers. On military installations, as many as 50%-60% of service personnel may be carriers. The duration of carriage varies and is generally longer in children than adults. In addition, the relationship of carriage to the development of natural immunity is poorly understood.