Introduction
Streptococcus pneumoniae is the most common cause of pneumonia and is an infectious agent posing a most serious threat to human health. Though mortality in developed countries has decreased significantly since the preantibiotic era, it is still considerable and morbidity has not changed substantially. Mortality in children in developing countries is still alarmingly high.
During the last few decades, a rapid emergence of resistance against most of the commonly used antimicrobial drugs and the identification of the seriousness of pneumococcal infections in asplenic individuals and in those with advanced HIV have added to the burden of pneumococcal disease. However, the introduction of a vaccine has provided some hope.
Though the clinical presentation of pneumonia has been known for at least 2500 years, the pathology for 170 years, and the importance of the pneumococcus in pneumonia for more than 100 years, major deficiencies still exist in our understanding of pathogenesis.
Microbiology
S. pneumoniae (pneumococcus) is a Gram-positive, often lanceolateshaped diplococcus with a polysaccharide capsule. Though the classification as streptococci has not always been accepted, the pneumococcus has several characteristics in common with other streptococci. The cell wall composition is similar, and like other streptococci it is catalase-negative and ferments glucose by the hexose monophosphate pathway to form lactic acid. Also, by nucleic acid homology it has been found to be closely related to many other streptococci and genetic material can be transferred to, and from, other streptococcal species by natural transformation.