RESEARCH REVIEW Summary. The study of Streptococcus pneumoniae (the pneumococcus) had

been a central issue in medicine for many decades until the use of antibiotics became generalized. Many fundamental contributions to the history of microbiol-ogy should credit this bacterium: the capsular precipitin reaction, the major role this reaction plays in the development of immunology through the identification of polysaccharides as antigens, and, mainly, the demonstration, by genetic transfor-mation, that genes are composed of DNA—the finding from the study of bacteria that has had the greatest impact on biology. Currently, pneumococcus is the most common etiologic agent in acute otitis media, sinusitis, and pneumonia requiring the hospitalization of adults. Moreover, meningitis is the leading cause of death among children in developing countries. Here I discuss the contributions that led to the explosion of knowledge about pneumococcus and also report some of the contributions of our group to the understanding of the molecular basis of three important virulence factors: lytic enzymes, pneumococcal phages, and the genes coding for capsular polysaccharides. [Int Microbiol 2006; 9(3):179-190] Key words: Streptococcus pneumoniae · capsular polysaccharide · cell wall hydrolases · bacteriophage · virulence factors Pneumococcus: the sugar-coated bacteri