Streptococcus pneumoniae, or pneumococcus, is an encapsulated gram-positive coccus. Pneumococci are a major cause of common upper respiratory infections and life-threatening, invasive infections in children, adolescents, and adults in the United States and worldwide resulting in much morbidity and mortality. Pneumococci cause an estimated 3000 cases of meningitis, 50,000 cases of bacteremia, and 500,000 cases of pneumonia in the United States each year.<sup>12 Centers for Disease Control and Prevention Prevention of pneumococcal disease: Recommendations of the Advisory Committee on Immunization Practices (ACIP) MMWR Morb Mortal Wkly Rep 1997 ;  46 (RR-8) : 1

Haga clic aquí para ir a la sección de Referencias</sup> Cases of bacterial pneumonia and meningitis account for more than 500,000 hospitalizations and more than 40,000 deaths in the United States each year. In the United States, pneumococci are the most common cause of bacterial meningitis in children 1 to 23 months of age<sup>54 Schuchat A., Robinson K., Wenger J.D. , y al. Bacterial meningitis in the United States in 1995 N Engl J Med 1997 ;  337 : 970  [cross-ref]

Haga clic aquí para ir a la sección de Referencias</sup> and pyogenic pneumonia in pediatric patients beyond the neonatal period. The spectrum of invasive infections caused by pneumococci also includes septic arthritis, osteomyelitis, bacteremic cellulitis, primary peritonitis, and occult bacteremia in highly febrile infants and young children, and sepsis that may be rapidly fatal in immunocompromised hosts, such as those with impaired splenic function. S. pneumoniae is the leading cause of the common bacterial upper respiratory tract infections, acute otitis media and sinusitis. Seven million cases of acute otitis media are diagnosed each year in the United States, and 62% of US infants have an episode of otitis media during the first year of life.<sup>12 Centers for Disease Control and Prevention Prevention of pneumococcal disease: Recommendations of the Advisory Committee on Immunization Practices (ACIP) MMWR Morb Mortal Wkly Rep 1997 ;  46 (RR-8) : 1

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The pneumococcal cell may be enveloped by 1 of at least 90 chemically distinct polysaccharide capsules. Landmark studies performed during the first half of the twentieth century demonstrated that protective immunity was conferred by serum from rabbits injected with heat-killed pneumococci, that the protective activity of the serum is pneumococcal type specific, and that polysaccharide capsular is the major virulence factor and is the target of type-specific immunity (i.e., antibody). These findings led to capsular typing of pneumococcal isolates using antiserum prepared in animals by the Quellung reaction, which is the capsular swelling noted microscopically after incubation of the isolate withtype-specific antiserum. In early studies, killed bacteria and later purified capsular polysaccharides were studied as vaccines. In addition, capsular type-specific horse antiserum was used for the treatment of pneumococcal infection in humans. Thus, by the mid-1900s, researchers had established that capsule serves as a major virulence determinant, allowing the bacterium to resist phagocytosis and killing in the absence of anticapsular antibodies. They had also established that serotype-specific, anticapsular antibodies can be generated by immunization and play a major role in type-specific protection against pneumococcal disease.