Streptococcus pneumoniae (the pneumococcus) is a gram-positive coccus that grows in chains, causes α-hemolysis on blood agar, is bile soluble, and is sensitive to optochin.
Nearly every clinical isolate has a polysaccharide capsule that protects the bacteria from phagocytosis in the absence of type-specific antibody; 98 distinct capsules have been identified.
In industrialized countries, children serve as the major vectors of pneumococcal transmission: 20–50% of children <5 years old have asymptomatic nasopharyngeal colonization with S. pneumoniae (compared with 5–15% of young and middle-aged adults). Colonization rates for all age groups are even higher in low-income countries.
Rates of pneumococcal disease vary by season (higher in winter), gender (higher for males), and underlying medical condition (e.g., higher with splenic dysfunction; chronic respiratory, heart, liver, and kidney disease; immunosuppression).
The introduction and widespread use (in industrialized countries) of pneumococcal conjugate vaccines have led to dramatic changes in the epidemiology of invasive pneumococcal disease; rates have fallen by >75% among infants and children in the United States.
Nasopharyngeal colonization can persist for many months, resulting in the development of type-specific serum IgG that ultimately leads to pneumococcal clearance from the nasopharynx. Accordingly, pneumococcal disease is usually associated with recent acquisition of a new colonizing serotype.
Once the nasopharynx has been colonized, the bacteria spread either via the bloodstream to distant sites (e.g., brain, joint, bones) or locally to contiguous areas (e.g., middle ear, lungs).
Local cytokine production, particularly after intercurrent viral infections, facilitates pneumococcal adherence. Bacterial factors such as peptidoglycan and teichoic acid induce inflammation, result in characteristic pathology, and permit bacterial invasion.
CLINICAL MANIFESTATIONS AND DIAGNOSIS
The clinical manifestations of pneumococcal disease depend on the site of infection and the duration of illness.
Pneumococcal pneumonia—the most common serious pneumococcal syndrome—is difficult to distinguish from pneumonia of other etiologies on the basis of clinical findings.
Pts often present with fever, abrupt-onset cough and dyspnea, and sputum production.
– Pts may also have pleuritic chest pain, shaking chills, or myalgias.
– Among the elderly, presenting signs and symptoms may be less specific, with confusion and malaise but without fever or cough.
On physical examination, adults may have tachypnea (>30 breaths/min) and tachycardia, crackles on chest auscultation, and dullness to percussion of the chest in areas of consolidation.
– In some cases, hypotension, bronchial breathing, a pleural rub, or cyanosis may be present.
– Upper abdominal pain may be present if the diaphragmatic pleura is involved.
Pneumococcal pneumonia is generally diagnosed by Gram’s staining and culture of sputum.
– While culture results are awaited, chest x-rays—which classically demonstrate lobar or segmental consolidation—may provide some adjunctive evidence, although they may be normal early in the course of illness or with dehydration.
– Blood cultures are positive for pneumococci ...