Morphology and culturing. Pneumococci are Gram-positive, oval to lancet-shaped cocci that usually occur in pairs or short chains (Fig. 4.5a). The cells are surrounded by a thick capsule.
When cultured on blood agar, S. pneumoniae develop a-hemolytic colonies with a mucoid (smooth, shiny) appearance (hence "S" form, Fig. 4.5b). Mutants without capsules produce colonies with a rough surface ("R" form).
Antigen structure. Pneumococci are classified in 90 different serovars based on the fine chemical structure of the capsule polysaccharides acting as antigens. This capsule antigen can be identified using specific antisera in a reaction known as capsular swelling.
Pathogenesis and clinical pictures. The capsule protects the pathogens from phagocytosis and is the most important determinant of pneumococcal virulence. Unencapsulated variants are not capable of causing disease. Other potential virulence factors include pneumolysin with its effects on membranes and an IgA1 protease.
The natural habitat of pneumococci is provided by the mucosa of the upper respiratory tract. About 40-70% of healthy adults are carriers. Pneumococcal infections usually arise from this normal flora (endogenous infections). Predisposing factors include primary cardiopulmonary diseases, previous infections (e.g., influenza), and extirpation of the spleen or complement system defects.
The most important pneumococcal infections are lobar pneumonia and bronchopneumonia. Other infections include acute exacerbation of chronic bronchitis, otitis media, sinusitis, meningitis, and corneal ulcer. Severe pneu-mococcal infections frequently involve sepsis.
— Streptococcus pneumoniae -
Fig.4.5 a Gram staining of a preparation of middle ear secretion: gram-positive, round-oval, encapsulated cocci; clinical diagnosis: otitis media.
b Culture on blood agar: gray colonies showing little intrinsic color, often mucoid (due to capsules); a zone of greening is often observed around the colonies, caused by a-hemolysis; the shiny appearance of the colonies is caused by light reflections from their mucoid surface.
Diagnosis. The laboratory diagnosis includes detection of the pathogen in appropriate test samples by means of microscopy and culturing. Pneumococ-ci can be differentiated from other a-hemolytic streptococci based on their greater sensitivity to optochin (ethyl hydrocuprein hydrochloride) in the disk test or their bile solubility. Bile salts increase autolysis in pneumococci.
Therapy. Penicillin is still the antibiotic of choice. There have been reports of high-frequency occurrence of strains resistant to penicillin (South Africa, Spain, Hungary, USA). These strains are still relatively rare in Germany, Switzerland, and Austria (5-10%). Macrolide antibiotics are an alternative to penicillins, but resistance to them is also possible.
Penicillin resistance is not due to penicillinase, but rather to modified penicillin-binding proteins (PBPs) to which penicillins have a lower level of affinity. PBPs are required for murein biosynthesis. Biochemically, penicillin re
sistance extends to cephalosporins as well. However, certain cephalosporins (e.g., ceftriaxone) can be used against penicillin-resistant pneumococci due to their higher levels of activity.
Epidemiology and prophylaxis. Pneumococcal infections are endemic and occur in all seasons, more frequently in the elderly. Humans are the natural pathogen reservoir.
The vaccine product Pneumovax® is available for immunization purposes. It contains 25 mg of the purified capsule polysaccharides of each of 23 of the most frequent serovars. Eighty to ninety percent of all isolated pneumococci have antigens contained in this vaccine, which is primarily indicated in persons with predisposing primary diseases. There is also a seven-valent conjugate vaccine that is effective in children under two years of age (p. 33). Exposure prophylaxis is not necessary.
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