Streptococcus pneumoniae
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Streptococcus pneumoniae ELAINE TUOMANEN
Introduction Pneumococcus is a fragile body and contains within itself enzymatic forces that lead to its disruption and disintegration, rob the substrate in which it lives of nutrient substances, and from these substances evolve chemical agents that arrest further growth, cause the death of the organism, and morbidly affect the cells of the animal body into which the microbe may find its way (B. White, 1938). Streptococcus pneumoniae is currently the leading cause of invasive bacterial disease in children and the elderly. Its original role in causing disease was appreciated by studies on lobar pneumonia in the late 1880s. By the turn of the 20th century, it was determined that antiserum from animals conferred excellent passive protection and even therapeutic benefit to humans stricken with disease. Thus was born the appreciation of the general role of antibodies in host defense and of capsular polysaccharide as a protective target. Antiserum also allowed classification of pneumococci into 2 different types initially (pneumococcus “I” and “Franz”). This was followed by the work of such great microbiologists as Lister, Neufeld, Dochez, Avery and Fleming, who demonstrated an increasing number of serotypes from 4, to 30, and finally to the 90 known today. Later, the natural transformability of pneumococci led to the discovery of DNA as the genetic material (Avery et al., 1944). Thus, both in terms of impact on medicine and on understanding of basic biology, the pneumococcus has been a formidable participant in the advancement of medical science.
Phylogeny S. pneumoniae are Gram-positive, a-hemolytic bacteria. Each bacterium is between 0.5 and 1.25 µ. They have no spores, vacuoles, visible granules or flagella, and they are nonmotile (White, 1938). Like other streptococci, they grow in pairs or short chains, lack catalase and ferment glucose to lactic acid. Unlike streptococci, they
do not display an M protein, they hydrolyze inulin, and their cell wall composition is characteristic both in terms of the peptidoglycan and the teichoic acid (alternatively termed C polysaccharide). The pneumococcal cell wall is roughly six layers thick and is composed of peptidoglycan with teichoic acid attached to approximately every third N-acetylmuramic acid residue (Garcia-Bustos and Tomasz, 1990; Fig. 1). Lipoteichoic acid is chemically identical to the teichoic acid but is attached to the cell membrane by a lipid moiety. Both the teichoic acid and the lipoteichoic acid contain phosphorylcholine, which has been recognized as an important element in the biology of pneumococcus (Tomasz, 1967). Two choline residues are covalently added to each carbohydrate repeat. The genetic locus for this process, lic, has been identified and mutations eliminating the ability to add choline to the surface appear to be crippling or lethal for the pneumococcus (Zhang et al., 1999). The peptidoglycan is synthesized by a set of at least five cell surface enzymes (penicillin-
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