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Chapter 46 : The Pneumococcus: Population Biology and Virulence
Streptococcus pneumoniae disease has been a major cause of mortality throughout human history, causing serious invasive diseases such as pneumonia, bacteremia, septicemia, and meningitis. In common with other bacteria colonizing the nasopharynx, S. pneumoniae rarely causes invasive disease despite its prevalence in the population. The precise regulation of virulence factors in pneumococci is essential as the organism changes from colonizing the nasopharynx and surviving in and invading the lung before entering the bloodstream and cerebrospinal fluid. Studies have found 13 putative two-component signal-transduction systems in S. pneumoniae, and early studies suggest that these systems regulate expression of virulence loci in response to environmental stimuli as has been found in other bacterial pathogens. The S. pneumoniae capsule is an obvious feature of the organism when viewed on blood agar, and it serves a key role in both virulence and immune evasion. Increasing rates of antibiotic resistance have been found in studies in many countries, prompting the most alarmist of commentators to speculate about a return to the preantibiotic era. The pneumococcal capsular polysaccharide was the first virulence factor identified in the species. Pneumococcal capsular polysaccharide genes are not always reliable markers of the strain genetic background, as early studies using multilocus enzyme electrophoresis (MLEE) showed. MLEE examines allelic diversity at a number of housekeeping gene loci by comparing the mobility of their gene products on starch gels. The development of automated DNA sequencing allowed the development of multilocus sequence typing (MLST), which is based on the sound evolutionary theory underlying MLEE.