Practice question

Virulence in Streptococcus pneumoniae depends on capsular polysaccharide surrounding cell wall. Capsule is hydrophilic, negatively charged, blocking complement deposition and opsonization, resisting neutrophil phagocytosis, and allowing bloodstream invasion and meningitis. Rough mutants lacking capsule enzymes are readily cleared and avirulent in mouse models. Different capsule serotypes vary in antigenicity and virulence strength. Genes for capsule synthesis cluster in cps locus, transferable via transformation. Restoring capsule genes converts rough to smooth with regained lethality, showing capsule as primary virulence determinant rather than pili, flagella, or toxins.