Encapsulated Bacteria

Overview

Most important encapsulated bacteria and fungi (YES Some Nasty Killers Have Pretty Big Capsules)

• Yersinia pestis
  • Escherichia coli
    • meningeal strains only
  • Salmonella typhi
  • Streptococcus pneumoniae and Streptococcus pyogenes
  • Neisseria meningitidis
  • Klebsiella pneumoniae 
  • Haemophilus influenzae type B (B polysaccharide)
  • Pseudomonas aeruginosa
  • Bordetella pertussis and Bacillus anthracis (contains poly D-glutamate capsule)
  • Cryptococcus neoformans
• only encapsulated fungal pathogen

Physiology

• Quellung reaction
  • antibodies bind to capsule
  • used to visualize
    • S. pneumoniae
    • H. influenzae
    • N. meningitidis
    • K. pneumoniae
• Capsules serve as antigen in vaccines
  • H. influenzae type B vaccine
  • meningococcal vaccines
  • S. pneumoniae vaccine (pneumovax)
• Capsules act as antiphagocytic virulence factors
• Encapsulated bacteria can generally
  • cause meningitis
  • produce IgA protease
  • undergo transformation
    • take up DNA from their environment
• Opsonization
  • antibodies bind to capsule to aid phagocytosis by macrophages and neutrophils
  • clearance of these pathogens depends on presence of C3 complement or IgG and a functional spleen