Streptococcus pneumoniae

Background

Microbiology

• Gram-positive, lancet-shaped diplococcus
• 90+ serotypes, based on capsular polysaccharide that is bound to peptidoglycan
• Lab identification is based on α-hemolysis of blood agar (from pneumolysin), optochin susceptibility, and bile salt solubility
• Via transformation, bacteria can exchange genetic material with each other

Susceptibility testing

• CLSI penicillin breakpoints for susceptibility changed in 2008
  • For meningitis: ≤0.06 μg/mL
  • For other infections: ≤2 μg/mL

Antibiotic resistance

• Penicillin resistance
  • S. pneumoniae has 6 PBPs: 1A, 1B, 2A, 2B, 2X, and 3
  • Resistance in any of the PBPs can increase the MIC
  • Mutations in PBP 2B are associated with low-level resistance
  • Mutations in PBP 2X are associated with high-level resistance
• Macrolide resistance
  • ermB encodes an enzyme that methylates the 23S subunit, blocking macrolides and giving very high MIC ≥64
  • mefA encodes an efflux pump that gives a relatively lower MIC ≤16

Epidemiology

• Present worldwide
• Major cause of morbidity and mortality in children
  • Leading cause of under-5 mortality worldwide

Pathophysiology

• Acquired by coughing and sneezing
• Asymptomatic carriage or colonization in the nasopharynx
• Invasion through epithelial cells into the bloodstream, using the PAF and pIg receptors
• Capsule and various proteins help it to evade immune system

Clinical Manifestations

Asymptomatic carriage

• 4-10% in the general adult population, usually lasting several weeks
• Highest in children, up to 30-60% depending on the situation, lasting up to 6 months

Pneumonia

• Acute onset of cough (92%), fatigue (63%), shortness of breath (47%), and dyspnea (23%) with documented or subjective fever (92%), chills (77%), sweats, purulent sputum, and pleuritic chest pain (79%)

Meningitis

• Most common cause of meningitis in adults
• Acquired by hematogenous spread from nasopharynx, or direct invasion from sinuses
• May be secondary to otitis media or sinusitis
• CSF leaks and other defects predispose to infection
• Diagnostic yield in CSF decrease significantly 4 hours after administration of antibiotics