Streptococcus Pneumoniae

a.k.a. pneumonococcus.

Characteristics/Epidemiology

Obligate parasite of human

Nasopharynx in carriers.

Pathogenesis/Transmission

Droplets from nose in carriers.

 

 

Pathogenicity

1. Capsule

Capsule is antiphagocytic (protection from PMN in absence of anti-capsular antibodies), and antigenic.

85 capsular serotypes, 20 of which accounts for majority of infections.

2. Autolysin

Peptidoglycan hydrolase in bacterial cell wall.

Normally inactive.

Can be triggered fo by surface-active agents, beta-lactam antibiotics, or aging.

Causes (s. pneumoniae) cell lysis, producing pneumolysin.

3. Pneumolysin

Normally within the cytosol of intact s. pneumoniae.

Released by autolysin.

Causes lysis by attacking mammalian cell membrane.

Clinical significance

Most common cause of pneumonia and otitis media

More susceptible individuals - malnutrition, alcoholism, post viral respiratory infections, immunocompromised, sickle cell disease, splenectomy.

1. Acute bacterial pneumonia

Often preceded by an upper respiratory viral infection.

-> due to increased volume and viscosity of secretion and inhibited action of bronchial cilia.

 

2. Otitis media

Most commonly caused by s. pneumoniae.

Other common causes are Haemophilus influenzae, and Moraxella catarrhalis

 

3. Bacteremia

 

4. Meningitis

Haemophilus influenzae used to be the leading cause, until vaccination.

S. pneumoniae is the second most common cause.

Neisseria meningitidis is the most common cause.

Laboratory identification

Lancet-shaped, tend to occur in pairs.

Encapsulated.

alpha-hemolytic.

No Lancefield type group.

Growth inhibited by low concentrations of the surfactant, optochin.

Lysed by bile.

Capsular swells when treated with type-specific antisera (Quellung reaction)

Treatment

First line: 3rd generation cephalosporins, e.g. cefotaxime, ceftriaxone

Second line: vancomycin

Resistence to penicillin G common.

Prevention/immunity

• 'Polyvalent' pneumococcal polysaccharide vaccine (23vPPV:
  * Uses capsular polysaccharide.
  * Useful against 23 serotypes (85 to 90% of infections)
  * For over 65y.o. or over 50y.o. if indigenous
  * Do not repeat dose within 3 years.
  * Booster every 5 years or longer.
  * Not for <2 y.o. due to limited immunity result.
• Heptavalent pneumococcal conjugate vaccine (7vPCV):
  * 7 pneumococcal antigens conjugated with non-toxic diphtheria toxin
  * 3 doses - given at 2, 4, and 6 months.
  * Add an additional dose at 18-24 months if indigenous.

 

PMN = polymorphonuclear leukocyte

 

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Things to revise/add later:

Bibliography: LWW microbiology

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