Streptococcus pneumoniae

September 15th, 2010

Streptococcus pneumoniae (Pneumococcus)

Diseases

The most common diseases are pneumonia and meningitis in adults and otitis media and sinusitis in children.

Characteristics

Gram-positive “lancet-shaped” cocci in pairs (diplococci) or short chains. Alpha-hemolytic. Catalase-negative. Sensitive to bile and optochin in contrast to viridans streptococci, which are resistant. Prominent polysaccharide capsule. 85 serotypes based on antigenicity of polysaccharide capsule. One of the three classical encapsulated pyogenic bacteria (Neisseria meningitidis and Haemophilus influenzae are the other two).

Habitat and Transmission

Habitat is the human upper respiratory tract. Transmission is via respiratory droplets.

Pathogenesis

Induces inflammatory response. No known exotoxins. Polysaccharide capsule retards phagocytosis. Antipolysaccharide antibody opsonizes the organism and provides type-specific immunity. IgA protease degrades secretory IgA on respiratory mucosa, allowing colonization. Viral respiratory infection predisposes to pneumococcal pneumonia by damaging mucociliary elevator; splenectomy predisposes to sepsis. Skull fracture with spinal fluid leakage from nose predisposes to meningitis.

Laboratory Diagnosis

Gram-stained smear and culture.

Alpha-hemolytic colonies on blood agar.

Growth inhibited by bile and optochin.

Quellung reaction occurs (swelling of capsule with type-specific antiserum).

Serologic tests for antibody not useful.

Latex agglutination test for capsular antigen in spinal fluid can be diagnostic.

Treatment

Penicillin G.

Low-level and high-level resistance is caused by alterations in penicillin-binding proteins.

No Beta-lactamase is made.

Prevention

Two vaccines are available.

The one used in adults contains capsular polysaccharide of the 23 serotypes that cause bacteremia most frequently.

The other, which is used primarily in children under the age of 2 years, contains capsular polysaccharide of 7 serotypes coupled to carrier protein (diphtheria toxoid).

Oral penicillin is used in immunocompromised children.