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Question:

A 46-year-old man comes to the office due to 3 days of fever, shortness of breath, pleuritic chest pain, and cough with purulent sputum.  The patient has no prior medical conditions and takes no medications.  He has smoked a pack of cigarettes daily for 20 years.  Temperature is 38 C (100.4 F).  On examination, there are crackles at the base of the left lung.  Chest x-ray reveals consolidation in the left lower lobe.  Microscopic analysis of a sputum sample shows gram-positive, lancet-shaped cocci in pairs.  Which of the following additional characteristics are these bacteria most likely to demonstrate?

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Explanation:

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The presence of lancet-shaped, gram-positive diplococci in the sputum sample of a patient with fever and cough is suggestive of community-acquired pneumonia due to Streptococcus pneumoniae.  This pathogen is typically identified in the laboratory based on the following:

  • Catalase testing adds hydrogen peroxide to a bacterial sample to determine whether oxygen bubbles form (catalase positive) or no reaction occurs (catalase negative).  Streptococci are catalase negative, which distinguishes them from staphylococci (catalase positive) (Choice E).

  • Hemolysis pattern, the degree of hemolysis on blood agar, broadly classifies streptococcal species:

    • Alpha-hemolytic streptococci (eg, S pneumoniae, viridans streptococci) cause partial hemolysis on blood agar and turn the agar green due to hemoglobin oxidation.

    • Beta-hemolytic streptococci (eg, Streptococcus agalactiae, Streptococcus pyogenes) cause complete hemolysis on blood agar, resulting in a clear halo of lysed red blood cells around the colonies (Choice B).

    • Gamma-hemolytic streptococci (eg, Streptococcus gallolyticus [formerly S bovis]) do not cause hemolysis.

  • Optochin and bile-solubility tests differentiate alpha-hemolytic species:

    • The antibiotic optochin inhibits the growth of S pneumoniae but not viridans streptococci (Choice F)

    • The addition of bile salts to a solution of S pneumoniae activates pneumolysin (autolytic enzyme) and results in cell lysis (clearing of the sample), whereas viridans streptococci are unaffected.

(Choice A)  Pyrrolidonyl arylamidase (PYR) enzyme testing is used to differentiate beta- and gamma-hemolytic streptococci.  S pyogenes and enterococci are PYR positive (ie, can hydrolyze with PYR), whereas S agalactiae and S gallolyticus are PYR negative (ie, do not produce PYR).

(Choice C)  Enterococci and some streptococcal species (eg, S gallolyticus) are gamma-hemolytic (no hemolysis), catalase-negative organisms.  Enterococci can grow in hypertonic saline (6.5% NaCl) and bile, whereas S gallolyticus can grow in the presence of bile but not 6.5% NaCl.  S gallolyticus bacteremia is associated with colon cancer.

Educational objective:
Streptococcus pneumoniae is a gram-positive, lancet-shaped diplococcus that is catalase negative, alpha-hemolytic, optochin sensitive, and bile soluble.  Viridans group streptococci are also alpha-hemolytic, but they are optochin resistant and bile insoluble.  Streptococcus pyogenes (a type of group A streptococcus) is beta-hemolytic and PYR positive and appears as gram-positive cocci in chains.