A 46-year-old man comes to the physician due to 3 days of fever, shortness of breath, pleuritic chest pain, and cough productive of green sputum. He has smoked a pack of cigarettes daily for 20 years. On examination, there are crackles at the base of the left lung. Chest x-ray reveals a left lower-lobe consolidation. Microscopic analysis of a sputum sample shows gram-positive, lancet-shaped cocci in pairs. Which of the following characteristics are these bacteria likely to demonstrate?

Question

A 46-year-old man comes to the physician due to 3 days of fever, shortness of breath, pleuritic chest pain, and cough productive of green sputum.  He has smoked a pack of cigarettes daily for 20 years.  On examination, there are crackles at the base of the left lung.  Chest x-ray reveals a left lower-lobe consolidation.  Microscopic analysis of a sputum sample shows gram-positive, lancet-shaped cocci in pairs.  Which of the following characteristics are these bacteria likely to demonstrate?

Quizplus · Accepted Answer

11ec160b_3892_35d0_9249_575a08c1a03e_MD0001_00 The presence of lancet-shaped, gram-positive diplococci in the sputum sample of a patient with fever and cough is suggestive of infection with Streptococcus pneumoniae. All streptococci are catalase-negative. The degree of hemolysis on blood agar distinguishes hemolytic (partial or complete) from nonhemolytic streptococci. The bile solubility and optochin tests are used to differentiate between partial (green) hemolytic species (α-hemolysis), namely S pneumoniae and viridans streptococci group. Autolysin produced by S pneumoniae is activated by bile, resulting in cell lysis. Therefore, addition of bile salts into a tube with S pneumoniae growth (turbid) leads to cell lysis (clearing of turbidity); this is known as bile solubility. S pneumoniae is bile-soluble and optochin-sensitive; viridans group streptococci are bile-insoluble and optochin-resistant (Choice F). (Choice A) Bacitracin susceptibility testing is used to differentiate Streptococcus pyogenes (bacitracin- susceptible) from other β-hemolytic streptococci. Many laboratories have replaced this test (which is not specific to S pyogenes) with the pyrrolidonyl arylamidase (PYR) test. Both S pyogenes and enterococci are PYR-positive. Streptococcus agalactiae is PYR-negative and enhances the area of Staphylococcus aureus hemolysis when grown in the same plate, resulting in a positive CAMP test (named after Christie, Atkins, and Munch-Petersen). (Choice C) Staphylococci are catalase-positive; streptococci are catalase-negative. (Choice D) Complete (or β) hemolysis is characteristic of Group A streptococcus (S pyogenes) and Group B streptococcus (S agalactiae). S aureus (catalase-positive) also demonstrates complete hemolysis, as noted by the arrows. (Choice E) γ-hemolytic (no hemolysis) catalase-negative organisms include enterococci and non-enterococci (S bovis). Enterococci are able to grow in hypertonic saline (6.5% NaCl) and bile. S bovis can grow in the presence of bile but not 6.5% NaCl. S bovis bacteremia is associated with colon cancer. Educational objective: Streptococcus pneumoniae are gram-positive, α-hemolytic, optochin-sensitive, bile-soluble diplococci. Viridans group streptococci are also α-hemolytic, but they are optochin-resistant and bile insoluble. Streptococcus pyogenes (Group A Streptococcus) appear as gram-positive cocci in chains with bacitracin susceptibility.

A 46-Year-Old Man Comes to the Physician Due to 3