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A 45-year-old white male presents with left lower leg pain. A physical exam reveals a well-demarcated region that is hot, erythematous, swollen, and exquisitely tender. Frank pus is noted and collected. Culture on sheep's blood agar produces a clear zone around colonies. You suspect infection with Group A Streptococcus. Which lab test will help confirm this diagnosis?
A cAMP test
B Grow in hypertonic NaCl
C Hydrolyze esculin in bile
D Hydrolyze hippurate
E Inhibition by bacitracin
F Inhibition by optochin
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It is E. Growth is inhibited by bacitracin.
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S. pyogenes is the only beta- hemolytic streptoccocus which is sensitive to bacitracin.
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Inhibition by bacitracin Bacitracin sensitivity is diagnostic of Streptococcus pyogenes, or Group A hemolytic Streptococcus. This bacterium is responsible for many cases of pharyngitis, cellulites, scarlet fever, and streptococcal toxic shock syndrome. The clear zone of hemolysis is consistent with beta-hemolysis, another characteristic of Streptococcus pyogenes
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cAMP test: Group B Streptococcus, or Streptococcus agalactaciae, is found in the female genital tract, and it often causes neonatal meningitis and sepsis. Group B Streptococcus is also bacitracin-resistant and demonstrates beta-hemolysis on sheep's blood agar.
Grow in hypertonic NaCl :Enterococci, a subgroup of Group D Streptococcus, are resistant to conditions in the harsh environment of the intestines. Enterococci often cause urinary tract infections
Hydrolyze esculin in bile Group D streptococci are potential agents for endocarditis. These bacteria demonstrate alpha-hemolysis on sheep's blood agar, leaving a green halo rather than clearing.
Hydrolyze hippurate Streptococcus agalactaciae can hydrolyze hippurate as a unique characteristic.
Inhibition by optochin Streptococcus pneumoniae, which is a gram-positive diplococcus, is inhibited by optochin. This bacterium is responsible for pneumonia characterized by rusty, brown sputum, and it is a major agent in upper respiratory infections. A 23-valent polysaccharide vaccine is used for prophylaxis.
