Bacteria: Structure and Pathogenesis · USMLE Step 1

Which bacterial shape and arrangement is characteristic for Streptococcus pyogenes under the microscope?

  1. Gram-negative rod in chains
  2. Gram-positive cocci in clusters
  3. Gram-positive cocci in chains
  4. Spiral-shaped gram-negative bacteria
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Correct answer: Gram-positive cocci in chains

S. pyogenes are Gram-positive cocci that grow in chains due to division in a single plane and failure of separation after division.

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