Which bacterium is Gram-positive cocci arranged in clusters and catalase-positive?

• A. Streptococcus pneumoniae
• B. Escherichia coli
• C. Bacillus subtilis
• D. Staphylococcus aureus

Answer: (D)

Gram-positive cocci arranged in clusters and catalase-positive point to Staphylococcus species. The cluster arrangement—grape-like clumps of round cells—is characteristic of Staphylococcus, unlike Streptococcus or Enterococcus, which tend to form chains or pairs. The catalase test further narrows it: Staphylococcus species produce the enzyme catalase, so they bubble when hydrogen peroxide is added, whereas Streptococcus species are catalase-negative. Among the Gram-positive cocci, Staphylococcus aureus is the primary example that is both cluster-arranged and catalase-positive. Escherichia coli is a Gram-negative rod, and Bacillus subtilis is a Gram-positive rod, not cocci, so they don’t fit.