A microbiologist receives a sample from a patient with a suspected skin infection. Gram stain reveals Gram-positive cocci in clusters. Further testing shows the organism is catalase-positive. Which of the following tests would be most crucial to differentiate *Staphylococcus aureus* from other *Staphylococcus* species?

Explanation

The Gram stain showing Gram-positive cocci in clusters and a positive catalase test indicates a *Staphylococcus* species. To differentiate the highly pathogenic *Staphylococcus aureus* from other coagulase-negative staphylococci (CoNS), the coagulase test is the most crucial and definitive test. *Staphylococcus aureus* is coagulase-positive, while CoNS are coagulase-negative. Optochin sensitivity is used to identify *Streptococcus pneumoniae*. Bacitracin sensitivity is used to identify *Streptococcus pyogenes*. The Bile esculin test is used for Group D streptococci and enterococci.