A 34-year-old patient has painful, red, and swollen skin lesions on the thigh. A wound swab is cultured. Gram stain shows Gram-positive cocci in clusters. Blood agar yields large golden beta-hemolytic colonies. MSA yellow; Catalase bubbling; Slide latex agglutination positive. Coagulase positive. DNAse halo around the organism. Which organism do you suspect?

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Multiple Choice

A 34-year-old patient has painful, red, and swollen skin lesions on the thigh. A wound swab is cultured. Gram stain shows Gram-positive cocci in clusters. Blood agar yields large golden beta-hemolytic colonies. MSA yellow; Catalase bubbling; Slide latex agglutination positive. Coagulase positive. DNAse halo around the organism. Which organism do you suspect?

Recognizing Staphylococcus aureus through its distinctive lab profile helps explain why it fits this scenario. The organism is Gram-positive cocci growing in clusters, and it is catalase positive, which separates Staphylococcus species from Streptococcus (the latter are catalase negative and typically appear in chains). On blood agar it often shows beta-hemolysis with golden, pigmented colonies, a hallmark clue for S. aureus. On Mannitol Salt Agar, the yellow color indicates mannitol fermentation, a trait typical of S. aureus and not of many other Staphylococcus species. Coagulase positivity is a critical feature, as S. aureus produces coagulase, helping distinguish it from coagulase-negative staphylococci like S. epidermidis and S. saprophyticus. The DNAse test showing a halo around the colonies further supports S. aureus, since many strains secrete DNase. A positive slide latex agglutination test typically detects factors associated with S. aureus, such as clumping factor or Protein A, reinforcing the identification. Clinically, painful, red, swollen skin lesions align with common S. aureus skin and soft tissue infections, such as boils or abscesses. In contrast, the other organisms would not match this combination of traits: a coagulase-negative staphylococcal species would lack coagulase, DNase, and the characteristic golden pigmented, mannitol-fermenting profile, and Streptococcus pyogenes would be Gram-positive cocci in chains, catalase negative, and lack the same Staphylococcus-specific test results. All these converging findings point to Staphylococcus aureus as the culprit.

Recognizing Staphylococcus aureus through its distinctive lab profile helps explain why it fits this scenario. The organism is Gram-positive cocci growing in clusters, and it is catalase positive, which separates Staphylococcus species from Streptococcus (the latter are catalase negative and typically appear in chains). On blood agar it often shows beta-hemolysis with golden, pigmented colonies, a hallmark clue for S. aureus. On Mannitol Salt Agar, the yellow color indicates mannitol fermentation, a trait typical of S. aureus and not of many other Staphylococcus species.

Coagulase positivity is a critical feature, as S. aureus produces coagulase, helping distinguish it from coagulase-negative staphylococci like S. epidermidis and S. saprophyticus. The DNAse test showing a halo around the colonies further supports S. aureus, since many strains secrete DNase. A positive slide latex agglutination test typically detects factors associated with S. aureus, such as clumping factor or Protein A, reinforcing the identification.

Clinically, painful, red, swollen skin lesions align with common S. aureus skin and soft tissue infections, such as boils or abscesses. In contrast, the other organisms would not match this combination of traits: a coagulase-negative staphylococcal species would lack coagulase, DNase, and the characteristic golden pigmented, mannitol-fermenting profile, and Streptococcus pyogenes would be Gram-positive cocci in chains, catalase negative, and lack the same Staphylococcus-specific test results.

All these converging findings point to Staphylococcus aureus as the culprit.

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