Teste de Catalase Falso Positivo em Ágar Sanguíneo: Por Que Acontece e Como Evitar

The catalase test differentiates catalase-positive bacteria (staphylococci, micrococci) from catalase-negative (streptococci, enterococci). Blood agar can sabotage the result.

Principle

Catalase decomposes H₂O₂ → H₂O + O₂ (bubbles). Reagent: 3% H₂O₂ applied to bacterial colony.

False Positive on Blood Agar

• Blood contains catalase in erythrocytes (heme catalase).
• When colony is picked from blood agar, red cells adhere.
• H₂O₂ contacts erythrocyte catalase → bubbles → false attribution to bacteria.
• Especially problematic for catalase-negative organisms (streptococci) that may appear positive.

Prevention

1. Use non-blood media (nutrient agar, chocolate agar, TSA) for catalase test.
2. If blood agar only available: pick colony carefully with minimal agar/blood; or use slide method with colony emulsion in saline.
3. Include controls: known catalase-positive (S. aureus) and negative (S. pyogenes).

Additional Pitfalls

• Old H₂O₂ (degraded) → false negative.
• Metal loop (catalase-like activity)—use platinum, nichrome, or wooden stick.
• Some anaerobes produce catalase slowly; wait 30–60 seconds.

Proper technique ensures accurate Gram-positive cocci differentiation.