🧪 Enterococci: Pseudocatalase Positive – Don't Let It Fool You

FRCPath Prep Medical Microbiology Consultants
Jul 26
2 min read

Published on: July 26, 2025

Author: Team FRCPathPrep

Category: Laboratory Diagnostics, Gram-Positive Cocci, FRCPath Essentials

🧬 What’s the Trick?

In the lab, Enterococcus spp. are traditionally classified as:

Gram-positive cocci, often in pairs/short chains
Catalase-negative organisms

But here’s the FRCPath-relevant caveat:

❗ Enterococci may show weak bubble formation on catalase testing – this is due to pseudocatalase activity, not true catalase production.

🔍 What Is Pseudocatalase?

Pseudocatalase activity refers to partial or weak breakdown of hydrogen peroxide observed during the catalase test, typically resulting in:

Few or small bubbles (unlike the brisk bubbling in Staphylococcus spp.)
Variable results depending on culture conditions, agar type, and age of colonies

This can lead to false interpretation as catalase-positive, especially by junior staff or during OSPE stations.

🧫 Lab Practical Tip

Test Feature	Staphylococcus spp.	Enterococcus spp.
Catalase activity	Strong (true positive)	Negative or weak (pseudo)
Bubble formation	Brisk, immediate	Sparse/slow, often delayed
Enzyme source	Catalase	Pseudocatalase (peroxidase)
Confirmatory action	Repeat on nutrient agar Use known control strains	Same – never trust catalase alone!

🧠 FRCPath Exam Tip

In both MCQs and OSPEs, remember:

If you see Gram-positive cocci in chains, weak catalase reaction, and growth in 6.5% NaCl → think Enterococcus, not Staph.
Catalase testing is not definitive for differentiating enterococci – use in combination with PYR, bile esculin, and NaCl growth.

❗ Never base clinical decisions on weak catalase results alone.

🔬 Why This Matters in Real Life

Misidentification of Enterococcus as Staphylococcus can lead to inappropriate therapy.
For example, E. faecalis may be mistaken for S. aureus and wrongly treated with flucloxacillin, which has no activity against enterococci.

💡 Key Takeaway

Enterococci = Catalase-negative Weak bubbling ≠ true catalase → it's pseudocatalase. Confirm with biochemical or MALDI-TOF identification.