Differentiating Streptococcus pyogenes (group A): Which single laboratory characteristic best distinguishes S. pyogenes from other beta-haemolytic streptococci on primary isolation?

Introduction / Context:
Beta-haemolytic streptococci on blood agar can belong to several Lancefield groups (A, B, C, G). Rapid, inexpensive bench tests help presumptively identify S. pyogenes (group A) in clinical microbiology labs.

Given Data / Assumptions:

• S. pyogenes is classically sensitive to a low-dose bacitracin disk (taxo A).
• Antibiotic class “sensitivity” (penicillin, erythromycin, aminoglycosides) is not a reliable differential screening tool among streptococci on plates.
• Definitive identification may use Lancefield grouping or molecular tests; bacitracin remains a classic presumptive screen.

Concept / Approach:

A simple bacitracin disk placed on a lawn of beta-haemolytic colonies produces a zone of inhibition with S. pyogenes, whereas most other beta-haemolytic groups (e.g., B) are resistant. This is a screening, not confirmatory, test but is widely taught and used.

Step-by-Step Solution:

Verification / Alternative check:

PYR positivity and Lancefield group A antigen testing also support identification; however, the question asks for the classical differentiator on haemolytic plates.

Why Other Options Are Wrong:

Erythromycin, penicillin, and aminoglycoside susceptibility are therapeutic concerns, not diagnostic differentiators in this context.

Common Pitfalls:

Over-reliance on bacitracin alone; always corroborate with antigen detection or molecular methods, especially where non-pyogenes strains may show atypical results.

Final Answer:

Bacitracin sensitivity