Selective media knowledge: Crystal violet blood agar is selective for which organism group by suppressing competing gram-positive flora such as staphylococci?

Introduction / Context:Selective and differential media streamline recovery of pathogens from mixed clinical specimens. Crystal violet blood agar incorporates crystal violet to inhibit certain gram-positive cocci while allowing growth of targeted streptococci with visible haemolysis.

Given Data / Assumptions:

• Crystal violet suppresses staphylococci and some other gram-positive flora.
• β-haemolytic streptococci (e.g., S. pyogenes) are intended targets to recover from throat swabs.
• Corynebacterium and mycobacteria require specialized media (e.g., Loeffler’s, Lowenstein-Jensen) rather than crystal violet blood agar.

Concept / Approach:

By inhibiting overgrowing competitors, the medium enhances detection of beta-haemolysis and facilitates selection of suspect streptococcal colonies for further testing (e.g., bacitracin, PYR, antigen detection).

Step-by-Step Solution:

Verification / Alternative check:

Laboratory manuals list crystal violet or colistin-nalidixic acid supplements to reduce staphylococcal overgrowth, improving isolation of group A strep.

Why Other Options Are Wrong:

Staphylococcus aureus is inhibited rather than selected.

Corynebacterium diphtheriae is better supported on tellurite media or Loeffler’s serum slope.

Mycobacterium tuberculosis requires egg-based or agar-based selective media and prolonged incubation.

Common Pitfalls:

Confusing “selective” with “differential”; while haemolysis is differential, crystal violet provides the selective component against competitors.

Final Answer:

β-haemolytic streptococci