In microbiology culture techniques, if you add a substance to a solid growth medium that inhibits the growth of unwanted bacteria while allowing the desired bacteria to grow, what is this type of culture medium called?

Introduction / Context:
Selecting the correct culture medium is fundamental in diagnostic microbiology and research. When mixed samples contain both target and non-target organisms, we often rely on media that either suppress competitors or help visually differentiate similar colonies. Understanding the definition of a selective medium versus enriched, enrichment, and differential media helps you choose the right tool for isolation and identification.

Given Data / Assumptions:

• The scenario explicitly states that a substance is added to a solid medium.
• This additive inhibits unwanted bacteria.
• The desired (wanted) bacteria are permitted to grow.
• We are classifying the medium by function.

Concept / Approach:
A selective medium contains inhibitory agents (for example, bile salts, dyes, antibiotics, high salt) that suppress specific groups while allowing the target group to grow. This differs from an enriched medium (extra nutrients for fastidious organisms, no inhibition), an enrichment medium (usually liquid, conditions that favor a target’s proliferation before plating), and a differential medium (indicators that distinguish colonies by metabolic traits without necessarily inhibiting others).

Step-by-Step Solution:

Verification / Alternative check:
Classic examples: MacConkey agar is both selective (bile salts, crystal violet inhibit Gram-positives) and differential (lactose fermentation indicator). The selective aspect directly addresses inhibition, validating the choice.

Why Other Options Are Wrong:

• Differential medium: distinguishes organisms (for example, color change) but does not necessarily inhibit others.
• Enriched medium: supplies additional nutrients; no inhibition intended.
• Enrichment medium: commonly liquid and designed to favor target growth pre-isolation; not merely about inhibition on solid plates.

Common Pitfalls:
Confusing “enrichment” with “selective.” Enrichment favors a target’s growth but is not defined strictly by inhibiting non-targets on a solid plate. Also, assuming “differential” implies selection; it does not.

Final Answer:
selective medium