Media definitions in bacteriology When a substance is added to a liquid culture medium that inhibits unwanted bacteria while favoring the growth of the target organism, this medium is best described as:

Introduction / Context:
Precise terminology for culture media guides correct laboratory choices. “Selective,” “enrichment,” and “differential” are commonly confused. The wording of the question emphasizes inhibition of unwanted flora plus relative advantage to the target organism in a liquid medium.

Given Data / Assumptions:

• A substance is added that suppresses competitors.
• The medium is liquid.
• The goal is to recover a “wanted” organism from a mixed specimen.

Concept / Approach:
Selective media contain agents (for example, bile salts, dyes, antibiotics, high salt) that inhibit unwanted microbes while permitting growth of the desired group. Differential media allow visual distinction (for example, pH indicators) without necessarily inhibiting others. “Enrichment medium” usually refers to a liquid formulation that favors a target by providing conditions or nutrients (for example, alkaline peptone water for Vibrio) and may not always rely on direct inhibitors; however, the explicit mention of inhibition aligns squarely with “selective.”

Step-by-Step Solution:
Read the key phrase: “inhibits the growth of unwanted bacteria.”Map this to the definition of selective medium.Confirm that differential and enriched media do not primarily act via inhibition.Choose “Selective medium.”

Verification / Alternative check:
Examples include MacConkey agar (bile salts and crystal violet inhibit Gram-positives) and mannitol salt agar (7.5% NaCl selects staphylococci). Liquid analogs can also be selective when they include inhibitors.

Why Other Options Are Wrong:

• A: Differential media reveal biochemical traits but are not primarily inhibitory.
• B: Enriched media add nutrients (for example, blood, serum) to support fastidious organisms without inhibiting others.
• C: Enrichment media favor a target organism but the question emphasizes inhibition, making “selective” the precise term.
• E: Transport media preserve viability, not selection.

Common Pitfalls:
Confusing enrichment with selective. Some enrichment broths can be selective, but the defining feature in this prompt is inhibition.

Final Answer:
Selective medium