Selective–differential media: Which medium best differentiates colonies of Vibrio cholerae and Vibrio parahaemolyticus in routine lab practice?

Introduction:
Distinguishing Vibrio cholerae from Vibrio parahaemolyticus is a core diagnostic task. The proper choice of selective–differential medium allows rapid presumptive identification by colony color and sugar fermentation characteristics.

Given Data / Assumptions:

• TCBS agar contains sucrose and indicators that differentiate sucrose fermenters from nonfermenters.
• V. cholerae ferments sucrose (yellow colonies), while V. parahaemolyticus typically does not (green colonies).
• Other listed media are not as specifically differentiating.

Concept / Approach:
Use the medium whose formulation is designed to separate Vibrio species by sucrose fermentation and bile tolerance. TCBS is the classic medium for this purpose in clinical microbiology.

Step-by-Step Solution:
Step 1: Recall the color reactions on TCBS: yellow for sucrose fermenters, green for nonfermenters.Step 2: Map species to reactions: V. cholerae → yellow; V. parahaemolyticus → green.Step 3: Note that alkaline bile salt agar is mainly enrichment/selective, not strongly differential for these two.Step 4: MacConkey targets lactose fermentation among Enterobacterales; Vibrio differentiation is limited.Step 5: Therefore, TCBS is best.

Verification / Alternative check:
Standard lab manuals and exam guides consistently teach TCBS agar for isolating and differentiating Vibrio species, especially V. cholerae and V. parahaemolyticus by colony color.

Why Other Options Are Wrong:

• Alkaline bile salt agar: useful enrichment but not reliably differential by colony color for these species.
• MacConkey agar: poor differentiation of Vibrio species and may inhibit some vibrios.
• All of the above: incorrect because the differential power is not equal.
• BCYE: for Legionella, not Vibrio.

Common Pitfalls:
Confusing lactose-based differentiation (MacConkey) with sucrose-based differentiation (TCBS).

Final Answer:
Thiosulfate–citrate–bile salts–sucrose (TCBS) agar.