Selective pH for cholera agent: To selectively recover Vibrio cholerae from mixed specimens, the culture medium is maintained at approximately what alkaline pH?

Introduction / Context:
Vibrio cholerae is an alkaline-tolerant, halophilic organism. Selective enrichment exploits its preference for higher pH to suppress competing gut flora and enhance recovery from clinical or environmental samples.

Given Data / Assumptions:

• We need the typical pH used in alkaline enrichment media.
• Common media include alkaline peptone water (APW) and thiosulfate citrate bile salts sucrose (TCBS) agar.
• Choices include alkaline and acidic values.

Concept / Approach:

Alkaline peptone water is typically adjusted to about pH 8.5–8.6 to favor V. cholerae. This environment inhibits many enteric competitors that prefer neutral pH, increasing the likelihood of isolating vibrios on subsequent subculture to selective agar plates.

Step-by-Step Solution:

Verification / Alternative check:

Laboratory manuals specify APW at pH near 8.5–8.6; V. cholerae grows rapidly within a few hours, after which plating on TCBS (also selective) reveals characteristic yellow sucrose-fermenting colonies for many strains.

Why Other Options Are Wrong:

8.90: also alkaline but not the standard textbook value; 8.5 is the commonly cited benchmark.

6.79 and 3.5: near neutral or acidic, inhibitory to vibrios and not used for selection.

Common Pitfalls:

Over-incubation in APW leading to overgrowth of non-target organisms; confusing enrichment pH with final plating conditions; ignoring transport media requirements.

Final Answer:

8.5