Alkaline tolerance in medical microbiology Among common enteric and gastrointestinal pathogens, which bacterium is well known to grow optimally in alkaline conditions (for example, pH about 8.5–9.5) and is deliberately enriched on alkaline peptone water during laboratory culture?

Introduction / Context:
Recognizing the preferred pH ranges of clinically important bacteria is essential for specimen processing and selecting the correct culture media. Vibrio cholerae, the cholera pathogen, has a notable ability to tolerate and even prefer alkaline conditions. Laboratories exploit this property to selectively enrich and recover the organism from fecal samples that contain mixed flora.

Given Data / Assumptions:

• We compare enteric organisms routinely encountered in clinical microbiology.
• Alkaline peptone water (approximately pH 8.5–9.5) is used as an enrichment broth for suspected Vibrio species.
• Typical intestinal commensals and many pathogens prefer near-neutral conditions.

Concept / Approach:
The key is to match organism physiology to laboratory methods. Vibrio cholerae grows rapidly in alkaline peptone water, outcompeting neutrophilic organisms. In contrast, Shigella and Salmonella are neutrophiles; Lactobacilli favor acidic environments (they produce lactic acid and are used in fermented foods). Therefore, the species that “thrives in alkaline pH” is Vibrio cholerae.

Step-by-Step Solution:
Identify the organism associated with alkaline enrichment: Vibrio cholerae.Recall selective/enrichment methods: alkaline peptone water followed by plating on thiosulfate–citrate–bile salts–sucrose agar.Exclude Shigella/Salmonella (neutrophiles) and Lactobacilli (acid-tolerant).Select Vibrio cholerae as the only option aligned with alkaline growth.

Verification / Alternative check:
Standard diagnostic algorithms instruct inoculating feces into alkaline peptone water for several hours and then subculturing; recovery rates for Vibrio increase because competing flora are suppressed at high pH.

Why Other Options Are Wrong:

• Shigella: prefers neutral pH; no alkaline enrichment advantage.
• Lactobacilli: acidophilic; thrive in acidic, not alkaline, media.
• Salmonella: grows best near neutral pH.
• None of the above: incorrect because Vibrio cholerae fits.

Common Pitfalls:
Confusing “selective” with “enrichment.” Alkaline peptone water is an enrichment that leverages pH preference; TCBS is the selective plate.

Final Answer:
Vibrio cholerae