Selective agents in intestinal bacteriology: effect of crystal violet and sodium deoxycholate A medium fortified with crystal violet dye and sodium deoxycholate will preferentially allow which group of organisms to grow while inhibiting many competitors?

Introduction / Context:Enteric bacteriology relies on selective media to recover target organisms from complex specimens. Crystal violet and bile salts (or sodium deoxycholate) are classic inhibitors of Gram-positive bacteria and many nonenteric species. Understanding which organisms are favored by such media is essential for diagnostic workflows.

Given Data / Assumptions:

• The medium contains crystal violet (inhibits many Gram-positive bacteria).
• It also contains sodium deoxycholate (a bile salt that suppresses many Gram-positives and nonenteric Gram-negatives).
• Target group is intestinal Gram-negative rods (Enterobacterales and related).

Concept / Approach:

Crystal violet plus bile salts form the basis of MacConkey-like or deoxycholate agars, which select for Gram-negative enteric bacteria while differentiating lactose fermenters. Gram-positive organisms are largely inhibited; environmental aquatic bacteria not adapted to bile are also suppressed.

Step-by-Step Solution:

Verification / Alternative check:

Standard manuals list MacConkey and deoxycholate agars as selective for enteric Gram-negative bacilli, exploiting bile tolerance and dye inhibition.

Why Other Options Are Wrong:

• Gram-positive intestinal bacteria: Inhibited by crystal violet and bile salts.
• Aquatic bacteria: Often sensitive to bile; not the selection target.
• None of these / obligate anaerobes: Anaerobes are usually not recovered on such aerobic, bile-containing media.

Common Pitfalls:

• Assuming crystal violet alone is differential; it is primarily selective.
• Confusing selectivity with lactose fermentation differentiation (which requires lactose and an indicator).

Final Answer:

Gram-negative intestinal bacteria