A clear halo around colonies on starch agar after iodine application indicates:

Question

BlackTom AI · Accepted Answer

Iodine reacts with intact starch to form a blue-black complex, so a clear zone indicates the starch around the colony has been broken down — starch hydrolysis.

1. Relevant knowledge points and concepts
- Starch agar contains soluble starch as the substrate and agar for solidifying.  
- Many bacteria secrete extracellular amylase (and related enzymes) that hydrolyze starch into smaller sugars (maltose, glucose).  
- Iodine (Lugol’s) binds to amylose/amylopectin and produces a dark blue-black color where starch is present. Where starch has been enzymatically degraded, the iodine will not form the dark complex, producing a clear halo (zone of hydrolysis) around colonies.

2. Step-by-step reasoning process
- Inoculate and incubate starch agar; organisms that produce extracellular amylase grow and secrete enzyme into the agar.  
- After incubation, flood the plate with iodine solution.  
- Observe color: areas with intact starch turn dark blue-black; areas where starch has been hydrolyzed remain unstained (clear).  
- The clear halo therefore corresponds to enzymatic breakdown of starch by the organism.

3. Intermediate procedural steps (no mathematical calculations required)
- Preparation: starch agar poured and solidified.  
- Inoculation and incubation under appropriate conditions.  
- Flood plate with iodine, allow 1–2 minutes for color development, then pour off excess and observe.

4. Why the correct answer is correct
- The clear halo is the classical indicator of starch hydrolysis because iodine only stains intact polysaccharide chains; hydrolyzed products do not produce the colored complex.

5. Why other options are incorrect
- No growth: would show no colony formation; here colonies are present with a halo.  
- Positive fermentation: refers to sugar fermentation in liquid media with pH indicators, not an iodine-starch reaction on agar.  
- Acid production: detected by pH indicators (color change), not by a clearing with iodine on starch agar.

6. Conclusion
The clear halo around colonies after iodine application indicates starch hydrolysis, matching the provided correct answer.

A clear halo around colonies on starch agar after iodine application indicates:单项选择题

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