Which characteristic best defines coliform bacteria?

Question

BlackTom AI · Accepted Answer

Coliform bacteria are defined as Gram-negative, non–spore-forming rod-shaped bacteria that can ferment lactose with acid and gas production within 48 hours under standard incubation conditions.

1) Relevant knowledge points and concepts
- Taxonomy/shape: Coliforms are Gram-negative rods (bacilli), not Gram-positive organisms.  
- Physiology: They are facultative anaerobes that ferment lactose.  
- Diagnostic criterion: Standard microbiological water and food testing defines coliforms by their ability to produce gas (and acid) from lactose fermentation within 48 hours at around 35–37°C.  
- Examples and significance: Typical genera include Escherichia, Enterobacter, and Klebsiella; they are used as indicator organisms for fecal contamination or sanitary quality of water/food.

2) Step-by-step reasoning
- Compare each option to the coliform definition.  
- Option saying “Gram-negative rods that ferment lactose with gas production within 48 hours” matches the diagnostic criteria used in standard tests (e.g., MacConkey agar, lactose broth, multiple-tube fermentation).  
- Other options propose traits (obligate anaerobe, Gram-positive, endospore formation, catalase-positive cocci) inconsistent with the coliform profile.

3) Intermediate steps / evidence
- Standard tests: A sample incubated in lactose-containing medium that shows acid and gas within 48 hours is counted as containing coliforms.  
- Gram stain: Coliforms stain Gram-negative (pink) and appear as rods microscopically.  
- Endospore formation is absent in coliforms; presence would indicate genera like Bacillus or Clostridium (Gram-positive rods).

4) Why the correct answer is correct
- The phrasing “Gram-negative rods that ferment lactose with gas production within 48 hours” precisely matches the accepted microbiological definition and testing criteria for coliform bacteria.

5) Why the other options are incorrect
- “Obligate anaerobic bacteria found in soil”: coliforms are facultative anaerobes and are not defined by soil habitat.  
- “Gram-positive rods that produce endospores”: endospore-forming rods (e.g., Bacillus, Clostridium) are Gram-positive and not coliforms.  
- “Gram-positive cocci that produce catalase”: Catalase-positive cocci describe Staphylococcus species (Gram-positive), not coliforms.

6) Conclusion
The correct answer is “Gram-negative rods that ferment lactose with gas production within 48 hours,” which matches the provided answer.

Which characteristic best defines coliform bacteria?多项选择题

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