Question

All of the following species are considered coliforms, except :

• A. Enterobacter aerogenes
• B. Klebsiella pneumonia
• C. Salmonella typhi
• D. Escherichia coli

Hint:

A key biochemical difference in microbiology is that true coliforms (like *E. coli* and *Enterobacter*) are lactose-fermenting bacteria, whereas major pathogens like *Salmonella* and *Shigella* are lactose non-fermenting bacteria.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Coliform bacteria are defined as Gram-negative, rod-shaped, non-spore-forming, aerobic or facultatively anaerobic bacteria that can ferment lactose with the production of acid and gas within 48 hours at 35 \(^\circ\)C.
They are widely used as bacterial indicators of water quality and sanitary conditions.

Step 2: Detailed Explanation:
The coliform group includes several genera of the family Enterobacteriaceae:
- Escherichia coli: A fecal coliform that is a primary indicator of direct fecal contamination from warm-blooded animals.
- Enterobacter aerogenes: A coliform often found in soil and vegetation, indicating organic enrichment.
- Klebsiella pneumoniae: A coliform that can survive in water and soil, often associated with industrial wastes.
All three species are lactose-fermenting, gas-producing organisms, classifying them as coliforms.
In contrast, Salmonella typhi is a Gram-negative, rod-shaped enteric bacterium, but it does NOT ferment lactose (it is a lactose non-fermenter).
Because it lacks the ability to ferment lactose with acid and gas production under standard conditions, it is not classified as a coliform bacterium, despite being a major intestinal pathogen.
Therefore, Salmonella typhi is the exception.

Step 4: Final Answer:
Among the options, Salmonella typhi is not considered a coliform bacterium.