Water microbiology — Coliform definition check: Which one of the following species is NOT considered a coliform indicator bacterium?

Introduction / Context:
Coliforms are operationally defined, lactose-fermenting, Gram-negative rods used as indicator organisms for fecal contamination in water and food safety testing. Identifying non-coliforms helps avoid misinterpretation of test results.

Given Data / Assumptions:

• Classical coliforms: Escherichia, Enterobacter, Klebsiella, and some Citrobacter.
• Salmonella typhi is a pathogenic enteric bacillus but not a coliform.
• Question asks for the exception.

Concept / Approach:
Coliforms are defined by fermentation of lactose with acid and gas at 35–37°C, oxidase negative, and belonging to Enterobacterales. Salmonella typhi does not ferment lactose and is not used as an indicator organism; it is a specific pathogen detected by targeted culture/serology.

Step-by-Step Solution:
List typical coliform genera: Escherichia, Enterobacter, Klebsiella. Recognize Salmonella typhi as a non-lactose fermenter and a frank pathogen. Select the non-coliform: Salmonella typhi.

Verification / Alternative check:
Standard water testing (MPN, membrane filtration on selective media) targets total and fecal coliforms; Salmonella requires different selective enrichment and serotyping.

Why Other Options Are Wrong:

• Enterobacter aerogenes: a classical coliform.
• Klebsiella pneumoniae: a classical coliform.
• Escherichia coli: the prototypical fecal coliform.

Common Pitfalls:
Assuming all enteric pathogens are coliforms; pathogenicity and indicator status are distinct concepts.

Final Answer:
Salmonella typhi