A microbiologist using phenol coefficient test to compare disinfectant efficacy must maintain a critical condition. The phenol coefficient represents the ratio of the dilution of test disinfectant to phenol required to kill bacteria in a specified time under which conditions?

• A Staphylococcus aureus ATCC 6538 killed in 10 minutes but not 5 minutes
• B E. coli ATCC 25922 killed at 20 minutes contact time
• C Bacillus subtilis spores killed in 30 minutes
• D Salmonella Typhi killed at 10 minutes and 5 minutes but not at 2.5 minutes ✓

Explanation

The phenol coefficient test uses Salmonella Typhi as the test organism (Rideal-Walker/AOAC method); the highest dilution of the test disinfectant that kills bacteria at 10 minutes and 5 minutes, but NOT at 2.5 minutes (subcultures from 2.5 min show growth), is compared to the same endpoint dilution of phenol; PC = dilution of disinfectant / dilution of phenol at equivalent endpoints. Staphylococcus aureus is used in some modified methods but the classic standard organism is S. Typhi. Spores are irrelevant as the test uses vegetative organisms.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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