18. Which one of the following organisms is used for the determination of phenol coefficient of a disinfectant?
(A) Salmonella typhi
(B) Escherichia coli
(C) Candida albicans
(D) Bacillus psychrophilus
Salmonella typhi is the correct organism used for determining the phenol coefficient of a disinfectant.
This standard test, known as the Rideal-Walker method, compares a disinfectant’s bactericidal activity against phenol using Salmonella typhi as the test organism.
Introduction
The phenol coefficient test determines a disinfectant’s potency by comparing it to phenol, using a specific test organism in microbiology labs worldwide. This key phrase—”phenol coefficient of a disinfectant organism”—guides the selection of Salmonella typhi as the standard bacterium, ensuring reliable evaluation of bactericidal efficacy against pathogens. Understanding each option clarifies why this Gram-negative enteric bacterium remains the gold standard.
Test Overview
The Rideal-Walker phenol coefficient method involves diluting phenol (1:95 to 1:115) and the test disinfectant (1:400 to 1:800), then exposing suspensions to kill the organism at 20°C. The coefficient is the ratio of the highest dilution killing the bacterium to phenol’s equivalent dilution; values >1 indicate superior efficacy. This test targets enteric pathogens relevant to water and hygiene disinfection.
Option Analysis
| Option | Explanation | Role in Test? |
|---|---|---|
| (A) Salmonella typhi | Gram-negative typhoid fever causative agent; standard test organism in Rideal-Walker method for phenol coefficient due to sensitivity mirroring real-world disinfection needs. | Correct |
| (B) Escherichia coli | Common Gram-negative coliform used in some broad-spectrum tests but not standard for phenol coefficient, which specifies Salmonella typhi. | Incorrect |
| (C) Candida albicans | Yeast (fungus) for antifungal testing; phenol coefficient focuses on bacteria, not fungi. | Incorrect |
| (D) Bacillus psychrophilus | Psychrophilic (cold-loving) spore-former; irrelevant as test requires mesophilic enteric bacteria at 20°C. | Incorrect |
Limitations and Modern Use
Though historically vital, the phenol coefficient test is less common today due to arbitrary conditions and omission of spores or fungi; alternatives like USEPA AOAC use Pseudomonas aeruginosa. It remains educational for disinfectant standardization in exams and basic labs.
