Page 40 - MNU PM502 Ph. Micreo practical

Pharm D- Clinical Pharmacy Program Third Level Pharmaceutical Microbiology& Antimicrobials (PM 502)

Results:

G: growth, NG: no growth

Calculation of the coefficient:

R. W. coefficient = Highest dilution of test disinfectant killing the microorganism

in 7.5 min not in 5 min / Highest dilution of phenol killing the
microorganism in 7.5 min not in 5 min.

For the above example, after 7.5 minutes, the test organism was killed by the test
disinfectant at a dilution of 1:600. In the same period the test organism was killed

by phenol at a dilution of 1:100.
Phenol coefficient= 1200/100=12

This means that the test disinfectant can be diluted twelve times as much as

phenol and still possess equivalent killing power for the test organism.

Disadvantages of the Rideal-Walker test:

1. No organic matter is included

2. the microorganism Salmonella typhi may not be appropriate

3. the time allowed for disinfection is short

4. It should be used to evaluate phenolic type disinfectants only.

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