PHENOL
COEFFICIENT
Phenol Co-efficient
 is a measure of the microbicidal activity of a chemical compound in relation
to phenol under identical conditions.
 When listed numerically, the figure expresses the disinfecting power of a
substance by relating it to the disinfecting power of phenol, which may be a
function of the standardized test performed.
 In this test, similar quantities of microorganisms are
added to rising dilutions of phenol & of the disinfectant
to be tested.
In U.K, the organism used is Salmonella typhi and in U.S.A,
Salmonella typhi, Staphylococcus aureus and Pseudomonas
aeruginosa are used.
The phenol coefficient test includes :
• Chick-Martin Test (CM Test)
• United States Food & Drug Administration Test (FDA
Test)
• The US Association of Official Agricultural Chemist Test
(AOAC Test)
• Rideal-Walker Test (RW Test)
Chick Martin Test :
 This test determines the phenol coefficient of the test
Parameter
disinfectant.
 In this method, the
disinfectants are
made to act in the
presence of a yeast
suspension (or 3%
dried human faeces)
to simulate the presence or organic matter.
 Time for subculture is fixed at 30 minutes and the
organism used to test efficacy is S.typhi as well as
S.aureus.
AOAC Test :
• Includes two test organisms (S. aureus and P.aeruginosa) and
included the disinfectant inactivators in the recovery medium.
• The recovery medium Letheen broth contains the inactivator
Lecithin and Polysorbate 80.
• In separate tests, the bacterial suspensions are added to
standard dilutions of pure phenol and several dilutions of the
test disinfectant.
• After contact time of 5, 10 and 15 minutes, samples are
transferred to the
recovery medium by a standard wire loop.
• When the positive and negative cultures have been recorded,
the result of the test is expressed as phenol coefficient.
NB: It is calculated by dividing the highest dilution of the
disinfectant that kills the test inoculum in ten minutes but not in
five minutes by the dilution of phenol that gives the same result.
Rideal Walker method:
• Phenol is diluted from 1:400 to 1:800 and the test
disinfectant is diluted from 1:95 to 1:115. Their
bactericidal activity is determined against
Salmonella typhi suspension.
• Subcultures are performed from both the test and
phenol at intervals of 2.5, 5, 7.5 and 10 minutes.
• The plates are incubated for 48-72 hours at 37°C.
That dilution of disinfectant which disinfects the
suspension in a given time is divided by that dilution
of phenol which disinfects the suspension in same
time gives its phenol coefficient.
 For 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.
• If a phenol coefficient or R.W coefficient of the given test
disinfectant is 1, it implies that it has the same
effectiveness as phenol.
• If the R.W coefficient is <1, it means that it’s less effective
& if it’s >1, it means that it’s more effective as compared to
phenol.
• Phenol coefficient test is the best known screening test in
which potency of a disinfectant is compared with that of
phenol.
INFERENCE :
This result indicates that the test disinfectant can be diluted six
times as much as phenol and still
possess equivalent killing power for the test organism.
Advantages of the Rideal-Walker test:
•
It’s inexpensive & can be performed quickly.
•
It gives reproducible results in the hands of experienced
workers.
•
It’s valuable so as to eliminate useless products & supply
standards for crude preparations.
Disadvantages of the Rideal-Walker test:
•
No organic matter is included; the microorganism
Salmonella typhi may not be appropriate.
•
The time allowed for disinfection is short.
•
It is used to evaluate phenolic type disinfectants only.
References :
 Dr.Chandrakant Kokre ; “Pharmaceutical
Microbiology – Principles & Applications” ;
Nirali Prakashan ; Page No : 17.14-17.17
 Joan.F.Gardner ; Margaret.M.Peel ;
“Introduction to Sterilization & Disinfection” ;
2 Edition ; Churchill Livingstone Publications
nd
 http://wiki.answers.com/Q/Phenol_coefficien
t_test_in_microbiology#ixzz1t6kiHZGL