Rideal-Walker test

Phenol no longer plays any significant role as an antibacterial agent. It is of historical interest, since it was introduced by Lister in 1867 as an antiseptic and has been used as a standard for comparison with other disinfectants, which are then given a phenol coefficient in tests such as the Rideal-Walker test. 

Testing Disinfectants (Phenol Coefficient Tests). The tests employed for determining the efficacy of all of the coal tar disinfectants include the Rideal-Walker test, the Chick-Martin test, the United States Food and Drug Administration (F.D.A.) method and the United States Association of Official Agricultural Chemists (A.O.A.C.) method, in which the antibacterial activity of the disinfectant is expressed in terms of its action relative to that of phenol. Each of these tests has much in common, and their main features are set out in the table below. It is necessary to follow precisely the directions as laid down in the approved method in order to get reproducible results. Each test wdll give a different coefficient... 

For the Rideal-Walker test, prepare four suitable serial dilutions of the disinfectant and one of the standard phenol in 5-ml amounts in 5 X f in. sterile test-tubes, and cool in a water-bath to 17-18°. At halfminute intervals, add 0-2 ml of the twenty-four-hour culture of Salm. typhi (also cooled to 17 18°) in turn to each of the tubes, and then at subsequent two and a half, five, seven and a half, and ten minute intervals remove one standard loopful of the mixture to a 5-ml tube of the medium, incubate at 37 for forty-eight to seventy-two hours and record the growths obtained. It may be necessary to repeat the test with another 5 dilutions, but using a different phenol dilution, in order to obtain a satisfactory end-point. Calculate the Rideal-Walker Coefficient by dividing the numerical value of the dilution of the disinfectant which... 

Croshaw B. (1981) Disinfectant testing—with particular reference to the Rideal-Walker and Kelsey-Sykes tests. In Disinfectants Their Use and Evaluation of Effectiveness (eds C.H. Collins, M.C. Allwood, S.F. Bloomfield A. Fox), pp. 1-15. London Academic Press. 

The effectiveness of disinfectants should also be evaluated. The approach to this is not as simple as it might seem. Very obviously, the simplest method is chemical analysis of the active ingredient. However, it is usual to find that this is supplem ted by some form of microbiological data. The principles of microbiological testing of disinfectants go back to the Rideal-Walker method of 1903 and probably earlier. Even with this long history, disinfectant testing is not free, nor ever has been free, from criticism and controversy. 

There are a number of tests designed to determine the effect of disinfectants on bacterial populations. Two tests which have been used widely in the past are the Rideal-Walker and Chick—Martin tests for determining the phenol coefficient of disinfectants in a hospital situation. These tests compare the efficiency of the disinfectant with phenol. They should only be used to assess a phenol based... 

The Rideal Walker and Chick Martin tests have been replaced by the Kelsey-Sykes test which assesses the disinfectant at its in use concentration under both clean and dirty conditions. There is a considerable amount of preparation involved and the tests are probably best carried out in specialist laboratories. 

Procedures. The principle of all of the above tests is that serial dilutions of the test disinfectant and of the standard phenol (dilutions in arithmetic series for the Rideal-Walker, F.D.A, and A.O.A.C. tests and in logarithmic series for the Chick-Martin test) are inoculated with a given amount of the culture and at intervals one 4-mm loopful of the mixture is transferred to a tube of the culture medium which is incubated at 37° and the growths noted. By equating the end-point dilution of the disinfectant with that of phenol giving the same response, the phenol coefficient of the disinfectant is obtained. 

The A.O.A.C and F,D.A, methods follow a similar procedure to that of the Rideal-Walker method, except that the test is made at 20° (or 37°), 0-5 ml of the culture is added to each of the 5 ml disinfectant or phenol dilutions, the time intervals for subculturing are five, ten and fifteen minutes, and ten dilution tubes are required for each test, comprising eight of the disinfectant and the two of phenol. For ordinary coal tar disinfectants, the medium as given in the table (p. 206) is satisfactory, but V here they contain surface-active compounds of the quaternary ammonium type a Letheen medium is specified, which is the same basic medium with the addition of 0-07 per cent lecithin (Azolectin) and 0 5 per cent Tween 80. The Staph, aureus test is made at 20° on disinfectants intended for external use, and at 37 for those intended for personal use or for application to wounds. 

Questions concerning disinfectant activity of essential oils, for example, the minimum time needed to kill a given microbial species or the determination of microbial survivors after short time contact, are not answered by agar diffusion or DILs. In older literature, the killing concentration relative to phenol was determined after 15 or 30 min exposure of the respective microbials species to the compound to be tested. The so-called carboxylic acid coef cient or phenol coef cient was introduced in 1903 (Rideal and Walker, 1903) and was also taken for the characterization of the killing activity of essential oils toward microorganism (Martindale, 1910). 

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