Disinfectants phenol coefficient tests

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... 

Phenol Coefficient Test. The first important attempt at standardizing testing methods was known as the phenol coefficient test (96). It has been modified several times, and is an official AO AC screening test recognized by EPA and PDA. The phenol coefficient test compares the activity of disinfectants to that of phenol, under specific conditions, to give a number that measures the activity of the chemical tested with respect to that of phenol, ie, the phenol coefficient. The AO AC method employs visual examination of bacterial growth in a nutrient medium. The Kelsey-Sykes test (1969) is a modified method popular in British circles. 

Available Chlorine Test. The chlorine germicidal equivalent concentration test is a practical-type test. It is called a capacity test. Under practical conditions of use, a container of disinfectant might receive many soiled, contaminated instniments or other items to be disinfected. Eventually, the capacity of the disinfectant to serve its function would be overloaded due to reaction with the accumulated organic matter and organisms. The chlorine germicidal equivalent concentration test compares the load of a culture of bacteria that a concentration of a disinfectant will absorb and still kill bacteria, as compared to standard concentrations of sodium hypochlorite tested similarly. In the test, 10 successive additions of the test culture are added to each of 3 concentrations of the hypochlorite. One min after each addition a sample is transferred to the subculture medium and the next addition is made 1.5 min after the previous one. The disinfectant is then evaluated in a manner similar to the phenol coefficient test. For equivalence, the disinfectant must yield the same number of negative tubes as one of the chlorine standards. 

Phenol coefficient tests were developed in the early 20th century when typhoid fever was a significant public health problem and phenolics were used to disinfect contaminated utensils and other inanimate objects. Details of such tests can be found in earlier editions of this book. However, as non-phenolic disinfectants became more widely available, tests that more closely paralleled the conditions under which disinfectants were being used (e.g. blood spills) and which included a more diverse range of microbial types (e.g. viruses, bacteria, fungi, protozoa) were developed. Evaluation of a disinfectant s efficacy was based on its ability to kill microbes, i.e. its cidal activity, under environmental conditions mimicking as closely as possible real life situations. As an essential component of each test was a final viability assay, removal or neutralization of any residual disinfectant became a significant consideration. 

In addition to the phenol coefficient tests, a Use-Dilution Confirmation test has been introduced in the United States.The purpose of this test is to confirm that the recommended use-dilution of the disinfectant (calculated from the phenol coefficient of the fluid to be equivalent to that of a 5 per cent phenol solution) is in fact satisfactory. If the response in the test is not satisfactory, then an adjustment must be made to the dilutions recommended for use. 

Other Cresol-type Disinfectants and Antiseptics. Various non-irritant germicides of low toxicity, and therefore particularly suitable for disinfecting the skin and for application to wounds, contain different amounts of chloroxylenol, dichloroxylenol, benzyl cresols, o-phenylphenol, chloro-benzylphenols, special narrow-range boiling fractions of coal tar distillates, etc. They are usually dissolved in a soap, such as castor oil soap, and essential oil solution as they are only sparingly soluble in water. The various phenol coefficient tests, and particularly the Chick-Martin test, are of value in standardising such preparations, but they are of little value in determining their practical efficacy as antiseptics. For such purposes, other more specific tests are required which are beyond the scope of this book. 

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. 

PHENOL COEFFICIENT. In determining the effectiveness of a disinfectant using phenol as a standard of comparison, the phenol coefficient is a value obtained by dividing the highest dilution of the test disinfectant by the highest dilution of phenol that sterilizes a given culture of bacteria under standard conditions of time and temperature. 

Joseph Lister introduced phenol (carbolic acid) as a disinfectant in 1967. It has been the standard disinfectant to which other disinfectants are compared under the same conditions. The result of this comparison is the phenol coefficient. Salmonella typhi, a pathogen of the digestive system, and Staphlococcus aureus, a common wound pathogen, are typically used to determine phenol coefficients. A disinfectant with a phenol coefficient of 1.0 has the same effectiveness as phenol (Dorland s Illustrated Medical Dictionar). A coefficient less than 1.0 means that the disinfectant is less effective than phenol and greater than 1.0 is more effective than phenol. Phenol coefficients are reported separately for the different test organisms. 

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... 

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 dilution tests. 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 coefficient or phenol coefficient was introduced in 1903 (Rideal et al., 1903) and was also taken for the characterization of the killing activity of essential oils toward microorganism (Martindale, 1910). 

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. 

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... 

---