Evaluation Of Antimicrobial Chemical Agents

Evaluation of antimicrobial chemical agents are conducted by one of three general procedure. In each of the technique chemical agent tested against a specified microorganism referred to as the test organism.

Tube Dilution And Agar Plate Technique

1 Liquid water soluble substances appropriately diluted are dispensed into sterile test tube, to which are added a measured amount of test organism. At specific interval, a transfer is made from this tube to the tubes having sterile media that are then incubated and observed for the appearance of growth. It is necessary in this type of procedure to ascertain whether the inhibitory action is bactericidal and not bacteriostatic. This approach can also be used to determine the number of organisms killed per unit time by performing a plate count on samples taken at appropriate intervals.

2 The chemical agent is incorporated into an agar medium or broth, inoculated with the test organism, incubated, and then observed for decrease in the amount of growth or complete absence of growth.

3 A plate of agar medium is inoculated with the test organism, and the chemical agent is placed on this medium. Following incubation, the plate is observed for zone of inhibition around the chemical agent.

For evaluation of gaseous substance, paper strips impregnated with known number of bacterial spores and exposed to the gas under prescribed conditions, after which they are culture for survival.

Phenol Coefficient Method

This procedure is suitable for testing disinfectants miscible with water and extending their antimicrobial action in a manner similar to that of phenol.

The test organism used in this procedure is a specific strain of either Salmonella typhi or Staphylococcus aureus. The temperature at which the test is performed, the manner of making subcultures, the composition of subculture medium, the size of the test tube, and other details of the test are spelled out in the official procedure. Briefly, the test is performed as follows.

 To a series of dilutions of the disinfectant being tested (5 ml per tube), similar additions, in the same amounts, are made to the series of dilution of phenol.  All tubes are placed at 27°C water bath. At intervals of 5, 10, and 15 min, subcultures and made with a koop transfer needle into sterile tubes of medium. The inoculated subculture tubes are incubated and further examined for growth. The greatest dilution of the disinfectant killing the test organism in 10 min but not in 5 min is divided by the greatest dilution of phenol showing the same result. The number obtained by this division is the phenol coefficient of the substance tested.

It should be emphasized that no single microbiological test method is suitable for the evaluation of germicidal chemical for all applications recommended .

Therefore one must exercise care in selecting the test method for a specific chemical agent.

Reference: Microbiology Pelczar