SPECTRUM ACTIVITY OF CHEMICAL STERILIZATION

PHYSIO-CHEMICAL METHOD

Mode of action: A physio-chemical method adopts both physical and chemical method. Use of steamformaldehyde is a physio-chemical method of sterilization, which takes into account action of steam as well as that of formaldehyde. Saturated steam at a pressure of 263 mm has a temperature of 70o C. The air is removed from the autoclave chamber and saturated steam at sub-atmospheric pressure is flushed in. Formaldehyde is then injected with steam in a series of pulses, each of 5-10 minutes. The articles are held at this holding temperature for one hour. Formaldehyde is then flushed by inflow of steam.

Disadvantages: Condensation of formaldehyde occurs and induction of large volume of formaldehyde wets the steam resulting in loss of latent heat. Sterilization control: using paper strips containing 106 spores of G.stearothermophilus.

TESTING OF DISINFECTANTS

A disinfectant must be tested to know the required effective dilution, the time taken to effect disinfection and to periodically monitor its activity. As disinfectants are known to lose their activity on standing as well as in the presence of organic matter, their activity must be periodically tested. Different methods are:

 1. Koch’s method

2. Rideal Walker Method

3. Chick Martin test

4. Capacity use dilution test (Kelsey-Sykes test)

 In-use test Koch’s method: Spores of Bacillus anthracis were dried on silk thread and were subjected to action of disinfectants. Later, it was washed and transferred to solid medium.

Rideal Walker method: This method relies on the estimation of phenol coefficient. Phenol coefficient of a disinfectant is calculated by dividing the dilution of test disinfectant by the dilution of phenol that disinfects under predetermined conditions. Both the phenol and the test disinfectant are diluted from 1/95 to 1/115 and 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.

Disadvantages of the Rideal-Walker test are: No organic matter is included; the microorganism Salmonella typhi may not be appropriate; the time allowed for disinfection is short; it should be used to evaluate phenolic type disinfectants only. Chick Martin test: This test also determines the phenol coefficient of the test disinfectant. Unlike in Rideal Walker method where the test is carried out in water, the disinfectants are made to act in the presence of yeast suspension (or 3% dried human feces). Time for subculture is fixed at 30 minutes and the organism used to test efficacy is S.typhi as well     as S.aureus. The phenol coefficient is lower than that given by Rideal Walker method.