Disinfection - Characteristics, Classification, Method and Evaluation

Disinfection - Characteristics, Classification, Method and Evaluation.

Disinfection is a process of destruction or removal of pathogenic micro-organisms capable of giving rise to infections. Disinfectants are capable of killing vegetative bacteria, fungi, viruses, and bacterial spores.
Disinfectants are an agent, usually a chemical, that kills the growing form but not necessarily the resistant spore forms of disease-producing micro-organisms.
Disinfection

Characters of ideal disinfectant:

An ideal disinfectant or antiseptic should have the following properties.
  • It should be able to kill all kinds of microbes, in a very diluted form, in a relatively short time.
  • Its efficacy should not be reduced in the presence of blood, feces or other organic matter.
  • It should be non-toxic to animals or humans.
  • It should be soluble in water.
  • It should have good penetrating properties.
  • It should not separate on standing.
  • Finally, it should be cheap and easily available.
  • It should not have a bad odor it should be odorless.
  • Should be speedily in action.
  • Once applied, it should be active for a long duration.

Classification of disinfectants

A. Natural – Air and sunlight prevent the growth of micro-organisms. UV rays present in sunlight also have disinfecting action.
B. Physical – Dry heat, Moist heat, and radiations discussed under Sterilization.
C. Chemical – Solid, liquid, and Gaseous.

A major group of disinfectants

According to drug and cosmetic rules, disinfectants are classified as :
1. Halogens
2. Phenol and phenolic compounds
3. Heavy metals
4. Alcohols
5. Dyes
6. Detergents
7. Quaternary ammonium compounds
8. Aldehydes
9. Gaseous agents

Explanation

1. Halogens

(I). Chlorine:- Chlorine is available in organic, inorganic and Gaseous form, mainly employed to keep bacteria population at low levels. It is used for municipal drinking water.
(II). Iodine:- It is more reactive and germicidal.
(III). Tincture iodine:- It is commonly used as antiseptic for wounds.

2. Phenols and phenolic compounds

phenol are used to reduce the infection of surgical wounds. phenol related compounds are called phenolics are also used as effective disinfectants.
Ex – Lysol, Detol.
It is used as disinfectants and in many commercial antiseptic and disinfectants preparation. Aqueous solutions of phenol are used to disinfect material like urine, faceses etc.

3. Heavy metals

Such as mercuric chloride ( HgCl2 ). As an antiseptic for treating skin disease. HgCl2 and oxycyanide are very toxic and inactivated by organic matter.
Mercury:- It is mostly used in ointments and antiseptics.
Copper:- Copper sulfate is most effective against algae, it is used in swimming pools and open water reservoirs.
Silver:- Silver nitrate, silver sulphadiazine cream are used in treatment of burns.

4. Alcohols

Aliphatic alcohol – ethanol, isopropanol used for disinfection and they are bactericidal against vegetative forms but are not sporicidal 70% ethanol kills mycobacterium and some viruses. 70% isopropyl alcohol used for skin disinfectants.

5. Dyes

Certain dyes can be used as antimicrobial agents such as aniline, acridine. Dyes are effective probably by binding to nucleic acid thus responsible for the mutation in DNA. Mostly inhibitory for gram-positive bacteria and fungi.
For the selective growth of microbes, dyes are used as media components.

6. Detergents

Surface tension reducers are employed primarily for cleaning surface is called detergents. The different form are cationic are popularly used as disinfectant. They are used to sanities food and beverage utensils in restaurants and homes. They are also used as a disinfectant in cosmetic preparation.

7. Quaternary ammonium compounds

Most compounds of the germicidal cationic- detergent class are quaternary ammonium salts . The bactericidal power of the quaternaries is exceptionally high against gram-positive bacteria, and they are also quite active against gram-negative organisms.
They are used as skin disinfectants, as a preservative in ophthalmic solution, and in cosmetic preparations.

8. Aldehydes

Two of the most effective are formaldehyde and glutaraldehyde. Both are highly microbicidal, and both have the ability to kill spores (sporicidal).
Formaldehyde in solution is useful for sterilization of certain instruments. Formaldehyde in Gaseous form can be used for disinfection and Sterilization of enclosed areas. Formaldehyde is an extremely reactive chemical.

9. Gaseous agents :

I. Ethylene oxide – it is a liquid at a temperature below 10.8°C. Above this temperature, it vaporizes rapidly. Vapors of this compound in the air are highly flammable even in low concentrations.
The mode of action of Ethylene oxide is believed to be an alkylation reaction with organic compounds such as enzymes and other proteins. Ethylene oxide has been established as an effective Sterilizing agent for heat and moisture-sensitive materials.

Factors affecting disinfection

1. Concentration
The effectiveness of disinfectants is generally related to concentration exponentially, not linearly. The optimum concentration of phenol is at about 1% beyond this concentration the effectiveness of phenol becomes increasingly less.
2. Time of contact
Sucient time of contact must be allowed for the disinfectant to exert its action. The contact depends on the nature of disinfectants, concentration, pH, temperature, nature of the organisms.
3. Temperature
The disinfection can be treated as a first-order reaction. As the temperature increased on bactericidal activity of disinfectants is increased.
4. The pressure of organic matter and other inactivators
The presence of organic matter may reduce the effect of a disinfectant by adsorbing or inactivating it.
5. pH
The bacterial growth is optimal in the pH range 6-8. A change of pH during the disinfection process can affect the rate of growth of inoculum, the potency of the disinfectant, the ability of the disinfectant to combine with the site on the cell surface.

Evaluation of disinfectants.

1. Minimum inhibitory concentration (MIC).
2. Phenol coeffcient test.
3. Chick Martin test.
4. Capacity test ( Kelsey and Sykes test)
5. Dilution test

Explanation :

1. Minimum inhibitory concentration (MIC):- This test measures the lowest concentration of the disinfectant that growth of salmonella typhi in a nutrient medium.

2. Phenol coefficient test :
1) In this test, a similar quantity of organisms is added to the rising dilution of phenol and disinfectant to be tasted.
2) The organisms used are Salmonella typhi, HIM typhi, Staphylococcus aureus, pseudomonas aeruginosa are used.
3) For testing two methods are mainly used i.e. Rideal- walkar test and chickmartin can be used.
4) Organic matter is also used to stimulate the natural situation.
5) Dilution of disinfectant in question which kills the organisms at the given time is divided by the dilution of phenol which kills the organisms at the same time. This gives the phenol coefficient.
6) A phenol coefficient of 1.0 means that the disinfectant in question has the same effectiveness as a phenol coefficient of less than 1.0 mean it is less effective and more than 1.0 means more effective.

3) Chick Martin test:- Disinfectants were required to act in the presence of organic matter, was pointed by chick and Martin in 1908 who recommended that dried human feces should be added to the test system. Later, Garrod suggested the use of dried yeast as a substitute.
1) The test is conducted by inoculating test organisms salmonella typhi into solution containing disinfectants or phenol.
2) All solutions containing dried yeast the contact period is 30min. At 20°C.
3) At the end of this period, subculture is made in duplicate into the nutrient broth.
4) Subculture tube are incubated for 48 hrs at 37°C .
5) The presence and absence and growth noted the concentration of phenol is disinfectant which prevents growth in broth is determined.

4. Capacity test ( Kelsey and Sykes test ):- capacity test is designed to simulate the natural conditions under which the disinfectants are used in hospitals.
1) Test organisms ( S. aureus, E coli, P. aeruginosa and Proteus vulgaris ) in both clean ( test bacteria in broth)  and dirty ( bacteria in 20% inactivated horse serum or 2% yeast suspension ) condition are added to the disinfectant in 3 successive lots at 0, 10 and 20 minutes.
2) Each addition is in contact with the disinfectant for 8 minutes, therefore, samples are transferred at 8, 18 and 28 minutes respectively to a recovery medium.

5. Dilution test:-

1) In this test, disinfectants are used in diluted form. This method employs in 3- strain of bacteria-       a. S.aureus.
b. Salmonella typhi.
c. P.aeruginosa.
2) Standarish culture of test organisms is grown in liquid media.
3) Standarish metal carrier ring is dipped into liquid culture media are removed and dried at 37°C for short time.
4) The dried culture is then placed into a diluted solution of disinfectant for 10 min.  at 20°C.
5) The effectiveness of disinfectant is then determined by the number of the organism in resulting culture.