CHEMICAL STERILIZATION (DISINFECTANTS AND ANTISEPTICS)

CHEMICAL STERILIZATION (DISINFECTANTS AND ANTISEPTICS)

Mode of Action of Chemical Agents
Chemical agents act on microorganisms in the following ways
MAJOR GROUPS OF DISINFECTANTS

• Alcohols
• Aldehydes

(a) Formaldehyde
(b) Glutaraldehyde

• Phenolic Compounds
• Chlorxylenols
• Salts of Heavy Metals
• Chlorhexidine (Hibitane)
• Halogens

(a) Hypochlorites
(b) Iodophores These consist of iodine
(c) Iodine As alcoholic tincture of iodine
Quaternary Ammonium Compounds
GASEOUS STERILIZATION

• Ethylene Oxide
• Formaldehyde

LABORATORY EVALUATION OF DISINFECTANTS

• the Kelsey Maurer's method.

CHEMICAL STERILIZATION (DISINFECTANTS AND ANTISEPTICS)

Various disinfectants and antiseptics are used for chemical sterilization. They are divided into a number of chemical groups for specific purposes. All chemical agents of disinfection take time to act. 

The rate of disinfection is approximately logarithmic, with a plot of the logarithm of the number of survivors against unit time giving straight line over most of the curve. This means that a constant proportion of surviving bacteria are killed per unit of time.

The actual rate of kill varies due to certain factors, e.g., the nature of the disinfectant and concentration used. Environmental conditions also affect the efficiency of the chemical disinfectants, e.g., the presence of organic matter, the pH of solution and the possible inactivation of chemical compounds such as soap.

Mode of Action of Chemical Agents

Chemical agents act on microorganisms in the following ways:

1. Interfering with the enzymatic system of the organism.

2. Coagulation of proteins.

3. Disruption of cell membrane.

4. Oxidative damage.

MAJOR GROUPS OF DISINFECTANTS:Many different compounds are used as disinfectants, the major groups are:

Alcohols

Alcohols act by denaturing proteins. Absolute alcohol is not a very effective disinfecting agent due to its poor penetrating power. When diluted with water to a concentration of 70 %, it has optimum activity as a skin antiseptic and it is commonly used prior to injection or vain puncture. It is used as an antiseptic.

Aldehydes

(a) Formaldehyde 

Formaldehyde is supplied as 40% solution and known as formalin. It is highly bactericidal and active against spores. It is used in liquid form or in gaseous form when humidity must be around 50%. Its extreme irritating and toxic property is the major disadvantage. It is useful for disinfecting rooms, safety cabinets (fumigation) and sometimes for aspirators and other anaesthetic equipment's.

(b) Glutaraldehyde 

This is used as a 2% buffered solution (Cidex)) and it is highly effective against all bacteria, tubercle bacilli and spores. It is much less irritant than formaldehyde but it is expensive. It has a low penetrating power but useful for items that cannot withstand heat such as endoscopes.

Phenolic Compounds

Phenol or carbolic acid is the oldest known disinfectant. Although, the original chemical is no longer used, some of the compounds derived from it constitute the phenolic substitutes. It is used usually at a concentration of 2-3%. Phenolics are also effective against spores. Their unpleasant odour and tissue irritation preclude their use on the skin. Phenolics such as 'Sudol', 'Hycolin', and 'Izal are however, widely used as good antiseptics when diluted.

Chlorxylenols

The best known of this group of disinfectants is dettol. They differ from other phenolic compounds by being non-irritating, and can be applied to skin and tissues. They are very active against Gram negative and Gram positive bacteria but not against tubercule bacilli and spores. They are moderately inhibited by organic matter.

Salts of Heavy Metals

Salts of heavy metals, e.g., mercuric chloride, are required in high concentrations to achieve any sig. nificant activity. They are slow acting and are mostly bacteriostatic.

Chlorhexidine (Hibitane)

This is a widely used skin disinfectant in alcoholic solution. It is very active against a wide range of organisms, but aqueous solutions are much less effective due to poor activity against spores, tubercule bacilli, and some Gram negative bacteria such as Alcaligenes, Flavobacteria and some Pseudomonas. It is usually used as hand cream in the laboratory after washing hands with soap and water.

Halogens

(a) Hypochlorites  

These include such popular antiseptics as milton, chloros and domestos. They are derivatives of chlorine which is liberated from the solution during their action. As disinfectants, hypochlorites are bactericidal against vegetative bacteria, spores and viruses, but less active against tubercule bacilli. They are relatively inactivated by organic matter.

(b) Iodophores These consist of iodine  

detergent complexes and include such well known brands as Wescodine, Betadine and Pevidine. They are much less irritant to the skin than free iodine and do not stain. They are used for cleaning wounds and as a general purpose laboratory disinfectant for discard jars

(c) Iodine As alcoholic tincture of iodine 

 it is highly active against Gram positive organisms and so is used as a skin antiseptic. It stains the skin.

Quaternary Ammonium Compounds  

These include agents such as cetrimide, bromide and benzalkonium chloride. They have antibacterial and detergent properties and are active against Gram positive bacteria and slightly active against tubercule bacilli and spores. 

This antibacterial activity is antagonised by soaps and certain organisms, e.g., Pseudomonas. They are very useful in catering industry for washing cutlery, and in hospitals for cleaning wounds. Savlon, a popular antiseptic, is a mixture of cetrimide and chlorhexidine and is active against Gram negative bacteria.

GASEOUS STERILIZATION:

Gaseous disinfectants (alkylating agents) are used for the sterilisation or disinfection of items of hospital equipment that is bulky or heat-labile. The most widely used gases are ethylene oxide and formaldehyde.

Ethylene Oxide  

Ethylene oxide has a boiling point of 10.8°C, so it is a gas at room temperature. It is highly inflammable and explosive in pure form, but is safe to handle when mixed with carbon dioxide. It is effective in the killing of all bacteria, including tubercle bacilli and spores. 

To be effective, however, the humidity and temperature must be carefully controlled within narrow limits. It is highly toxic on contact with the skin or mucous membrane.

Materials that have been sterilized with ethylene oxide must be set aside in detoxification chambers for a few days to allow the gases to dissipate. It is frequently used to sterilize heart lung machines and plastic items, e.g., catheters.

Formaldehyde 

It is highly bactericidal. As gas, it is used as 40 % formalin with humidity at around 50%. It is highly irritant. it is used occasionally to fumigate rooms and disinfect respirators.

LABORATORY EVALUATION OF DISINFECTANTS

The aim of evaluation of a disinfectant is to define the capabilities of the disinfectant and to determine reasonable and practicable concentrations for routine use. There are many factors that influence bactericidal activity, and so, conditions of testing must simulate practical usage conditions.

 A suspension test, the 'Rideal Walker or the 'Phenol Coefficient test,' is an old method but hardly related to conditions obtainable in the laboratories. For this reason, it is not very satisfactory

.

Other tests 

such as, 'In-Use Test' and 'Capacity Test' are quite satisfactory. All quantitative tests for bactericidal activity involve the serious technical difficulty of enumerating the few surviving organisms as disinfection proceeds.The In-Use Test, which is simple and easy to perform, is outlined below: In-Use test method This is the Kelsey Maurer's method.
 

Stage 1 l ml disinfectant solution in use is taken from the jar with a sterile pipette.

Stage 2 The 1 ml sample is added to 9 ml diluent in a sterile universal container. The diluent should be selected according to the group to which the disinfectant belongs. This in effect means that the action of the disinfectant should be stopped using the appropriate inactivator. For general purposes, use 0.5% Tween  80

Stage 3 The bottle of diluted disinfectant is transported to the laboratory within an hour of the addition of the disinfectant. A separate sterile Pasteur pipette is used to withdraw a small quantity of the disinfectant/diluent/mixture and to place 10 drops, separately, on the surface of each of two well-dried nutrient agar plates. A 50 dropper pipette may be used in place of graduated pipette.

Stage 4 The two plates are incubated:

(a) One plate is incubated for three days at 32 or 37°C, whichever is more convenient. (The optimum temperature for most pathogenic bacteria is 37°C but those which have been damaged by disinfectants often recover more readily at 32°C.)

(b) The second plate is incubated for seven days at room temperature. Stage 5 After incubation the plates are examined and bacterial growth is recorded. In-Use test result The presence of bacterial colonies on one or both of a pair of plates is evidence of the survival of bacteria in the particular jar, from which the sample was taken. One or two colonies on the plate may be ignored. A disinfectant is not a sterilant. The presence of a few live bacteria is to be expected. The growth of five or more colonies should arouse suspicion and may be indicative of poor bactericidal activity.