Chemical Methods of Sterilization: Antiseptics and Disinfectants- Definition, Types, Properties, and Uses of Common Disinfectants and Disinfectants (e.g. Formaldehyde, Ethylene oxide, Phenol Compounds, Alcohol, hypochlorite). Definition of Phenol Coefficient and Determination Phenol Coefficient by Rideal Walker Method.

Chemical Methods of Sterilization

There are two types of chemical sterilization. These are

• 1. Sterilisation by heating with bactericides

• 2. Gaseous sterilisation

Sterilization by heating with bacteriocides: It is used to sterilize aqueous preparations not stable at moist heat sterilization temperature. Aqueous preparation is heated at 98 to 100 degrees C with bacteriocides like Chlorocresol, Benzalkonium chloride, phenylmercuric nitrate, chlorhexidine acetate, etc.

Gaseous sterilization: Gaseous chemicals such as formaldehyde, ethylene oxide, Beta propiolactone, etc are used in gaseous sterilization. Formaldehyde is an alkylating agent. Ethylene oxide is a better choice than formaldehyde. Beta propiolactone is liquid at room temperature, noninflammable, and has alow penetration power than ethylene oxide.

Antiseptics and Disinfectants

Antiseptics are antimicrobial agents applied on living surfaces to kill or prevent the growth of microorganisms to reduce or prevent microbial infection or sepsis.

Disinfectants are antimicrobial agents applied on non-living surfaces (inanimate surfaces) to kill or prevent microorganism growth to reduce their number to prevent infection. They do not kill spores.

Classification/Types of antiseptic and disinfectants

• · Acids: Ethanol, Isopropyl Alcohol

• · Aldehydes: Formaldehyde, Glutaraldehyde,

• · Phenol Derivatives: Phenol, Cresol, Paraben, Thymol, Resorscinol, Chlorhexidine, Chlorhexidine Gluconate, Chloxylinol, Hexachlorophene

• · Halogens: Chlorine, Iodine, Iodophores, Chloramine

• · Oxidising Agents: Hydrogen peroxide, Potassium permanganate, Benzoyl peroxide

• · Dyes: Gentian Violet, Methylene Blue, Brilliant Green

• · Heavy Metals: Mercury Compounds, Silver Compounds, Zinc Compounds

Formaldehyde: Formaldehyde is an alkylation agent. It was used in the old days to sterilize syringes, catheters, and other heat-sensitive materials. Nowadays it is only used for the fumigation of empty rooms.

• Properties:

• 1. Physical Properties: Formaldehyde is a colorless gas at room temperature. highly soluble in water. Normally it is stored as an aqueous solution called formalin. Formalin contains 37 to 50% formaldehyde in water. Formaldehyde and formalin have a strong pungent distinctive odor and are highly flammable in nature.

• 2. Chemically it acts as a reducing agent.

• Uses:

• 1. Sterilisation: It was used in the old days to sterilize syringes, catheters, and other heat-sensitive materials. Nowadays it is only used for fumigation of empty rooms.

• 2. Disinfectant: It is used as a disinfectant in various healthcare and cosmetic industries

• 3. Preservative: It is used to preserve personal care products and biological samples like tissues, organs, etc

• 4. Fumigation: It is used to control pests in agricultural and storage facilities

• 5. Preservative: It is used in personal care products as preservatives

Ethylene oxide

• Properties: It is a colorless, flammable gas at room temperature. It has a sweet and ether-like odor. It can be liquefied easily. The boiling temperature of ethylene oxide is 10.8 degrees C. It is highly soluble in water and organic solvents. It is not used alone due to its highly inflammable properties. It is mixed with some inert gases such as carbon dioxide, dichlorodifluoromethane, etc.

• Uses:

• 1. Sterilization: It is used to sterilize delicate diagnostic equipment. It is highly effective at 40 degrees C and in the presence of moisture.

• 2. It has high penetration power. Iy is suitable for sterilization of materials packed in plastic oe cellophane envelops.

• 3. It is suitable to sterilise heat sensitive materials such as rubber and plastic items.

• Phenol compounds: Phenol compounds are also called carbonic acid or hydroxybenzene compounds.

• Properties: It is white crystalline solid at room temperature. It is also available as colorless liquid. It has characteristic sweet odor slightly soluble in water aand readily soluble in organic solvents like ether, acetone and alcohol.

• Uses: Antiseptic and disinfectant. It has bactericidal properties.

• Alcohol: Alcohol, especially ethyl alcohol and isopropyl alcohol is commonly used in medical laboratories.

• Properties: Alcohol evaporates at room temperature and does not leave residue on the surface. It is a colorless liquid with good solvent properties.

• Uses

• 1. Disinfectant: It is the most commonly used disinfectant for working surfaces, equipment, and glassware.

• 2. Hand Sanitizer: cleaning hands and killing living microbes on hands and palm surfaces is called hand sanitization. Alcohol is a good hand sanitizer. Laboratory personnel sanitize their hands before working to reduce the risk of contamination.

• 3. Cleaning and Decontamination: Alcohol is used to clean and decontaminate glass slides cover slips and other medical equipment. It removes dirt, stains, and residues from the surface without leaving any residues.

• 4. Tissue processing: Alcohol is used in tissue processing, tissue fixation, or tissue embedding. Alcohol helps to dehydrate and preserve tissue specimens.

• 5. Preservation of microorganisms: It is used to preserve and store microbial culture in laboratories.

• 6. SpecimenTransport: Alcohol is used to preserve and transport clinical specimens such as urine samples.

• 7. Staining Procedure: Alcohol is used as a solvent to prepare a stain solution.

• 8. Fixation of blood smear: Alcohol is used to fix the blood smear in hematology.

Hypochlorite Compounds: The most commonly used hypochlorite is Sodium Hypochlorite also called bleach.

• Properties: It is a strong oxidizing agent and bleaching agent. It releases oxygen on its breakdown. Its pH is above 11. Its alkaline property makes it a strong disinfectant. It can damage metals and act as corrosive.

• Uses

• 1. Surface disinfectant

• 2. Sterilization

• 3. Instrument and apparatus decontamination

• 4. Sample decontamination

Definition of Phenol Coefficient:

The Phenol coefficient is a numerical value to express the ability of disinfectant and antiseptic to kill or inhibit the growth of the microorganism. It is measured by comparing the concentration of test disinfectant or antiseptic to the concentration of phenol required to achieve the same level of antimicrobial activity.

• Phenol coefficient = Minimum bactericidal concentration of phenol/ Minimum bactericidal concentration of test disinfectant.

• A phenol coefficient greater than one indicates that the test disinfectant is stronger than phenol. A phenol coefficient lower than one indicates that the test disinfectant is weaker than phenol.

Determination Phenol coefficient by Rideal Walker method

Materials and Equipment

• 1. Phenol

• 2. Test disinfectant or antiseptic

• 3. Sterile culture Medium (Nutrient Agar)

• 4. Test microorganism

• 5. Glass apparatus: Petri dish, Test tubes and Pipettes

• 6. Incubator

• 7. Microbiological loop or spreader

• 8. Gloves and other protective kits.

Procedure

• 1. Prepare solutions of different strengths like 0.1%, 0.05%, 0.025%, etc of both phenol and test disinfectant or antiseptic separately

• 2. Inoculate sterile culture medium with test microorganisms such as Staphylococcus aureus using a sterile microbiological loop.

• 3. Allow the culture media to solidify

• 4. Careat a well in inoculated culture media.

• 5. In each well, add a specified volume of a particular dilution of phenol and test disinfectant.

• 6. Incubate the culture plates at 37 degrees C for 24 hours

• 7. After incubation, observe the bacterial growth around the well.

• 8. The concentration at which no bacterial growth is observed is called Minimum Bactericidal Concentration (MBC).

• 9. Calculate the phenol coefficient by the following formula

• 10. Phenol coefficient = MBC of Phenol / MBC of the Test Disinfectant.

It is an older method and not in use.

Dr Pramila Singh