15. DISINFECTANTS

Introduction

Disinfectants are chemical substances used to kill or inhibit harmful microorganisms present on non-living surfaces. They are essential in hospitals, laboratories, pharmaceutical industries and households to control infections. Disinfection does not always kill spores, but it reduces microbial contamination to a safe level.

Difference Between Antiseptics and Disinfectants

  • Antiseptics: Safe to apply on living tissues such as skin and mucous membranes.
  • Disinfectants: Used only on non-living objects like floors, equipment and surfaces.

Ideal Properties of a Disinfectant

  • Broad antimicrobial activity
  • Fast action even at low concentration
  • Stable during storage
  • Non-corrosive and non-staining
  • Safe for users and environment
  • Active in presence of organic matter
  • Odourless and economical

Factors Affecting the Action of Disinfectants

  • Concentration: Higher concentration usually increases effectiveness.
  • Contact time: Longer exposure improves microbial kill.
  • Temperature: Warm temperature enhances disinfectant action.
  • pH: Some work better in acidic or alkaline conditions.
  • Organic matter: Blood, pus and dirt can reduce activity.
  • Type of microorganism: Spores and viruses may be more resistant.
  • Surface condition: Rough surfaces may protect microbes.

Mechanism of Action

Disinfectants act through one or more of the following mechanisms:

  • Coagulation and denaturation of microbial proteins
  • Disruption of cell membranes
  • Oxidation of essential cell components
  • Interference with metabolic processes

Common Types of Disinfectants

1. Alcohols

Ethyl alcohol and isopropyl alcohol (60–90%) are widely used. They act by protein denaturation and membrane damage. They work well on bacteria but cannot kill spores. Used for skin swabbing and disinfecting small instruments.

2. Aldehydes

Formaldehyde and glutaraldehyde are strong disinfectants. They destroy microorganisms by cross-linking proteins.

  • Formaldehyde: Used for room fumigation.
  • Glutaraldehyde (2%): Used for endoscopes and heat-sensitive instruments.

3. Phenolic Compounds

Phenol and its derivatives damage cell membranes. They are effective against bacteria, fungi and some viruses.

  • Cresol
  • Chloroxylenol (Dettol)
  • Hexachlorophene

4. Halogens

Iodine and chlorine compounds act by oxidation of microbial components.

  • Iodine tincture: Used as antiseptic on skin.
  • Povidone-iodine: Broad spectrum and less irritant.
  • Sodium hypochlorite: Used for surface disinfection (0.5% to 1%).

5. Heavy Metals

Silver and mercury compounds inhibit microbes by protein precipitation.

  • Silver nitrate (1%): Eye prophylaxis in newborns
  • Mercuric chloride: Historical use, now replaced due to toxicity

6. Surface Active Agents (Quaternary Ammonium Compounds)

These damage cell membranes and reduce surface tension.

  • Benzalkonium chloride
  • Cetrimide

7. Oxidizing Agents

Hydrogen peroxide and potassium permanganate act by releasing oxygen, which destroys cell components.

  • Hydrogen peroxide (3%): Used for wound cleansing, surface cleaning.
  • Potassium permanganate: Used in skin infections and water treatment.

8. Gaseous Disinfectants

Used for sterilizing rooms, equipment and heat-sensitive materials.

  • Formaldehyde gas
  • Ethylene oxide

Ethylene oxide is useful for plastics, catheters and surgical instruments.

Tests for Evaluating Disinfectants

1. Rideal-Walker Test

This classical test compares the disinfecting strength of a chemical with phenol against Salmonella typhi. Results are expressed as a phenol coefficient. Higher coefficient means stronger activity.

2. Chick-Martin Test

A modified version of the Rideal-Walker test that includes organic matter to simulate real conditions.

3. Kelsey-Sykes Test

This is a practical test used in modern labs. It checks how disinfectants perform when repeatedly challenged with microbes.

Applications of Disinfectants

  • Cleaning hospital floors and surfaces
  • Disinfecting medical devices
  • Pre-cleaning before sterilization
  • Water treatment and waste management
  • Laboratory bench disinfection
  • Pharmaceutical manufacturing rooms

Safety Precautions

  • Use correct concentration as recommended.
  • Avoid mixing different disinfectants.
  • Wear gloves and protective clothing.
  • Ensure proper ventilation during use.
  • Store chemicals safely, away from sunlight and heat.

Key Points

  • Disinfectants are used on non-living surfaces, while antiseptics are for living tissues.
  • Factors like concentration, time and organic matter affect disinfectant activity.
  • Common disinfectants include alcohols, phenols, halogens and aldehydes.
  • Tests like Rideal-Walker and Chick-Martin help compare disinfectant effectiveness.
  • Proper handling and correct concentration are essential for safe use.

Detailed Notes:

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