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Sterilization is the process by which an article, surface, or medium is freed of all living microorganisms either in the vegetative or spore state.

Disinfection is a process of destruction or degeneration of vegetative forms of pathogenic organisms that are capable of producing infection but not necessarily resistant spores.

Chemical disinfectants can be safely applied to your skin or mucous membrane and are used to prevent infection by inhibiting the growth of bacteria.

It is a process designed to prevent microorganisms from reaching a protected environment.

It is a process of removing soil and other dirt and reducing the microbial burden.

It is a process of rendering an article or area free of danger from contaminants, including microbial, chemical, radioactive, and other hazards.


Physical agents:

  • Sunlight: It is due to ultraviolet rays and heat rays.
  • Drying: It is an unreliable, only theoretical interest.
  • Heat (Moist heat and dry heat):
    • It is the most reliable method
    • This method of choice unless contraindicated.
  • Filtration
  • Radiation
  • Ultrasonic and sonic vibrations

Chemical agents:

  • Alcohols
  • Aldehydes
  • Dyes
  • Halogens
  • Phenols
  • Surface-active agents
  • Metallic salts
  • Gases: ethylene oxide, formaldehyde, beta propiolactone


Dry heat:
Mechanism of Action: destructive oxidation of intracellular components of an organism.
They include:
A. Flaming: For Scalpels, needles, mouths of culture tubes, glass slides, coverslips, etc.
B. Heating to read hot: For Inoculatnng loops or wires, points of forceps,
searing spatulas, etc.
C. Incineration: For Contaminated cloth, animal carcasses, and pathological materials Substances are reduced to ashes by burning.
D. Hot air oven:

  • Holding period: 160°C for 1 hour(170°C for 40minutes,180°C for 20 minutes).
  • For glassware, forceps, scissors, scalpel, syringes, swabs, pharmaceutical products include liquid paraffin, fats, grease, etc.
  • Sterilization control: Browne’s tube (green spot), Spores of Bacillus subtilis.

Moist Heat:
Mechanism Of Action: Denaturatnon and coagulation of proteins of an organism.
At temp. below 100°C: For vaccine bath, pasteurization, fractional sterilization (inspissator).
At temp. of 100°C: boiling at 100°C, steam at atmospheric pressure at 100°C for 90 minutes, Steaming at 100°C for 3 successive days (Tyndallisation)
At temperature above 100°C: autoclave

Domestic pressure cooker– The simplest form of an autoclave.

Simple iron-jacketed;

  • Low-pressure low temperature: 121°C for 15 minutes at 15 lb/inch2.
  • High pressure-Vaccum autoclaves: 133°Cfor 3minutes at 30 lb/inch2.

Sterilization control:
A. Bacteriological test- spore test: Spores of Bacillus stearothermophilus
B.Chemical indicators (Browne’s tube)

It acts not only mechanically but by electrostatic adsorption of particles to
their surface.
It is useful for substances that get damaged by heat e.g., antibiotic solutions, sera, and carbohydrate solutions used in the preparation of culture media.

1. Bacteriological: Earthenware candles, Asbestos disc filter, sintered glass.
2. Membrane filters
3. Sand filter
4. Air filters: HEPA filters Candle filters

Two types: Ionizing and non-ionizing
Ionising (Cold sterilization):

  • X- rays, gamma rays, and accelerated electrons.
  • It is more effective than UV rays.
  • Kill microbes by ionizing their DNA.
  • Most plastics, syringes, swabs, culture plates, catheters, animal feeds, various types of rubber, cardboard, oils, grease, fabrics, and metal foils.

Non-ionizing radiation:
1. Infrared radiation: The mass sterilization of syringes.
2. Ultraviolet radiation: Disinfection of surfaces and air like bacteriological laboratories, operation theatre.


  • Variable degree of sensitivity but not useful.

Mechanism Of Action:

  • Protein coagulation of microorganisms.
  • Disruption of the cell membrane resulting and the removal of free sulphydryl groups essential for the functioning of cellular enzymes.
  • Substrate competition

Properties of ideal disinfectants:

  • A wide spectrum of activity. -Posses high penetrating power.
  • Should not corrode metals.
  • Should not cause irritation and sensitization.
  • Should not be toxic.
  • Stable, effective in different pH, compatible with other disinfectants, active in the presence of organic matter.

Chemical Disinfection:
It is based on the effectiveness, they are;

  1. High-level disinfectants:
    • It is nearly equivalent to sterilization
    • For Endoscopes, cystoscopes, surgical instruments with plastic.
    • Glutaraldehyde, hydrogen peroxide, peracetic acid, chlorine compounds.
  2. Intermediated level disinfectants:
    • It is semicritical instruments (laryngoscopes, fibreoptic endoscopes, vaginal specula, anesthesia breathing circuits).
    • Alcohols, iodophor compounds, phenolic compounds
  3. Low-level disinfectants:
    • Stethoscopes, electrocardiogram electrodes, etc.

Factors determining effectiveness:

  • Type of chemical agent
  • The concentration of the agent
  • a pH of the medium
  • Temperature
  • Nature of the organisms
  • Presence of extraneous maternal


  • Works in less than 30 seconds
    • Isopropyl alcohol, ethyl, or methyl alcohol
  • Disadvantages:
    • Relatively inactive against spores and fungi.
    • Inflammable
    • Concentration: 65-80% (70% common)
    • Uses Skin antiseptics, Disinfection of clean inanimate surfaces.

Aldehydes e.g, Glutaraldehyde and formaldehyde

Advantages: Bactericidal, sporicidal, and lethal to viruses and fungi

Disadvantages: Most vegetative bacteria in < 10 min (3 hours desirable). -It is toxic and may cause a sensitivity reaction

Uses:– Glutaraldehyde (2%): endoscopes, the cystoscope, rubber tubes, plastic
endotracheal tubes, metal instruments, etc. -Formaldehyde gas: woolen blankets, footwear, ward and operation theatre

A. Hypochlorite and Chlorine:

  • Extra strong’(10000 ppm): blood spillage.
  • Strong (2500 ppm): laboratory pipette jars & as an antiseptic for certain dirty products or infected wounds.
  • 1000 ppm: for general environmental disinfection.

B. Iodophors (Iodine):


  • Highly active against vegetative bacteria (some spore)
  • Compatible with alcohol.
  • less likely to sensitize the skin.


  • Inactivation by blood and other organic matter.
  • Iodine: local reaction and staining of the skin.


  • Iodine: excellent preoperative skin disinfectant
  • Povidone-iodine: as a surgical scrub
  • Povidone-iodine: as an antiseptic (topical use)


Cause cell damage, thus releasing cell contents and cause lysis
I. Carbolic acid: a phenol of historic interest only.
II. Chloroxylenol(an active ingredient of Dettol): Not good against Gram-negative bacteria. -Not recommended for hospital use.

Phenolic compounds: e.g, Chlorhexndnne, Savlon (chlorhexidine with cetrimide)


  • Bactericidal at (more effective in high concentration but also effective in low concentration).
  • Less toxic and effective in presence of blood.


  • Poor activity against spores, fungi, and viruses.
  • Inactivated by pus, soap, etc.


  • Skin and mucous membranes.
  • Preoperative skin preparation.
  • Chlorhexidine- cetrimide (Savlon): useful for dirty wounds.
  • Preoperative scrub for surgeons

Quaternary Ammonium Compounds (Quarts)
E.g. Cetrimnde, Benzalkonium chloride

  • Quaternary ammonium germicidal detergent solutions: for hospital floors.

Gas Vapour Sterilization:
Ethylene oxide gas:

  • Highly toxic inflammable gas, highly effective (spores )
  • Good penetration power
  • Plastic materials, swabs, paper, cardiac catheters and pacers, respiratory ventilators, certain types of complex heat-sensitive instruments, etc.

Formaldehyde gas:

  • Formaldehyde in aqueous solution.
  • Limited use: carcinogenic, pungent smell.
  • Bedding, furniture.

Hydrogen peroxide vapors:

  • Oxidizing nature of the gas.
  • Use in Plasma sterilization. 


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