Sterilization

Dry heat sterilization typically involves exposing an item to a temperature of 170°C under normal air pressure for around an hour. That time period ensures that even the most resistant spores get killed off via oxidation of their cellular components

These methods consist of;

Red heat:Sterilization is done by holding materials in a bunsen burner flame until they become red hot. It is used to sterilize inoculation, straight wires, wire loops, tips of forceps and spatulas
Flaming:This is a very quick simple method of killing microorganisms on an inoculating loop or needle. The loop or needle is held inside a flame for a few seconds to bring it to redness and then cooled. Once cool, the loop or needle can be used for various culture manipulations.
Hot air ovens use extremely high temperatures, in a special oven, over several hours to destroy microorganisms and bacterial spores. The ovens use conduction to sterilize items by heating the outside surfaces of the item, which then absorbs the heat and moves it towards the center of the item.
Incineration involve complete burning of the substance into ash in an incinerator

Outline of the properties of dry heat sterilization methods.

Method	Principle	Advantages	Disadva	Uses

Dry Heat	• Thermal inactivation: destroys by oxidation	• Non-corrosive Simple design and principle	• Less effective than moist heat; • Requires longer times and/or higher temperatures	• Materials that are damaged by, or are impenetrable to, moist heat
Hot Air Oven	• 160-180^oC for 2-4 hours	• Penetrates water-insoluble materials (e.g., grease and oil) • Less corrosive to metals and sharp instruments than steam	slow diffusion, penetration • Loading, packing critical to performance • Not suitable for reusable plastics	• Anhydrous materials, such as oils, greases and powders • Laboratory glassware, instruments • closed containers
Red-heat Flame	• Oxidation to ashes (burning)	• Rapid	• Initial contact with flame can produce a viable aerosol • Possibility of accidental fire	• Inoculating loops, needles
Incineration	• Oxidation to ashes (burning) • 1-60 minutes: temperatures may exceed 1000?C	• Reduces volume of waste by up to 95%	• Improper use may lead to emission of pathogens in smoke • Requires transport of infectious waste • Excess plastic (>20%) content reduces combustibility	• For decontamination of waste items prior to disposal in landfill

Moist heat method of sterilization

Moist heat sterilization involves methods such as

Pasteurization
Tyndilization
Autoclaving

Outline of the properties of moist heat sterilization methods.

Method	Principle/ Conditions	Advantages	Disadva	Uses
Pasteurization	Heating to below boiling point (generally 77^oC) for up to 30 minutes	• Can be used on heat sensitive liquids and medical devices Low cost	Not reliably sporicidal	• Milk and dairy products • some heat-sensitive medical equipment
Tyndallization (Fractional Sterilization)	Heating to 80-100^oC for 30 mins on successive days, with incubation periods in between	Resistant spores germinate and are killed on the second and third days	Time consuming not reliably sporicidal	• Heat sensitive materials such as bacteriologic media, solutions of chemicals, biological materials
Autoclaving	Apply Steam under pressure usually at 121^oC/15 psi for 15 -90 mins (gravity displacement autoclave) or at 132^oC/27 psi for 4-20 minutes (pre-vacuum autoclave)	Minimal time required Most dependable sterilant for lab use	Loading and packing critical to performance Shielding dirt must first be removed Maintenance and quality control essential	• Penetration of sterile glassware, media and instruments • Decontamination of reusable supplies and equipment • Decontamination of infectious waste

Ultraviolet Light (Germicidal Lamps)

The light (approximately 260 nm wavelength) emitted by UV lamps is germicidal, and can be used to reduce the number of pathogenic microorganisms on exposed surfaces and in air.

However, UV light has poor penetrating power; accumulations of dust, dirt, grease or clumps of microorganisms may shield microorganisms from the direct exposure required for destruction.

UV light presents skin and eye burn hazard, and factors such as lamp age and poor maintenance can reduce performance.

Membrane filtration:

This involves physically removal of particulates (e.g., microorganisms) from heat-sensitive pharmaceutical and biological fluids. The size of the particles removed is determined by the pore size of the filter membrane.

Disinfection

Although physical methods are often superior to chemical disinfection / sterilization, it is not practical to autoclave or subject many items to high heat, especially if the items can be damaged through repeated exposure to heat.

Instruments or materials, which cannot withstand sterilization in a steam autoclave or dry-air oven, can be sterilized with a gas such as ethylene oxide or broad-spectrum liquid chemical disinfectants.

Treatment of inert surfaces and heat labile materials can be accomplished through appropriate use of this disinfectants.

Selection of Disinfectants

Choice of a chemical disinfectant for use on contaminated equipment, supplies, laboratory surfaces or biohazardous waste depends upon a number of factors, including:

1) Type and level of microbial contamination

2) Concentration of active ingredient

3) Duration of contact between disinfectant and item to be disinfected

4) Presence of organic matter or soil load

5) Toxicity to individuals, culture systems, environment, residual toxicity on items,

6) PH, temperature, hardness of available dilution water,

7) Cost.

Table showing Common disinfectants

Disinfectant

Effective Conc and Contact Times

Advantages

Disadvantages

Examples of Laboratory Uses

Alcohols

• 70-80% ethanol

• 60-95% isopropanol

• 10-30 minutes

• low toxicity

• rapid action

• low residue

• non-corrosive

• rapid evaporation limits contact time

• flammable, eye irritant

• may damage rubber, plastic, shellac

• ineffective against bacterial spores

• skin disinfectant (antiseptic)

• surface decontamination

• benchtop, cabinet wipedown

Phenolic Compounds

400-50,000 ppm (.05-1.5%)

10-30 minutes

• tolerant of organic load, “hard” dilution water

• leaves an active residue (may be desirable on some surfaces)

• biodegradable

• pungent odour, corrosive, some forms toxic

• not sporicidal; limited activity against viruses

• leaves a residual film (undesirable in culture systems)

• may support growth of bacteria¹

· instruments and equipment disinfection

· disinfection of floors and other surfaces

· antiseptic soaps and lotions

Quaternary Ammonium Compounds

• 500-15,000 ppm (.05-1.5%)

• 10-30 minutes

• combined detergent and germicidal activity

• stable

• working dilutions have low toxicity

• Non-sporicidal, limited activity against viruses, mycobacteria

• most formulations not readily biodegradable

• may support growth of bacteria²

· surface decontamination

· equipment wipedown

· antiseptic formulations available

· floors and walls

Hydrogen Peroxide

• 3-30% aqueous solution

• 10-60 minutes

• 6% for 30 minutes may kill spores

• rapid action

• no residue

• low toxicity

• environmentally safe

• limited sporicidal activity

• corrosive to some metals

• potentially explosive at high concentrations

• stock solutions irritating to skin and eyes

· surface decontamination

· instruments and equipment

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