Sterilization

Sterilization-
Definition -

Sterilization is the process of destroying all micro organisms including the spores.
Sterilization is making a substance free from all micro organisms both in vegetative and sporing states.

(Spores form part of the lifecycles of many bacteria, plants, algae , fungi and some protozoa.)

Disinfection-

The destruction or removal of all pathogenic organisms capable of giving rise to infection. (Disinfection does not affect spore state organisms.)
Disinfection is the process of destroying micro organisms by direct exposure to chemical or physical agents.

(Disinfectant is an agent that kills growing or vegetative forms of the bacteria.)

Antisepsis- The term is used to indicate the prevention of infection, usually by inhibiting the growth of bacteria in wounds or tissues.

Bactericidal agents / germicides- Those which able to kill bacteria.

Bacteriostatic Agents- Only prevent multiplication of bacteria, but they remain alive.

Cleaning- Important preparatory step before sterilization or disinfection, by removing soil and other dirt.

Decontamination- The process of rendering an article or area free of contaminants, including microbial, chemical, radioactive and other hazards.

Methods of Sterilisation-

Classification of Sterilization-

Physical methods
Chemical methods

A. Physical Agents-

Sunlight
Drying
Heat

Dry Heat- Flaming, Incineration, Hot air
Moist Heat- Pasteurization, Boiling, Steam under pressure.

Filtration- candles, asbestos pads, membranes
Radiation
Ultrasonic and sonic vibrations.

B. Chemicals agents-

Alcohol
Ethyl, isopropyl, trichlorobutanol
Aldehydes
Formaldehyde, Glutaraldehyde
Dyes
Hypochlorites
Phenols
Gases- Ethylene oxide, formaldehyde, beta propiolactone

Physical methods of Sterilization

Sunlight- Action primarily due to UV rays however, effects vary due to places. This is one of the natural method of sterilisation.

Bacteria in water are readily destroyed by sunlight.

Advantage

Natural and easy method.
Low Cost.

Disadvantage-

Not effective
Enable to destroy germ.

Drying-

Moisture is essential for growth of bacteria.
Drying in air has deleterious effect on many bacteria.
However, spores are unaffected.
Therefore, it is not really unreliable.

Heat-

Most reliable method of sterilization and should be the method of choice.

Dry Heat
Moist Heat.

The factors influencing sterilization by heat-

Nature of heat-dry or moist.
Temperature and time
Number of microorganisms present
Characteristics of organisms –species, strain, sporing capacity Type of material from which organism have to be eliminated.
Killing effect is due to protein denaturation, oxidative damage and toxic effect of elevated level of electrolytes.
Killing effect of moist heat due to denaturation and coagulation of proteins.

Dry heat-

Flaming-

Heating over fire, till they become red hot.
Instruments like: Point of Forceps, Spatulas, Inoculating loops and Wires. (Inoculating loop is better dipped in disinfectant first before flaming to prevent spattering)

Incineration-

It is a process that involves the combustion of organic substances contained in waste materials.
Items: contaminated cloth, animal carcasses and pathological material. PVC, polythene can be dealt. (However, polystyrene will emit black smoke. Hence should be autoclaved in appropriate container.)

Hot Air Oven-

Hot air ovens are electrical devices used in sterilization.
The oven uses dry heat to sterilize articles.
Generally, they can be operated from 50 to 300 °C (122 to 572 °F) . There is a thermostat controlling the temperature.
This is the most widely used method of sterilization by dry heat.
Items- glassware, forceps, scissors, scalpels, all-glass syringes, swabs, liquid paraffin, dusting powder, fats, grease.

(Materials should be properly arranged to allow free circulation of air )

Precautions-

Glass wares should be dry.
Oven should not be over loaded.
Articles are to be arranged in a manner to allow free circular of air.
Door of the Oven should be opened after it cools down.

Advantages & Disadvantages

They do not require water and there is not much pressure build up within the oven, unlike an autoclave, making them safer to work with.
Suitable to be use in a laboratory environment.
They are much smaller than autoclaves but can still be as effective.
As they use dry heat instead of moist heat, some organisms like prions, may not be killed by them every time.

Moist Heat-

Moist heat can be categorized into 3 groups-

Temperature below 100°C
Temperature at 100 °C
Temperature above 100 °C

Pasteurisation of milk

Holding period- 63 °C , 30 minutes (holder method) or 72 °C, 15-20 minutes followed by cooling quickly to 130°C or lower.
Target- All nonsporing pathogens

Eg: mycobacteria, brucellae, salmonella.Coxiellaburnetti, relatively heat resistant, may survive the holder method.

Temperature at 100 °C-

Boiling -

Not recommended for sterilising but used for disinfection. sterilization may be promoted by addition of 2% sodium bicarbonate to the water.
Holding period: 10-30 minutes.

Steam at atmospheric pressure (100 °C)

Used to sterilize culture media.
This is an Inexpensive method
Holding period: 100 °C , 20 minutes on three succesive days (intermittent sterilization).
Principle- first exposure kills vegetative bacteria and then the next exposure will kill vegetative bacteria that matures from the spore.

Steam under pressure Autoclave/steam sterilizer-

Autoclave is the name given to a 'steam under pressure sterilizer'. In this apparatus, material for sterilization is exposed to 121°C (250°F) (rapped 30 minute and unrapped 20 minute)

Many autoclaves are used to sterilize equipment and supplies by subjecting them to pressurized saturated steam at 121 °C (250 °F) for around 30-60 minutes at a pressure of 15 psi(103 kPa or 1.02 atm) depending on the size of the load and the contents.

Autoclave is a device that uses steam to sterilize equipment and other objects.
This means that all bacteria, viruses, fungi, and spores are inactivated.

Principle- Water boils when its vapour pressure equals the surrounding atmosphere. Thus, when pressure inside closed vessels increases, the temperature at which water boils increases too.

Advantages

Nontoxic to patient, staff, environment.
Cycle easy to control and monitor.
Rapidly microbicidal.
Most reliable.
Rapid cycle time.
Penetrates medical packing, device lumens.

Disadvantages

Deleterious for heat sensitive instruments.
Microsurgical instruments damaged by repeated exposure.
May leave instruments wet, causing them to rust.
Potential for burns.
Some plastic ware melts in the high heat, and sharp instruments often become dull.
Moreover, many chemicals breakdown during the sterilization process and oily substances cannot be treated because they do not mix with water.

Filtration-

Helps to remove bacteria from heat labile liquids.
Items- sera and solutions of sugars or antibiotics.

Principle- as viruses pass through the ordinary filters, filtration can be used to obtain bacteria-free filtrates of clinical samples for virus isolation.

Radiation-

2 types of radiation-

Ionising radiation
Non-ionising radiation

Non-ionising radiation.

Infrared-Used for rapid mass sterilization of prepacked items such as Syringe,Cathaters.
UV
Used for disinfecting enclosed area such as entryways, operation theatres and labs.

Ionising radiation

Gamma rays
X-rays-

Used for sterilising plastics, syringes, swabs, catheters, animal feeds, cardboard, oils, greases, fabric and metal foils.

Chemical methods of Sterilization Chemical agents-

Commonly used chemical

Alcohol-

Frequently used are Ethyl alcohol ,Isopropyl alcohol
These must be used at concentration 60-90%.
Isopropyl alcohol used in disinfection of clinical thermometer. Methyl alcohol is effective against fungal spores, treating cabinets and incubators.
Methyl alcohol is also toxic and inflammable.

Aldehyde Formaldehyde-

Having Bactericidal, sporicidal and has lethal effect on viruses.
Used to preserve anatomical specimens, destroying anthrax spores on hair and wool.

Glutaldehyde-

Effective against tubercle bacilli, fungi, viruses.
Less toxic and irritant to eyes, skin.
Used to treat anaesthetic rubber, face masks, plastic endotracheal tubes, metal instruments and polythene tubing.

Dyes-

2 groups of dyes

Aniline dye
Acridine dye

Both are bacteriostatic in high dilution but are of low bactericidal activity.

Aniline dye is more active against gram +ve than gram-ve organisms.

Halogens

Iodine-

Used as Skin disinfectant
Having Active bactericidal activity &moderate action on spores.

Chlorine

Used to disinfect Water supplies, swimming pools and food and dairy industries.
Along with hypochlorides are bactericidal. Also act on viruses.

Phenols -

It is used for sterlizing surgical instruments and killing culture accidentally split over in the laboratory. It is generally used in 3% solution.

Capacity to cause cell membrane damage, releasing cell contents and causing lysis.
Low concentration will precipitate proteins.

Gases-

Types of gases used for sterilization-

Ethylene oxide-Action is due to its alkylating the amino, carboxyl, hydroxyl and sulphydryl groups in protein molecules
Formaldehyde gas-This is widely employed for fumigation of OT and other rooms. After fumigation, the doors should be sealed and left unopened for 48 hours.
Beta propiolactone (BPL)-Capable of killing all microorganisms and is very active against viruses.