STERILIZATION - PHYSICAL METHOD

 STERILIZATION

·     Sterilization is defined as a process in which an article, surface or medium is free of all living organisms.

·       Sterilization is the process of killing of microorganisms.

·       Sterilization was carried out by two methods.

A) Physical method

a)     Drying

b)     Dry heat

i.       Flaming

ii.     Incineration

iii.   Hot air (Hot air oven)

c)     Moist heat

i.       Pasteurization (Temperature below 100 °C)

ii.     Boiling (Temperature at 100 °C)

iii.   Tyndallization (Temperature above 100 °C)

iv.   Autoclaving (Temperature above 100 °C)

d)     Filtration

i.       Candle filter

ii.     Asbestos filter

iii.   Sintered glass filter

iv.   Membrane filter

e)     Radiation

i.       Non – ionizing radiations (Infrared radiation and Ultraviolet radiation)

ii.     Ionizing radiations (X – rays, Gamma rays and Cosmic rays)

PHYSICAL METHOD OF STERILIZATION

(a) DRYING

·       Moisture is essential for the growth of bacteria. Four – fifth of the weight of the bacterial cell is due to bacteria.

·       Drying in has a deleterious effect on many bacteria.

·       Spores are unaffected by drying.

(b) DRY HEAT

i) Flaming

·       In Flaming, inoculation loops, tips of forceps and spatulas are held in a Bunsen burner flame till they become red hot.

Sterilization of Inoculation loop by Flaming (Source: microbeonline.com)

ii) Incineration

·     Incineration is an excellent method for safely destroying materials such as contaminated cloth, animal carcasses and pathological materials.

·       The equipment used for the Incineration process is Incinerator.

 

Laboratory Incinerator (Source: indiamart.com)

iii) Hot air oven

·       Sterilization by Hot air oven is the most widely used method for sterilization by Dry heat.

·       Hot air oven is used to sterilize glasswares, forceps, scissors, scalpels, glass syringes and swabs.

·      “Temperature required for sterilization is inversely proportional to Time” is the principle of Hot air oven.

Hot Air Oven

·      During the sterilization of materials in Hot air oven, the sterilization time was reduced when the sterilization temperature was increased.

·       Temperature and duration for Hot air oven sterilization is

§  140 °C for 1_1/2 hours

§  160 °C for 1 hours

§  180 °C for _1/2 hours

·      Fan is present in the Hot air oven and circulates the hot air. The circulating hot air sterilizes the materials which were kept for sterilization.

·       Sterilization by Hot air oven kills the microorganisms by denaturation of proteins. 

·      Overloading of materials should be avoided during Hot air oven sterilization.

·      The products to be sterilized in Hot air oven should be wrapped completely.

·     The Hot air oven must be allowed to cool slowly for about two hours before the door is opened because the glassware may crack due to sudden or uneven cooling.

(c) MOIST HEAT

i) Pasteurization

·       Pasteurization is the fractional sterilization process of milk.

·       Pasteurization technique was introduced by Louis Pasteur.

·       Pasteurization was carried out by two methods.

1.     Holder method (63 °C for 30 minutes)

2.     Flash method (72 °C for 15 - 20 seconds)

·       During Pasteurization, all the spore forming bacteria and heat resistant bacteria were killed.

Milk Pasteurization Plant in Industries (Source: exportersindia.com)

ii) Boiling

·   Vegetative bacteria are killed almost immediately at 90 – 100 °C, but spore forming bacteria required prolonged period of boiling.

·       Boiling is not recommended for the sterilization of instruments.

·       Boiling is usually preferred for cooking the food materials.

Boiling of Water (Source: bloggingheros.com)

iii) Tyndallization

·      Tyndallization is a process dating from the nineteenth century for sterilizing substances, usually food, named after its inventor, scientist John Tyndall.

·       It is still occasionally used.

·       Tyndallization can be used to destroy the spores.

·       Tyndallization essentially consists of heating the substance to boiling point (or just a little below boiling point) and holding it there for 15 minutes, three days in succession. 

·    After each heating, the resting period will allow spores that have survived to germinate into bacterial cells; these cells will be killed by the next day's heating.

iv) Autoclave

·    An autoclave is a pressure chamber used to carry out the sterilization under pressure (Moist heat).

Autoclave

·       The mechanism of the autoclave is similar to the pressure cooker.

·      Autoclave works under the principal of Boyl’s Law – “When volume is constant, temperature is directly proportional to time”.

·       Temperature and duration for Autoclave sterilization is 121 °C or 15 lbs for 20 minutes.

·  During autoclaving, water boils when its vapor pressure equals that of the surrounding atmosphere. Hence when the pressure inside a closed vessel increases, the temperature at which water boils also increase.

·       At the time of autoclaving, steam was produced and sterilizes the materials which were kept for sterilization.

·       Sterilization by autoclaving kills the microorganisms by coagulation of proteins. 

·       Overloading of materials should be avoided during autoclaving.

·       The products to be sterilized in autoclaving should be wrapped completely.

·       The autoclave must be allowed to free for 15 minutes for the release of pressure.

·   Autoclave is used to sterilize the dressings, instruments, laboratory ware, culture media and pharmaceutical products.

(d) FILTRATION

·      Filtration helps to remove the bacteria from heat liable substances such as serum, sugar solutions or antibiotics used for the preparation of culture medium.

·       Four types of filtrations are commonly used for the sterilization process. They are

i.  Candle filters

ii.   Asbestos filters

iii. Sintered glass filters

iv.   Membrane filters

i) Candle filters

·      Candle filters are manufactured in different grades of porosity.

·   They have been widely used widely for the purification of water for drinking and industrial process.

·       They are of two types:

1.     Unglazed ceramic filters (e.g) Chamberland filter and Doulton filter

2.     Diatomaceous earth filters (e.g.) Berkefeld filter and Mandler filter

ii) Asbestos filter

·       Asbestos filters are disposable, single – use discs.

·       They have high absorbing capacity and tend to alkalinize filtered liquids.

·       The carcinogenic potential of asbestos has discouraged their uses.

·       Examples of Asbestos filter include Seitz filters and Sterimat filters.

iii) Sintered glass filter

·       Sintered filters are prepared by heat fusing finely powdered glass particles of graded size.

·       They have low absorptive property and can be cleaned easily but are brittle and expensive.

Sintered Glass Filter

iv) Membrane filters

·       Membrane filters are made up of cellulose esters or other polymers have largely replaced other types of filters.

·       Membrane filters are routinely used in water purification and analysis, sterilization and sterility testing and for preparation of solutions for parental use.

Membrane Filter

(e) RADIATION

·   Two types of radiations are used for sterilization. They are

a)     Non – ionizing radiation

b)     Ionizing radiation

i) Non – ionizing radiation

·     Infra red and Ultraviolet rays are the non – ionizing low energy type.

·   Infra red radiation is used for rapid mass sterilization of prepacked items such as syringes and catheters.

·     Ultraviolet radiation is used for disinfecting enclosed areas such as entry ways, operation theaters and laboratories.

ii) Ionizing radiation

·       X – rays, Gamma rays and Cosmic rays are high energy ionizing type.

·       Ionizing radiations are lethal to DNA and other vital constituents.

·      They have high penetrative power.

·      Ionizing radiations are used to sterilize plastics, swabs, catheters, animal feeds, cardboards, oils, greases, fabrics and metal foils.

QUALITY CONTROL AND STERILITY CHECKING

·        Scientists use several means to ensure that an autoclave has sterilized its contents.

·  A biological indicator of sterility uses endospores of the bacterium Bacillus stearothermophilus impregnated into tape.

·       After autoclaving, the tape is aseptically inoculated into sterile broth.

·      If no bacterial growth appears, the original material is considered sterile. If bacterial growth appears, the material is considered non-sterile.