steriization

8/19/2019 SterIization

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CONTROL OF MICRO ORGANISMS

BY PHYSICAL AND CHEMICALAGENTSSTERILISATION BY PHYSICAL METHODS

28/10/2015

BIMIC 1 LFDC


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IMPORTANT TERMS :

Sterilization is the process by which all living cells, spores, and acellular entities (e.g., viruses, viroids, and prionsare either destroyed or removed from an object or habitat. A sterile object is totally free of viable microorganisms

spores, and other infectious agents. When sterilization is achieved by a chemical agent, the chemical is called

sterilant.

Disinfection is the killing, inhibition, or removal of microorganisms that may cause disease. The primary goal is todestroy potential pathogens, but disinfection also substantially reduces the total microbial population.

Disinfectants are agents, usually chemical, used to carry out disinfection and are normally used only on inanimateobjects. A disinfectant does not necessarily sterilize an object because viable spores and a few microorganisms may

remain.

Sanitization is closely related to disinfection. In sanitization, the microbial population is reduced to levels that arconsidered safe by public health standards. The inanimate object is usually cleaned as well as partially disinfected.

Antisepsis is the prevention of infection or sepsis and is accomplished with antiseptics. These are chemical agentsapplied to tissue to prevent infection by killing or inhibiting pathogen growth; they also reduce the total microbia

population. Because they must not destroy too much host tissue, antiseptics are generally not as toxic as disinfectants.

Chemotherapy is the use of chemical agents to kill or inhibit the growth of microorganisms within host tissue.SUFFIXES

Substances that kill organisms often have the suffix – cide [Latin cida, to kill]; a germicide kills pathogens (and many

non pathogens) but not necessarily endospores. A disinfectant or antiseptic can be particularly effective against a

specific group, in which case it may be called a bactericide, fungicide, algicide, or viricide.

Other chemicals do not kill, but they do prevent growth. If these agents are removed, growth will resume. Their

names end in -static [Greek statikos, causing to stand or stopping] — for example, bacteriostatic and fungistatic.

The various agents used in sterilization are classified as follows:

SUNLIGHT: Sunlight possesses appreciable bactericidal activity and provides important support in

sterilization that occurs under normal conditions.

It has germicidal effect due to mixed content of ultraviolet and heat rays. Bacteria suspended in water are

readily destroyed by the action of sunlight.


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DRYING: Moister is essential for growth of bacteria. Four-fifth of bacterial cell weight is due to presence

of water. Drying has deleterious effect on growth on bacteria but spore forming organisms are unaffected

by this method.

HEAT: it is the most reliable method for sterilization. Materials that can be damaged by heat are sterilized

under lower temperatures, for longer periods of time.

The factors influencing sterilization by heat are :

Nature of heat- dry or moist heat

Temperature and time

Number of organisms present

Type of materials from which organisms have to be eradicated

DRY HEAT:

Hot air oven: it is widely used method of sterilization by dry heat. A holding period of 160°C for one hour

is used to sterilize glass ware, forceps, scissors , scalpels.

Hot air is a bad conductor of heat and penetration power is low.

At 180°C cotton plugs may get charred and the oven must be allowed for 2 hours before the door is opened

as the glass material may crack due to sudden or uneven cooling.

Because heat is so useful in controlling microorganisms, it is essential to have a precise measure of the heat-killingefficiency.

Initially effectiveness was expressed in terms of thermal death point (TDP), the lowest temperature at which a

microbial suspension is killed in 10 minutes. Because TDP implies that a certain temperature is immediately lethal

despite the conditions ,thermal death time (TDT) is now more commonly used. This is the shortest time needed to

kill all organisms in a microbial suspension at a specific temperature and under defined conditions. However, such

destruction is logarithmic, and it is theoretically not possible to completely destroy microorganisms in a sample

even with extended heating. Therefore an even more precise figure, the decimal reduction time (D ) or D value has

gained wide acceptance.

D values are used to estimate the relative resistance of a microorganism to different temperatures through

calculation of the z value. The z value is the increase in temperature required to reduce D to 1/10 its value or to


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reduce it by one log cycle when log D is plotted against temperature . Another way to describe heating effectiveness

is with the F value. The F value is the time in minutes at a specific temperature (usually 250°F or 121.1°C) needed

to kill a population of cells or spores.

MOIST HEAT:

The autoclave is a sealed heating device that uses steam under pressure to kill microorganisms . Killing of heat

resistant endospores requires heating at temperatures above the boiling point of water at 1 atm. The autoclave

places steam under a pressure of 1.1 kg/cm2 (15 lb/in2), which yields a temperature of 121°C. At 121°C, the time

to achieve sterilization of small amounts of endospore-containing material is about 15 min. If the object to be

autoclaved is bulky or large volumes of liquids are to be sterilized, heat transfer to the interior is retarded, and

thus the total heating time must be extended. Note that it is not the pressure inside the autoclave that kills the

microorganisms but the high temperatures that are achieved when steam is placed under pressure.


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Autoclave :

Autoclave consists of steel casing and a lid that covers the body. the lid is attached to the body by the

butterfly shaped valves. The water is poured in the container and a plate is placed over the water

level where the equipment to be sterilized is placed.

The water is introduced through a pipe attached to the casing.

Attached to the lid is a pressure indicator which displays the pressure inside the chamber during the

process.

The water that overflows is drained away from the drain outlet.

A safety valve is present which is used to maintain the pressure

When the required pressure is gained the valve is closed

Advantages of autoclaving:-

1. Autoclaving destroys microorganisms more efficiently than dry heat and hence the material is

exposed to a lower temperature for a shorter period.

2 .It is used for sterilization of a large number of official injections.

3. Equipment or parts of rubber and plastic ,such as ,Nylon and PVC can withstand the temperature

and the pressure required for sterilization.

4. A large quantity of material can be sterilized in one batch using a big autoclave.


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LAMINAR AIR FLOW

In a laminar flow hood the air is passed through a HEPA (High Efficiency Particulates Air)filter

which removes all airborne contamination to maintain sterile conditions.

A laminar flow hood consists of a filter pad, a fan and a HEPA (High Efficiency Particulates Air)

filter. The fan sucks the air through the filter pad where dust is trapped. After that the

prefiltered air has to pass the HEPA filter where contaminating fungi, bacteria, dust etc areremoved. Now the sterile air flows into the working (flasking) area where you can do all your

flasking work without risk of contamination.

Important parameters to make sure that the hood works efficiently:

• the HEPA filter has to remove all airborne materials

• the air speed in the working area has to be about 0,5 m/s

The two types of laminar flow hoods

Before you start building your flow hood you have to decide if you prefer a vertical or

horizontal air flow in the flasking area. In a vertical flow the air moves from the top of the

working area to the bottom and leaves the flasking area through holes in the base. When you

use a flow hood with horizontal air flow the air moves from the back of the working area to

the front.


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The unit of ionizing radiation is the roentgen , and the standard for biological applications such as sterilization is the absorbed

radiation dose , measured in rads (100 erg/g) or grays (1 Gy = 100rad). Ionizing radiation is typically generated from X-ray

sourcesor the radioactive nuclides 60Co and 137Cs, which are relativelinexpensive by-products of nuclear fission. These nuclide

produceX-rays or gamma rays (γ-rays), both of which have sufficientenergy and penetrating power to efficiently kil

microorganismsin bulk items such as food products and medical supplies.


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steriization

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