STERILIZATION AND DISINFECTION

On the variouse sterilizing agents at our disposal today, that of moist heat in the form of saturated steam under pressure is the most dependable medium known for the destruction of all forms of microbial life. Unlike many of the chemical bactericides in solution form which are used in hospitals, and other gaseous sterilizing agents such as formaldehyde or ethylene oxide, steam leaves no toxic residue on the materilas following the sterilization process.

When steam is admitted to the sterilizing chamber, it promptly condenses upon contacting cold objects. As steam condenses, it liberates a great amount of latent heat, simultaneously heating and wetting the materials and thereby providing the two requisites for the thermal destruction of microbial life, namely moisture and heat. These two factors of moisture and heat must always be present for effective sterilization.

The process by which bacteria are destroyed when subjected to heat is not clearly understood. The traditional theory is that death at elevated temperatures is closely linked with the alteration of proteins, involving some irreversible protoplasm change within the bacterial cell.

To be sure, it is known that moist heat is a more efficient sterilizing agent than dry heat because in the presence of moisture, bacteria are destroyed at considerably lower temperatures than when moisture is absent. This phenomenon has been explained on the basis that all chemical reactions, including the coagulation of proteins, are catalysed by the presence of water. In brief, the generally accepted theory is that death by moist heat is caused by the denaturation and coagulation of some essential protein or enzyme-protein system within the bacterial cell, whereas death by dry heat is primarly an oxidation or slow burning up process.

Today, in every modern hospital, there may be found a variety of sterilizers, each performing a vital service in protecting the patient against infection, but all are dependant upon the application of certain fundamental principles allied with the use of steam as a sterilizing agent. We use steam under pressure, rather than atmospheric steam, for the sole purpose of attaining higher temperatures. It should be understood that pressure of itself has nothing whatsoever to do with the microbicidal properties of steam.

Since atmospheric, or what may be called “non-pressure” steam has a maximum temperature of 100 C, it is no value for the sterilization of surgical supplies.

Boiling water should not be employed as a sterilizing agent. Several investigations have shown that heat-resistant bacterial spores will withstand boiling water for many hours of continuous exposure.

At best, the boiling water processes could be described as a sanitizing medium. Those factors which have established pressure steam as the most realible medium for the sterilization of surgical supplies are : its power of penetration; its microbiological efficiency; and the ease of regulation or control for economical operation. Saturated steam possesses the singular property of being able to heat materials and permeate porous substances by the rapid process of condensation as opposed to the very slow process of heat absorption, as in the case of hot air or when any other gas is used as the heating medium. Also, saturated steam at a temperature of 134 C equivalent to 210 kPa pressure will destroy the most heat-resistant form of microbail life within a brief interval of exposure. Bacterial spores are recognised as the most resistant of all living organisms in their capacity to withstand external destructive agents. Anthrax spores, for example, dried on silk threads, have been found viable after sixty years. Other viable spore-formers have been recovered from canned and hermetically sealed food after a lapse of 115 years. The magnitude of resistance to saturated steam is illustrated by the fact that certain spore cultures all withstand a temperature of 115 C for more than three hours, whereas the vegetative forms of most bacteria are killed in a few minutes at temperatures ranging from 54 C to 65 C. Today, most authorities concur that no living thing can survive 10 to 15 minutes direct exposure to saturated steam at 121 C. To my knowledge, none of the pathogenic organisms have been known to be resistant to an exposure of even three minutes at 134 C.

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WHY STERILIZERS USE STEAM

It is obvious that heat and moisture are necessary to achieve the death of microorganisms. The most convenient vehicle of both heat and moisture is saturated steam, because it has the following :

1.Heat
2.Moisture
3.Its action is rapid
4.It is relatively inexpensive
5.It is plentiful or easily created
6.It is easily controlled
7.It is easily removed after it has been used by a drying process
8.It is non toxic

The bulk of the articles that need to be sterilized can stand temperatures of 134 C and are not ruined by moisture. There is a small percentage of articles that are either heat sensitive, moisture sensitive, or both, and an equally small percentage that cannot be sterilized by steam due to their physical properties. An example of items falling into these categories is as follows :

1.Heat sensitive – plastics
2.Moisture sensitive – ink and some types of paper products
3.Cannot be sterlized by steam – oil

Adapted from : Atherton

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PRINCIPALS OF STERILIZATION

An aura of mystery needlessly the art of sterilization. Modern pressure steam sterilizers function by simple application of ancient physical laws.

A sterilizer is a mechanical device for coagulating the protoplasm in microorganisms. The protoplasm of the microorganism dies and, of course, ceases to reproduce.

The protoplasm in microorganisms is very similar to the white of an egg or egg albumin. The coagulation of egg albumin is a coomon phenomena. We fry eggs and poach eggs almost daily.

When we fry an egg, the temperature reaches approximately 370 to 425 C in the pan.

When we poach an egg, due to the water in the pan, we never reach a temperature of more than 100 C.

In both instances we have coagulated the egg albumin,except, when poaching an egg we do it at a much lower temperature.

The only difference between the two methods of cooking the egg is that the fried egg is exposed to a dry heat the poached egg is exposed to a moist heat. We can assume then, that coagulation occurs at a lower temperature in the presence of moisture.

We know the different of microorganisms have varying resistance to unfavourable changes in their environment. This includes the degree of heat they can stand.

We know which organisme is the most resistant to destruction by heat. We know the conditions under which it is destroyed. We duplicate the destructive conditions in sterilizers, and therefore assume that all other organisms have been killed.

The most resistant pathogenic organism (one which is harmfull to man) is killed in hot air, after an exposure of one hour at 170 C. This same organism is killed at 134 C after only two minutes exposure in saturated steam.

Adapted from : atherton

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STEAM STERILIZATION

One of the efforts to avoid infection control in hospital is through an effective sterilization process. One of the most efficient and effective sterilization method is steam sterilization. Steam can kill microorganism by denaturizing and coagulating protein cell irreversibly. To produce sterile goods, pre-sterilization treatment (decontaminated and good cleaning, good packing) and post-sterilization (storage) must be well implemented. The perfect of steam sterilization process depends on decreasing process of the number of microorganism before sterilization through good cleaning and avoiding the occurred of recontamination before being used.

Steam sterilization machine
Basically there are two kinds of steam sterilization machine:
a.Steam sterilization machine gravity type: air is taken out from chamber by gravity.
b.Steam sterilization machine pre-vacuum type: air is taken out from chamber by a vacuum pump. In a sterilization process using pre-vacuum system usually sterilization time runs faster because the speed of air expulsion runs better.

Steam quality
Steam quality is very important toward the success and effectiveness of sterilization process. If the steam is too dry or wet, its penetration ability will be disturbed. A good quality steam has 97% of dry fraction ( on 0-100% scale, 0 refers to a very high water content while 100% refers to steam without water content at all).

Pressure of steam supply
It’s better to be checked every week by engineering department of the hospital so that it meets the specific requirements of the manufacturer.

Putting goods into machine
Goods arranging in the sterilization machine is one of the decisive factors in the success of sterilization process. A right goods arranging will make the air expulsion process from the chamber runs easily, will make steam easily penetrates into the packing and will avoid the forming of too much condensate that may cause the packing moist. Linen packing should be positioned vertically so that steam will penetrate into the packing easily. The goods loading into machine’s chamber should use 75% of chamber capacity.

Taking out goods from steam sterilization machine
Let goods be in the machine’s chamber until all steam out from the chamber and the sterile goods are in cooling down process. Forcing the cooling down process by using fan or AC blower is not recommended. Goods in cooling down process have to be placed in an area which is not too crowded with activities. To avoid microorganism infiltrating to the packing, the sterile goods can be handled only after perfectly cold. When cooling down process is still taking place , the sterile goods are not allowed to be placed on the metal surface for the condensation process will occurred toward goods so that there will be recontamination. Sterile goods must be stored in a wire rack until becoming cold.

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DRY HEAT STERILIZATION

Dry Heat Sterilization process works through the heat conduction mechanism, where heat will be absorbed by outer surface of tools sterilized and spreads to the inner surface until the sterilization temperature attained. Dry heat sterilization is usually used for materials or tools which steam can not easily penetrate or for tools made from glass.

In this method, microorganism killing works through oxidation mechanism until cell protein coagulating. Dry heat sterilization needs a longer time with higher temperature and works in a dry heat oven convection.

Working circle of dry hot sterilization machine :
1.Warming – Hot air produced through electricity mechanism and circulated at a chamber.
2.Plateau period (sterilization) begins when censor detects the attained temperature of sterilization process in chamber.
3.When all of the chamber has the same temperature so it is the end of equilibrium phase and starts “holding time” phase or sterilization.
4.Chamber cooling down, done by circulating cold air and filtrated into the chamber.

Advantages of dry heat sterilization :
1.It can sterilize several materials which can not be penetrated by steam like dry powder and oil material.
2.Non-corrosive toward metal.
3.Conduction mechanism can reach the entire non-overhauled tools surface.

Weaknesses of dry heat sterilization :
1.Penetration toward material moves slowly and not spread evenly.
2.Needs longer warming to attain sterile condition.
3.High temperature may damage rubber material and some cloth materials.

Manual instruction must be comprehensively prepared by machine supplier. The maintenance department must provide all information needed for all recommended procedure at the time of installation trial or routine maintenance. Manual instruction must be available at CSSD.
Reason: For the maintenance matter, repairing and calibration might be handled by other person, not only by the supplier.

Putting goods into the dry heat sterilization machine.
Before putting goods into the chamber, it must be heated first to the temperature of 160 C. Between one good and another there must be space to keep a good air circulation so that the thermal contact may run well and every item doesn’t stick to the chamber.

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