The 6 Most Important Practical Methods of Sterilization of Microbes by Heat

The method may be used in an emergency for sterilizing a strip of adhesive tape or the tips of forceps or other instruments.

The inside of a medicine glass may be sterilized by first wiping it out with alcohol or ether and then touching a flame to the adherent liquid, the fluid will burn and leave the glass sterile. A small instrument might be sterilized in a similar way.

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2. Burning or Incineration:

Burning is a safe and cheap method of disposal of small objects of no value, such as used swabs, or dressings, or paper sputum cups. Whenever possible, incineration of articles contaminated with disease germs is to be recommended.

3. Exposure to Hot Air (dry heat oven):

In bacteriological laboratories, one of the most commonly used pieces of apparatus is a sterilizer called the dry heat, or hot air, oven. This oven is similar to the ordinary banking oven used in the home. It is a double-walled chamber heated by gas or electricity and so constructed that it will stand a very high temperature.

This apparatus is used in the laboratory almost entirely for the sterilization of test- tubes, flasks, Petri dishes, and other kinds of glassware. An exposure of at least one hour at a temperature of about 160°C is necessary to assure the complete sterilization of objects in the dry heat oven.

As a routine method glassware is held in the oven at 170°C for one and one half hours. Cotton and paper may be charred if the temperature runs over 180°C; this must be avoided. After the period of heating, the oven must not be opened until the temperature has fallen at 100°C or less, because too sudden cooling of the glassware may crack it.

4. Boiling:

Boiling is such a commonplace, everyday procedure that its use is sometimes neglected for a more complicated method. Boiling water has a temperature of 100°C (212°F). The vegetative forms of most bacteria are killed by a few minutes exposure to this temperature, but it must be remembered that spores may resist boiling for as long as one hour or more.

The time required to sterilize by boiling will vary a great deal with the nature of the material. It must be remembered that, in order for an object to be sterilized, it must be completely immersed in the boiling water and the boiling must be continued long enough for all parts of the object to reach the sterilizing temperature.

Boiling is especially useful for the disinfection of dishes and tableware, handkerchiefs, bed linen, bedclothes, and other things contaminated by patients, and for die sterilization of surgical instruments, hypodermic needles, and syringes.

5. Heating in Flowing Steam:

Live steam is one of the most effective sterilizing agents. Steam kills micro-organisms much more rapidly than hot air; the protoplasm coagulates and dies more quickly. Also steam penetrates substances readily, carrying its heat along with it.

Flowing steam, that is, steam which is not confined under pressure, has the same temperature as boiling water, or 100°C, and never rises above this temperature. For this reason flowing steam is not effective against bacterial spores unless it is applied for a very long time.

In practice a method of intermittent exposure to flowing steam, called the fractional or discontinuous method of steam sterilization, is employed. In this method the material to be sterilized is exposed to the steam for half-hour period on each of three to five successive days.

The apparatus used in the laboratory is called the Arnold sterilizer. An ordinary double-boiler or steamer might be used at home. At the first heating, most, of die bacteria in the active vegetative stage are killed, but not the spores.

Then, in the interval between the first and the second heating, the spores which survived the first heating germinate into vegetative cells. These should be killed the second day, but in order to sterilize the material surely, the process of intermittent heating is usually continued for another day or two

The method is somewhat uncertain at the best and most laboratories at the present time the Arnold sterilizer is not often used, except for the sterilization of certain solutions or culture media which would lie altered in some undesirable way by temperatures above 100CC.

6. Heating in steam under pressure:

When steam is confined under pressure in a closed chamber, it is a much more effective sterilizing agent than flowing steam, because it penetrates better and particularly because it attains a higher temperature.

It is the temperature, not the pressure which is the more important.