Lesson 3: Practical Application of Microbiology

Section II: Disinfection and Sterilization

3-4

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3-4. PHYSICAL AGENTS

a. Mechanical Means. Mechanical means may be used to achieve a high degree of cleanliness and microbe removal, but not to accomplish sterilization. Mechanical means include the following:

(1) Scrubbing. Scrubbing thoroughly with soap and water, and often with an additional chemical, removes microorganisms mechanically, while the soap acts upon them chemically.

 

(2) Filtration. Filters made of unglazed porcelain, diatomaceous earth, asbestos, and other porous substances are used to remove microorganisms from fluids. Sterility mayor may not be achieved, depending upon the microorganisms present. The smaller viruses and rickettsiae are capable of passing through such filters.

 

(3) Sedimentation. Sedimentation is the process by which bacteria and other particles suspended in a liquid coagulate and settle to the bottom. This process finds its practical application in water purification and sewage treatment.

b. Heat. Heat is the most widely used and most effective means of sterilization. All species of microorganisms are killed by heat. The conditions under which the heat is applied, the temperature achieved, the length of time the temperature is maintained, and the species of microorganism involved are all factors affecting sterilization by heat. The temperature that will kill a species of microorganism in 10 minutes is known as the thermal death point. The time required to kill all microorganisms at a given temperature is known as the thermal death time.

(1) Dry heat. Dry heat is used primarily for sterilizing objects made of glass, metal, and substances that are damaged by moisture. For complete sterilization, a temperature of 160º-170ºC (320º-338ºF) must be maintained for at least 1 to 3 hours. Most fabrics are damaged by this much dry heat.

 

(2) Moist heat. Moist heat provides complete sterilization at lower temperatures and in a shorter time than does dry heat. Whereas dry heat kills by oxidation, a relatively slow process, moist heat kills by coagulation of the protein in the microorganism--a relatively fast process. Moist heat may be applied in three ways.

(a) Boiling. Boiling is the most common means of sterilization. Boiling will kill vegetative forms of pathogenic microorganisms in 5 minutes or less. Spores are much more heat resistant, but they can be destroyed by boiling for 15 minutes. Boiling is not considered a completely effective sterilizing agent, since the spores of certain thermophiles (heat-loving organisms) can survive prolonged boiling.

 

(b) Steam. Free flowing steam provides about the same degree of effectiveness as boiling, if applied in sufficient volume to maintain a temperature of 100ºC (212ºF).

 

(c) Steam under pressure. The most effective means of

sterilization is by means of the autoclave. The autoclave is an airtight cylinder in which steam may be introduced under pressure. Where as free-flowing steam has a temperature of 100ºC, steam under 15 pounds of pressure per square inch has a temperature of 121.5ºC (251ºF). Steam under 15-20 pounds of pressure per square inch will kill all organisms and spores in 15-45 minutes.

 

(d) Flaming. Some instruments or objects, such as platinum wire loops used in inoculating laboratory cultures, may be sterilized by holding the object in a gas or alcohol flame until it glows.

 

(e) Incineration. Incineration is the approved method for sterilizing and disposing of contaminated materials that have no further value. Care must be exercised that complete combustion of all fragments occurs before any of the residue (ash) is removed from the incinerator.

c. Cold. At very low temperatures most microorganisms lose their viability and multiply very slowly, if at all; but most are not killed. Refrigeration has become an indispensable adjunct to the food industry, the medical

laboratory, and many other areas in which growth of microorganisms must be inhibited. Some organisms may be killed at low temperatures--a point of importance in the storage, handling, and shipment of laboratory specimens. On the other hand, many microorganisms can be frozen for long periods (as spores) and regain their viability when the temperature is again favorable.

d. Desiccation. Natural or artificial drying results in the destruction of most microorganisms. However, bacterial spores resist drying for long periods. In dehydrated foods, bacterial growth does not take place; however, microbial action begins after the foods are reconstituted with water.

e. Radiation. Sunlight and artificially produced ultraviolet radiation have a germicidal effect on microorganisms. However, neither one is rapid in its effect nor complete in its action. Both sunlight and ultraviolet lamps are beneficial when used along with other sound procedures for disinfections. X-rays and other ionizing radiation are known to be lethal to microorganisms, but their use for this purpose has not been completely developed.

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