CONTROL OF MICROORGANISMS

The following are definitions of commonly used terms in the control of microbes.

STERILIZATION is the destruction of all microorganisms and viruses, as well as endospores. Sterilization is used in preparing cultured media and canned foods. It is usually performed by steam under pressure, incineration, or a sterilizing gas such as ethylene oxide.

ANTISEPSIS: is the reduction of pathogenic microorganisms and viruses on living tissue. Treatment is by chemical antimicrobials, like iodine and alcohol. Antisepsis is used to disinfect living tissues without harming them.

DISINFECTION: is the destruction or killing of microorganisms and viruses on nonliving tissue by the use of chemical or physical agents. Examples of these chemical agents are phenols, alcohols, aldehydes, and surfactants.

ASEPTIC: means to be free of pathogenic contaminants. Examples include proper hand washing, flame sterilization of equipment, and preparing surgical environments and instruments.

Any word with the suffix -cide or –cidal indicates the death or destruction of an organism. For example, a bactercide kills bacteria. Other examples are fungicides, germicides and virucides. Germicides include ethylene oxide, propylene oxide, and aldehydes.

Any word with the suffix -stat or –static indicates the prevention of growth of an organism. For example, a bacteriostat prevents growth and multiplication of bacteria.






METHODS OF STERILIZATION


PHYSICAL AGENTS

I. HEAT

Heat is the most common, inexpensive, simplest, reliable, and effective method used to destroy microorganisms. Heat denatures the proteins and enzymes of the microorganisms. Most pathogens will be destroyed at temperatures between 50° and 70°C for duration of 10 minutes, except endospores which may survive 1 to 2 hours at 100°C.

The heat used in sterilization is either moist or dry heat.

A. MOIST HEAT

(i). BOILING - Bacteria, fungi, and many viruses are destroyed by boiling at 100°C for I0 to 30 minutes. Some viruses and endospores may require boiling for up to 20 hours. Sterilization is accomplished by denaturing the proteins and enzymes of the microorganisms.

(2). STEAM UNDER PRESSURE - Water is heated under pressure which raises the temperature above 100°C which denatures proteins and amino acids. The most common device used is an autoclave for sterilizing surgical bandages, instruments, media, and contaminated material. At a pressure of 15 pounds/square inch and a temperature of 121°C for 15 to 20 minutes it will destroy microorganisms and endospores.

(3) PASTEURIZATION - It is mainly used in the food and dairy industries and it involves raising the temperature highenough to destroy pathogens or inhibit their growth without affecting the quality of the product.




B. DRY HEAT

Dry heat penetrates substances more slowly than moist heat.

(I) INCINERATION: The burning of disposable substances in a chamber.

(2). DIRECT FLAMING - Sterilization of inoculating loops, needles, and rims of test tubes with a bunsen burner. Wire heated to a red glow is 100% effective.

(3). HOT-AIR OVEN - Sterilization of glassware, test tubes, petri dishes, instruments, syringes, and needles requires higher temperatures for a longer period of time than other methods. Most endospores will be destroyed at 160-165"C for a period of 2 hours.

2. COLD

The effect of low temperature on microorganisms depends on the type of microorganism and the intensity of the application. Temperature in a refrigerator ranges from 0-8oC and has a bacteriostatic effect which reduces the metabolic rate of most organisms so that they cannot reproduce or synthesize toxins. Freezing at -20"C kills most bacteria, but some may survive in a frozen state.

3. DRYING OR DESICCATION

To grow and multiple, microorganisms require water. The removal of water by evaporation or freeze-drying (solid to gas) inhibits growth and reproduction of microorganisms by inhibiting enzymes. They may be viable for years so when water is made available they resume growth and reproduction.

4. ULTRA-VIOLET RADIATION AND IONIZINQ RADIATION (X-RAYS AND GAMMA RAYS)

Both types of radiation damage the DNA of microorganisms and denatures their proteins. U-V radiation cannot penetrate materials such as glass, clothing, dirt, paper, or pus and is used to kill microorganisms on surfaces. Germicidal lamps in operating rooms, nurseries, and communicable disease wards reduce the number of bacteria in the air, but they do not sterilize.


5. FILTRATION

Filtration is the passage of material or liquids through a filter containing small pores that retain microorganisms. Membrane filters of cellulose acetate are the most common, but gauze, cotton, and paper serve as filters for air. Vaccines that require the presence of live viruses, such as polio, are passed through filters which prevent bacteria from going through the pores. Air filters are used in operating rooms to lower the number of airborne microorganisms.

CHEMICAL AGENTS

Chemical agents are used to control the growth of microorganisms on inanimate objects and on living tissue. Most chemical agents reduce the number of microorganisms (disinfect), but do not achieve sterility. Selection of a chemical agent depends on the mode of action, concentration necessary, the type of microorganism, and the number of microorganisms present, the type of material to be disinfected, temperature, and pH.

I. PHENOLS (CARBOLIC ACID)

Phenol and derivatives called phenolics disrupt the plasma membrane, denature proteins, and inactivate enzymes. Phenol is rarely used anymore because it causes skin irritation and has a disagreeable odour. Common phenolics are cresols such as that found in Lysol, and hexachlorophene which is used as an antiseptic.

2. ALCOHOLS

Ethanol and isopropanol are widely used as skin antiseptics, but they are not effective against enveloped viruses or bacterial endospores. They effectively destroy bacteria and fungi by disrupting the lipids in the plasma membrane resulting in lysis, and denature proteins.




3. SURFACE-ACTIVE AGENTS (SURFACTANTS)

They include soaps and detergents that lower the surface tension of liquid molecules and make microorganisms accessible to other agents. The pH of soaps is usually alkaline which destroys some bacteria. Detergents are more effective against gram-positive than gram-negative bacteria and they disrupt the plasma membrane.

4. HALOGENS

The major halogens are iodine and chlorine. Iodine is an effective antiseptic for the skin and superficial wounds. It destroys many kinds of bacteria, some endospores, fungi, and some viruses. Iodine inhibits protein function in the microorganism. Chlorine is used as a disinfectant and it is a strong oxidizing agent which affects functions of the microorganisms’ enzymes.

5. IONS OF HEAVY METALS

The major metal ions are mercury and silver, and they function in denaturing proteins and enzymes. Both are used in combination with other substances as antiseptics.

6. CHLORHEXIDINE

Chlorhexidine is used as an antiseptic on skin and mucosal membranes and functions in disruption of the plasma membrane of gram-positive and gram-negative bacteria resulting in lysis.

7. ALKYLATING AGENTS

Alkylating agents disrupt the structure of proteins and nucleic acids. Formaldehyde (.2 - .4%) is used to inactivate viruses and toxins used in vaccines, it also destroys spores in bacteria and fungi. Glutaraldehyde is less irritating and more effective than formaldehyde. It kills many microorganisms, viruses, and endospores.


Ethylene oxide is a gas used in a closed chamber to sterilize materials. It kills all bacteria, including endospores. Beta propiolactone kills spores in concentrations not much more than required for killing cells. The effect is rapid and it disappears in a few hours. It is used to sterilize bone, cartilage, and artery grafts.

BIOLOGICAL AGENTS

This is the use of living organisms to destroy other living organisms. Examples are as follows