Sterilization And Disinfection

Sterilization is a process whereby all forms of microbial life; including bacterial spores, are killed. Sterilization may be accomplished by physical or chemical means. Disinfection is a process whereby pathogenic organisms, but not necessarily all microorganisms or spores, are destroyed. As with sterilization, disinfection may be accomplished by physical or chemical methods.

Methods of Sterilization

The physical methods of sterilization include the following:

  • Incineration
  • Moist heat
  • Dry heat
  • Filtration
  • Ionizing (gamma) radiation

Incineration is the most common method of treating infectious waste. Hazardous material is literally burned to ashes at temperatures of 870° to 980° C. Toxic air emissions and the presence of heavy metals in ash have limited the use of incineration in most large U.S. cities, however.

Moist heat (steam under pressure) is used to sterilize biohazardous trash and heat-stable objects; an autoclave is used for this puipose. An autoclave is essentially a large pressure cooker. Moist heat in the form of saturated steam under 1 atmosphere (15 psi [pounds per square inch]) of pressure causes the irreversible denaturation of enzymes and structural proteins. The most common type of steam sterilizer in the microbiology laboratory is the gravity displacement type shown in Figure 4-1. Steam enters at the top of the sterilizing chamber and, because steam is lighter than air, it displaces the air in the chamber and forces it out the bottom through the drain vent. The two common sterilization temperatures are 121° C (250° F) and 132°C (270° F). Items such as media, liquids, and instruments are usually autoclaved for 15 minutes at

Physical SterilizationPhysical Sterilization

Chamber wall

Outer shell

Heat exchanger

Steam to jacket

Steam from jacket to chamber

Chamber drain screer*

Water supply

Water/steam ejector

Figure 4-1 Gravity displacement type autoclave. A, Typical Eagle Century Series sterilizer for laboratory applications. B, Typical Eagle 3000 sterilizer piping diagram. The arrows show the entry of steam into the chamber and the displacement of air. (Courtesy AMSCO International Inc., a wholly owned subsidiary of STERIS Corp., Mentor, Ohio.)

121° C. Infectious medical waste, on the other hand, is often sterilized at 132° C for 30 to 60 minutes to allow penetration of the steam throughout the waste and the displacement of air trapped inside the autoclave bag. Moist heat is the fastest and simplest physical method of sterilization.

Dry heat requires longer exposure times (1.5 to 3 hours) and higher temperatures than moist heat (160° to 180° C). Dry-heat ovens are used to sterilize items such as glassware, oil, petrolatum, or powders. Filtration is the method of choice for antibiotic solutions, toxic chemicals, radioisotopes, vaccines, and carbohydrates, which are all heat-sensitive. Filtration of liquids is accomplished by pulling the solution through a cellulose acetate or cellulose nitrate membrane with a vacuum. Filtration of air is accomplished using high-efficiency particulate air (HEPA) filters designed to remove organisms larger than 0.3 pm from isolation rooms, operating rooms, and biological safety cabinets (BSCs). Ionizing radiation used in microwaves and radiograph machines are short wavelength and high-energy gamma rays. Ionizing radiation is used for sterilizing disposables such as plastic syringes, catheters, or gloves before use. The most common chemical sterilant is ethylene oxide (EtO), which is used in gaseous form for sterilizing heat-sensitive objects. Formaldehyde vapor and vapor-phase hydrogen per oxide (an oxidizing agent) have been used to sterilize HEPA filters' in BSCs. Glutaraldehyde, which is spori-cidal (kills spores) in 3 to 10 hours, is used for medical equipment such as bronchoscopes, because it does not corrode lenses, metal, or rubber. Peracetic acid, effective in the presence of organic material, has also been used for the surface sterilization of surgical instruments. The use of glutaraldehyde or peracetic acid is called cold sterilization.

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