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Chapter: Medical Microbiology: Sterilization and Disinfection

Sterilization and Disinfection: Definitions

Death/killing as it relates to microbial organisms is defined in terms of how we detectthem in culture.

Sterilization and Disinfection

From the time of debates about the germ theory of disease, killing microbes before theyreach patients has been a major strategy for preventing infection. In fact, Ignaz Semmel-weiss successfully applied disinfection principles decades before bacteria were first iso-lated .


Death/killing as it relates to microbial organisms is defined in terms of how we detectthem in culture. Operationally, it is a loss of ability to multiply under any known condi-tions. This is complicated by the fact that organisms that appear to be irreversibly inacti-vated may sometimes recover when appropriately treated. For example, ultraviolet (UV) irradiation of bacteria can result in the formation of thymine dimers in the DNA with loss of ability to replicate. A period of exposure to visible light may then activate an enzyme that breaks the dimers and restores viability by a process known as photoreactivation. Mechanisms also exist for repair of the damage without light. Such considerations are of great significance in the preparation of safe vaccines from inactivated virulent organisms.

Sterilization is complete killing, or removal, of all living organisms from a particularlocation or material. It can be accomplished by incineration, nondestructive heat treatment, certain gases, exposure to ionizing radiation, some liquid chemicals, and filtration.

Pasteurization is the use of heat at a temperature sufficient to inactivate importantpathogenic organisms in liquids such as water or milk but at a temperature below that needed to ensure sterilization. For example, heating milk at a temperature of 74°C for 3 to 5 seconds or 62°C for 30 minutes kills the vegetative forms of most pathogenic bacteria that may be present without altering its quality. Obviously, spores are not killed at these temperatures.

Disinfection is the destruction of pathogenic microorganisms by processes that fail tomeet the criteria for sterilization. Pasteurization is a form of disinfection, but the term is most commonly applied to the use of liquid chemical agents known as disinfectants, which usually have some degree of selectivity. Bacterial spores, organisms with waxy coats (eg, mycobacteria), and some viruses may show considerable resistance to the com-mon disinfectants. Antiseptics are disinfectant agents that can be used on body surfaces such as the skin or vaginal tract to reduce the numbers of normal flora and pathogenic contaminants. They have lower toxicity than disinfectants used environmentally but are usually less active in killing vegetative organisms. Sanitization is a less precise term with a meaning somewhere between disinfection and cleanliness. It is used primarily in house-keeping and food preparation contexts.

Asepsis describes processes designed to prevent microorganisms from reaching a protected environment. It is applied in many procedures used in the operating room, in the preparation of therapeutic agents, and in technical manipulations in the microbiology lab-oratory. An essential component of aseptic techniques is the sterilization of all materials and equipment used.

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