Sterilization & Disinfection

Sterilization & Disinfection Introduction • Sterilization is the killing or removal of all microorganisms, including bacterial spores, which are highly resistant. Sterilization is usually carried out by autoclaving, which consists of exposure to steam at 121°C under a pressure of 15 lb/in2 for 15 minutes. Surgical instruments that can be damaged by moist heat are usually sterilized by exposure to ethylene oxide gas, and most intravenous solutions are sterilized by filtration. • Disinfection is the killing of many, but not all, microorganisms. For adequate disinfection, pathogens must be killed, but some organisms and bacterial spores may survive. Disinfectants vary in their tissue-damaging properties from the corrosive phenol containing compounds, which should be used only on inanimate objects, to less toxic materials such as ethanol and iodine, which can be used on skin surfaces. Chemicals used to kill microorganisms on the surface of skin and mucous membranes are called antiseptics. Rate of Killing of Microorganisms • Death of microorganisms occurs at a certain rate dependent primarily upon two variables: the concentration of the killing agent and the length of time the agent is applied. The rate of killing is defined by the relationship which shows that the number of survivors, N, is inversely proportionate to the concentration of the agent, C, and to the time of application of the agent, T. Collectively, CT is often referred to as the dose. Stated alternatively, the number of microorganisms killed is directly proportionate to CT. The relationship is usually stated in terms of survivors, because they are easily measured by colony formation. Death is defined as the inability to reproduce. In certain circumstances, the physical remains of dead bacteria can still cause problems (see Endotoxins)