Sterilization and Disinfection. Chapter 12

 

Terms related to disinfection and sterilization - Table 12.1

·         Disinfectants are typically applied to inanimate objects to reduce the number of pathogenic organisms.

·         Antiseptics are generally applied to living organisms to destroy microorganisms or limit their growth.

·         Sterilization is the complete killing of all microorganisms in a material or on an object.  A substance is either sterile or not sterile - there is no intermediate.

·         A chemical substance is bacteriostatic if it inhibits the growth of organisms.

·         A chemical substance is bacteriocidal if it kills the organisms

·         In like manner viro, sporo and fungi can be substituted for bacterio in the terms bacteriocidal and bacteriostatic.

Disinfection

·         Figure 12.1 shows a method of determining efficacy of disinfection.

·         A concept - the number of organisms present at the beginning of disinfection affects the time needed to achieve disinfection.

·         Selection of disinfectant

·         Must be fast-acting in the presence of organic materials

·         Must be effective against all microorganisms without destroying tissue or acting as a toxin if ingested.

·         Easily penetrate the material to be disinfected without discoloration or damage.

·         Easy to prepare and stable in the environment where it is to be used.

·         Inexpensive and easy to use.

·         Not have an unpleasant odor.

·         There is no perfect disinfectant that meets all criteria.

Mechanisms of action

·         Denature proteins - Figure 12.2 - acids, alkalis, halogen compounds

·         Affect membranes - surfactants, detergents, quaternary ammonium compounds - dissolve lipids - Figure 12.3.

·         Affect other cell components - alkylating agents - nucleic acids (e.g. formaldehyde).

·         Examples of some of the simpler disinfectants are in Figure 12.4.

·         Heavy metals - Figure 12.5 - silver nitrate solution in infants eyes to prevent infection by Neisseria during delivery.  Tincture of merthiolate is an organomercury compound used as an antiseptic.

·         Properties of chemical antimicrobial agents are summarized in Table 12.3.

·         A method of expressing efficacy is the phenol coefficient.  The phenol coefficient is determined by testing dilutions of the disinfectant relative to that of phenol.

Physical antimicrobial agents - summarized in Table 12.5

·         Dry heat

·         Useful for substances not damaged by dry heat - 171oC for one hr.

·         Moist heat

·         Autoclave - Figure 12.10 - 121oC, 15 lb/in2 for 15 min - time is dependent on configuration of the materials to be sterilized - a one liter flask of broth is easy - a 30 gal bag stuffed with insulating materials may require one hr or more!!

·         Refrigeration, freezing excellent to preserve food and other materials - retards microbial growth, but does not effectively kill microorganisms.  Microorganisms can be stored in this way.

·         Drying - many microorganisms are sensitive to drying - but, many are not.  For example, Treponema pallidum dies if dried.  Thus, if toilet seats are dried, there is little chance of spread of syphilis by that route.  Freeze drying is used for storing many microorganisms.

·         Radiation is an effective means to sterilize materials. 

·         Ultraviolet light is effective to sterilize surfaces, but it does not penetrate - Figure 12.15.

·         Ionizing radiation is an effective sterilizer.  It damges DNA and generates peroxides in the cells.  Both effectively sterilize the material.  The material does not become radioactive.  Gamma rays and x-rays pass through the material without causing the material to become radioactive.  Very effective for the preservation of food.

·         Microwave radiation - does not sterilize materials - effective only where watermolecules are present.  Therefore, bacterial endospores are not killed - endospores do not contain water.  Ordinary microwave ovens are not effective.  Specialized microwave ovens can be used for sterilization.

·         Filtration is an effective way to remove microorganisms from a liquid.  However, the porosity of the filter is very important.  To effectively remove bacteria, a pore size of 0.45 mm is essential, 0.22 mm is preferred.  Figure 12.17 - Table 12.4

·         Osmotic pressure can create conditions that result in plasmolysis - the loss of water from cells - bacteriostatic.  Prevents growth of microorganisms in foods with high salt or sugar content - Figure 12.13.