Physical Methods of Disinfection
UV rays are long wavelength and low energy. They do not penetrate well and organisms must have direct surface exposure, such as the working surface of a BSC, for this form of disinfection to work. '
Chemical Methods of Disinfection
When chemicals are used to destroy all life they are called chemical' sterilants, or biocides; however, these same chemicals used for shorter periods are disinfectants. Disinfectants used on living tissue (skin) are called antiseptics.
Resistance to disinfectants varies with the type of microorganisms present Bacterial spores such as Bacillus spp., are the most resistant, followed by mycobacteria (acid-fast bacilli); nonlipid viruses, for example, poliovirus; fungi; vegetative (nonsporulating) bacteria, for example, gram-negative rods; and, finally, lipid viruses, for example, herpes simplex virus, which are the most susceptible to the action of disinfectants. The Environmental Protection Agency (EPA) registers chemical disinfectants used in the United States and requires manufacturers to specify the activity level of each compound at the working dilution. Therefore, microbiologists who must recommend appropriate disinfectants should check the manufacturer's cut sheets (product information) for the classes of microorganisms that will be killed. Generally, the time necessary for killing microorganisms increases in direct proportion with the number of organisms (microbial load). This is particularly true of instruments contaminated with organic material such as blood, pus, or mucus. The organic material should be mechanically removed before chemical sterilization to decrease the microbial load. This is analogous to removing dried food from utensils before placing them in a dishwasher and is important for cold sterilization of instruments such as bronchoscopes. The type of water and its concentration in a solution are also important.
Hard water may reduce the rate of killing of microorganisms, and surprisingly, 70% ethyl alcohol is more effective as a disinfectant than 95% ethyl alcohol.
Ethyl or isopropyl alcohol is nonsporicidal (does not kill spores) and evaporates quickly. Therefore, either is best used on the skin as an antiseptic or on thermometers and injection vial rubber septa as a disinfectant. Because of their irritating fumes, the aldehydes (formaldehyde and glutaraldehyde) are generally not used as surface disinfectants. The halogens, especially chlorine and iodine, are frequently used as disinfectants. Chlorine is most often used in the form of sodium hypochlorite (NaOCl), the compound known as household bleach. The Centers for Disease Control and Prevention (CDC) recommends that tabletops be cleaned following blood spills with a 1:10 dilution of bleach. Iodine is prepared either as tincture with alcohol or as an iodophor coupled to a neutral polymer, for example, povidone-iodine. Both iodine compounds are widely employed antiseptics. In fact 70% ethyl alcohol, followed by an iodophor, is the most common compound used for skin disinfection before drawing blood cultures or surgery. Because mercury is toxic to the environment, heavy metals containing mercury are no longer recommended, but an eyedrop solution containing 1% silver nitrate is still instilled in the eyes of newborns to prevent infections with Neisseria gonorrhoeae. Quaternary ammonium compounds are used to disinfect bench-tops or other surfaces in the laboratory. However, organic materials, such as blood, may inactivate heavy metals or quaternary ammonium compounds, thus limiting their utility. Finally, phenolics, such as the common laboratory disinfectant amphyl, are derivatives of carbolic acid (phenol). The addition of detergent results in a product that deans and disinfects at the same time, and at concentrations between 2% and 5% these products are widely used for cleaning bench-tops.
The most important point to remember when working with biocides or disinfectants is to prepare a working solution of the compound exactly according to the manufacturer's package insert. Many people think that if the manufacturer says to dilute 1:200, they will be getting a stronger product of they dilute it 1:10. However, the ratio of water to active ingredient may be critical, and if sufficient water is not added, the free chemical for surface disinfection may not be released.
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