Disinfection refers to treatments that reduce the numbers of living microorganisms and viruses (which are not considered to be alive, but which can cause disease when they infect a host cell) to a safe level. Disinfection is not intended to kill all the microbes present, which is the process that is called sterilization. Nonetheless, disinfection is a key component in infection control.
Health care facilities maintain three different levels of disinfection, based upon patient care levels and the purpose for which equipment and surfaces are used. High-level disinfection destroys all microorganisms on a surface, with the exception of high numbers of bacterial spores. Intermediate-level disinfection kills Mycobacterium tuberculosis, most viruses and fungi, and bacteria, but it does not kill bacterial spores. Low-level disinfection kills most bacteria, certain viruses and fungi, but does not reliably kill bacterial spores or the bacteria that causes tuberculosis.
Until the middle of the nineteenth century, surgeries and hospitalization frequently resulting in infections.The importance of personal hygiene and clean clothing had yet to be realized by health care providers. As a result, microbial infections easily spread from patient to patient. The French chemist Louis Pasteur (1822–1895) proposed that infections were connected with the presence of microorganisms. This idea prompted an English surgeon named Joseph Lister (1827–1912) to study this suggestion. Lister became convinced that infections following surgery often did involve microorganisms infecting the incision. To minimize this risk, Lister sprayed a film of carbolic acid over the patient during surgery. The treatment effectively disinfected the wound and helped reduce post-surgical infections. As Lister's findings became accepted, the importance of disinfection to medicine was recognized.
WORDS TO KNOW
BIOFILM: Biofilms are populations of microorganisms that form following the adhesion of bacteria, algae, yeast, or fungi to a surface. These surface growths can be found in natural settings such as on rocks in streams, and in infections such as can occur on catheters. Microorganisms can colonize living and inert natural and synthetic surfaces.
HIGH-LEVEL DISINFECTION: High-level disinfection is a process that uses a chemical solution to kill all bacteria, viruses, and all other disease-causing agents except for bacterial endospores and prions. High-level disinfection should be distinguished from sterilization, which removes endospores (a bacterial structure that is resistant to radiation, drying, lack of food, and other things that would be lethal to the bacteria) and prions (misshapen proteins that can cause disease) as well.
INTERMEDIATE-LEVEL DISINFECTION: Intermediate-level disinfection is a form of disinfection that kills bacteria, most viruses, and mycobacteria.
LOW-LEVEL DISINFECTION: Low-level disinfection is a form of disinfection that is capable of killing some viruses and some bacteria.
STERILIZATION: Sterilization is a term that refers to the complete killing or elimination of living organisms in the sample being treated. Sterilization is absolute. After the treatment the sample is either devoid of life, or the possibility of life (as from the subsequent germination and growth of bacterial spores), or it is not.
Disinfection uses a chemical or other type of agent (typically ultraviolet light) to kill microorganisms. All of these agents are termed disinfectants.
Ultraviolet light disinfects because of the high energy of the waves of light. The energy is sufficient to break the strands of genetic material of the microbes. When many breaks occur in the deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) the damage is lethal, as it cannot be repaired by the microorganism. Ultraviolet light can be used to disinfect liquids of small volume, surfaces, and some types of equipment.
Alcohol is a liquid disinfectant that tends to be used on the skin to achieve short-term disinfection. It kills microbes, such as bacteria, by dissolving the membrane around the organisms. It can be sprayed on surfaces; the droplets of alcohol will kill microbes on contact. The spray needs to be fairly heavily applied to a surface to ensure disinfection because the alcohol evaporates quickly. If it evaporates within a few seconds, the microorganisms may not be exposed long enough to be killed. Alcohol-based hand washes are also available, and are becoming more widely used in hospitals because a busy doctor or nurse need only rub their hands for 10–15 seconds with an alcohol-based solution to adequately disinfect their hands between seeing patients. Typical disinfectant soaps such as those used in the home require skin contact of 30 seconds or more to be effective disinfectants.
The compound iodine is another disinfectant. In hospitals, surgical scrubbing is often accomplished using an iodine-containing soap. As with alcohol-based handwashing, the intent is to lower the number of living bacteria on the surface of the skin, although iodine is a more efficient disinfectant than alcohol.
Another liquid disinfectant that remains on a surface much longer is sodium hypochlorite. The active component of the disinfectant is chlorine, and it is also the disinfectant agent in household bleach. Water can also be treated using chlorine, and this is the basis of drinking water chlorination. The concentration of sodium hypochlorite used is important—too much chlorine can dissolve metal surfaces and can irritate the cells in the eye and the nose. A sodium hypochlorite solution (bleach) is used by medical personnel in the field when investigating outbreaks of diseases that can be spread by contact with infected body fluids, droplets, or contaminated surfaces, and when local infrastructure will not support high-tech disinfection methods. For example, the Centers for Disease Control and Prevention (CDC) recommended household bleach diluted with water in a 1:100 ratio to disinfect areas contaminated with blood and body fluids in a makeshift isolation ward hospital during a 2003 outbreak of Ebola in the Cuvette West region of the Democratic Republic of Congo.
Surfaces can also be disinfected using compounds that contain a phenol group. A popular example is Lysol®. In a hospital, phenol-based disinfectants are not used in certain cases, such as in an operating theater. This is because some disease-causing bacteria and viruses are resistant to phenol.
Chlorhexidine is a chemical disinfectant that kills fungi and yeast much more effectively than bacteria and viruses. Formaldehyde and glutaraldehyde possess a chemical group called an aldehyde, which is a very potent disinfectant. Glutaraldehyde is a general disinfectant, which means it is effective against a wide array of microbes after only a few minutes of contact. Another effective general disinfectant is quaternary ammonium.
The disinfection strategy that is selected depends on a number of factors. These include the surface being disinfected and the intended use of that surface (a doctor's hands should be disinfected rigorously, for example). A smooth crack- or crevasse-free surface is easier to disinfect, and so typically requires less time to disinfect than does a rougher surface. A rough surface, which has niches that microorganisms can fit into, is not an appropriate surface to disinfect with a rapidly evaporating spray of alcohol. The surface material is also important. For example, a wooden surface may soak up liquids and reduce the concentration of the disinfectant that acts on the microorganisms.
The number of microorganisms present can determine the type of disinfectant used and how long it should be used for. Higher numbers of microbes usually require a lengthier exposure time to reduce the number of living organisms to a level that is considered safe. How the organisms grow is also important. For example, many disease-causing bacteria can grow in a slimeencased community known as a biofilm. Biofilm bacteria are much more resistant to disinfectants than they are when dispersed from the biofilm. As another example, bacteria such as Bacillus anthracis, the organism that causes anthrax, and Clostridium botulinum, a neurotoxinproducing bacterium that can contaminate foods, can form a hardy structure called a spore, which often survives exposure to disinfectants.
Many disinfectants act against a variety of microbes; they are known as broad-spectrum disinfectants. Glutaraldehyde, sodium hypochlorite, and hydrogen peroxide are broad-spectrum disinfectants. Other disinfectants act on specific microorganisms, while the activity of other disinfectants is in between these extremes. An example of the latter is alcohol. It dissolves cell membranes that are made of lipids, and so is effective against many bacteria and viruses. Spores or viruses that do not have a lipid membrane, however, are not as affected by alcohol as bacteria.
Disinfectants are a vital defense against infectious disease, especially in the health care, cosmetic, and food service industries. Still, the full benefits of disinfection have yet to be realized. Surveys conducted in North America and Europe have shown that health care providers do not wash their hands between patient visits as often as they should. Transfer of infection from patient to patient via the hands of medical personnel and their equipment (such as a stethoscope) still occurs, even though it could be avoided in many cases. Alternatively, overuse of disinfectants can cause microorganisms to develop resistance to disinfectants, if, for example, the compound is not applied for an adequate amount of time. This resistance can make it more difficult to eliminate sources of infection, which allows for their spread.
On a broader scale, wide-scale disinfection of drinking water supplies was one of the most significant public health accomplishments of the twentieth century. Out-breaks of waterborne diseases such as typhus and cholera were common in both the United States and abroad before modern disinfection methods were put into place. In the 1990s, researchers recognized that while disinfectants neutralized many pathogens (disease-causing organisms) in water, some disinfectants also reacted with naturally occurring organic and inorganic matter in water sources and municipal water delivery systems. These reactions produced potentially harmful compounds called disinfection byproducts (DBPs). After DBPs were found to cause cancer and adverse reproductive effects in laboratory mice, the Environmental Protection Agency (EPA) set in place in 2001 new regulations to maximize disinfection of drinking water supplies while minimizing public exposure to DBPs.
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