Disinfection and disinfectants

Disinfection and the use of chemical disinfectants is one key strategy of infection control . Disinfection refers to the reduction in the number of living microorganisms to a level that is considered to be safe for the particular environment. Typically, this entails the destruction of those microbes that are capable of causing disease.

Disinfection is different from sterilization , which is the complete destruction of all microbial life on the surface or in the liquid. The steam-heat technique of autoclaving is an example of sterilization.

There are three levels of disinfection, with respect to power of the disinfection. High-level disinfection will kill all organisms, except for large concentrations of bacterial spores, using a chemical agent that has been approved as a so-called sterilant by the United States Food and Drug Administration. Intermediate level disinfection is that which kills mycobacteria, most viruses , and all types of bacteria . This type of disinfection uses a chemical agent that is approved as a tuberculocide by the United States Environmental Protection Agency (EPA). The last type of disinfection is called low-level disinfection. In this type, some viruses and bacteria are killed using a chemical compound designated by the EPA as a hospital disinfectant.

There are a variety of disinfectants that can be used to reduce the microbial load on a surface or in a solution. The disinfectant that is selected and the use of the particular disinfectant depend on a number of factors. The nature of the surface is important. A smoother surface is easier to disinfect, as there are not as many crevasses for organisms to hide. Generally, a smoother surface requires less time to disinfect than a rough surface. The surface material is also important. For example, a wooden surface can soak up liquids that can act as nutrients for the microorganisms, while a plastic surface that is more hydrophobic (water-hating) will tend to repel liquids and so present a more hostile environment for microbes.

Another factor in the selection of a disinfectant is the number of living microorganisms present. Generally, more organisms require a longer treatment time and sometimes a more potent disinfectant. The nature of the microbial growth is also a factor. Bacteria growing a slime-encased biofilm are hardier than bacteria that are not growing in biofilms. Other resistance mechanisms can operate. A general order of resistance, from the most to the least resistant, is: bacterial spores, mycobacteria (because of their unusual cell wall composition), viruses that repel water, fungi , actively growing bacteria, and viruses whose outer surface is mostly lipid.

Alcohol is a disinfectant that tends to be used on the skin to achieve a short-term disinfection. It can be used on surfaces as a spray. However, because alcohol evaporates quickly, it may not be present on a surface long enough to adequately disinfect the surface. A type of disinfectant known as tamed iodines, or iodophors, are also useful as skin disinfectants. In hospital settings, iodophors are used as a replacement for hand soap.

A better choice of disinfectant for surfaces is sodium hypochlorite. It can also be added to drinking water, where dissociation to produce free chlorine provides disinfection power. Bacteria such as Escherichia coli are susceptible to chlorine. Chlorination of drinking water is the most popular choice of water treatment in the world. If left for five minutes, sodium hypochlorite performs as an intermediate level disinfectant on surfaces. However, chlorine bleach can be corrosive to metal surfaces and irritating to mucous membranes of the eye and nose.

Another surface disinfectant is compounds that contain a phenol group. A popular commercial brand known as Lysol is a phenolic disinfectant. Phenolics are intermediate level disinfectants, derived from coal tar, that are effective on contaminated surfaces. However, certain types of viruses and some bacteria are resistant to the killing action of phenolic compounds.

Another disinfectant is chlorhexidine. It is effective against fungus and yeast , but is not as effective against Gramnegative bacteria. Nor will it inactivate viruses whose surfaces are water loving. In situations where the contaminant is expected to be fungi or yeast, chlorhexidine is a suitable choice of disinfectant.

Aldehyde compounds, such as formaldehyde and glutaraldehyde, are very effective disinfectants. Glutaraldehyde has other uses as well, such as preserving specimens prior to their examination by the technique of electron microscopy. Glutaraldehyde kills many microorganisms, and all known disease-causing microorganisms, after only a few minutes exposure. Another effective general disinfectant is those that contain quaternary ammonium.

Many disinfectants are non-specific in their action. They will act against any biological material that is present. These are referred to as broad-spectrum disinfectants. Examples of broad-spectrum disinfectants are glutaraldehyde, sodium hypochlorite (the active ingredient in common household bleach), and hydrogen peroxide. Disinfectants such as phenolics and quaternary ammonium compounds are very specific. Other disinfectants lie in between the highly specific and broadly based categories. For example, alcohol is effective against actively growing bacteria and viruses with a lipid-based outer surface, but is not effective against bacterial spores or viruses that prefer watery environments.

The potency of a disinfectant can also be affected by the concentration that is used. For example, pure alcohol is less effective than alcohol diluted with water, because the more dilute form can penetrate farther into biological specimens than the pure form can.

Another factor that can decrease the effectiveness of disinfectants can be the presence of organic (carbon-containing) material. This can be a great problem in the chlorine disinfection of surface water. The vegetation in the water can bind the chlorine, leaving less of the disinfectant available to act on the microorganisms in the water. Proteins can also bind disinfectants. So, the presence of blood or blood products, other body fluids, and fecal waste material can compromise disinfectant performance.

Microorganisms can develop resistance to disinfectants, or can even have built-in, or intrinsic, resistance. For example, application of some disinfectants to contaminated surfaces for too short a time can promote the development of resistance in those bacteria that survive the treatment.

See also Bacteriocidal and bacteriostatic; Fungicide