There are a number of different types of alcohol, but the ones generally used as antiseptics are ethanol — also known as ethyl alcohol — and isopropyl alcohol, or isopropanol. Both chemicals are effective against a wide range of bacteria, viruses, protozoa, and fungi, and have relatively low toxicity to humans. They are most commonly used as skin antiseptics, often in the form of wipes or gels, and for disinfecting surfaces, but they are not generally used for sterilizing instruments. Their main effect on microorganisms seems to be to coagulate essential proteins, rendering them ineffective, and causing cell death or inhibiting reproduction. They may also have a dehydrating effect and may interfere with the functioning of cell membranes.
Mode of Action
Proteins are common to all living things and perform many essential functions within cells. For example, a class of proteins known as enzymes allow cells to carry out the chemical reactions that keep them alive. Protein molecules have complex shapes that are crucial to the way they behave and, if a protein loses its shape, it will probably not be able to perform its function. The loss of shape in a protein is known as denaturation, and it happens when the bonds that maintain its form are broken. Proteins that have been denatured tend to coagulate, meaning that the molecules clump together into a formless, solid mass.
Alcohols denature proteins by breaking the hydrogen bonds that link oppositely charged hydrogen and oxygen atoms on different parts of the chain-like molecules. They do this because they themselves form hydrogen bonds at these locations, causing the protein molecule to lose its shape. Their effectiveness depends on how concentrated they are. Ethanol, for example, works best against microorganisms at about 70% concentration, as it is easily absorbed by cells. At much higher concentrations, it coagulates proteins on the cell surface, preventing further penetration; microbes are often able to survive this, although they may be temporarily inactivated.
Although ethanol and isopropanol are both useful antiseptics with similar modes of action, there appear to be some differences in their effectiveness against different types of microbe. Ethanol seems to be slightly more effective than isopropyl alcohol against viruses, while the reverse appears to be true for bacteria. In addition to denaturating proteins, these alcohols can have a dehydrating effect, and can also dissolve lipids — fats and oils — to some extent, which can damage cell membranes. Ethanol has a greater dehydrating effect, while isopropanol has a greater affinity for lipids.
Alcohols are often used in hand washing products, both in hospitals and for the home. In this context, they are often marketed as bottles of alcohol-based gel to be rubbed on the skin. Hospital staff are encouraged to frequently wash their hands with antiseptic preparations to minimize the risk of dangerous infections in vulnerable patients. Another common form is wipes, where the chemical is impregnated into pieces of cotton cloth that can be used to wipe surfaces or wash hands. Ethanol and isopropanol are often used to disinfect the skin just prior to surgery.
Although these substances may cause drying and minor irritation of the skin, they do not damage living tissue through normal use, as it is protected by an outer layer of dead skin cells. They should not, however, be used internally or on sensitive areas of the body. Alcohols are not normally used for sterilizing wounds because they can cause damage to the internal tissues, although they have occasionally employed for this purpose in emergency situations. Other biocides may sometimes be combined with alcohol to increase the effectiveness of both substances. One of the oldest and best-known examples is tincture of iodine, which is a solution of iodine in ethanol.
Advantages and Disadvantages
As antiseptics, alcohols have the advantages of being fast acting and very effective against the majority of harmful microorganisms. They also evaporate quickly, so that they do not leave the skin wet for long, making hand drying unnecessary. This can also be a disadvantage, as it means that the effect is short-lived, so that microbes can survive on surfaces treated with this form of antiseptic just a short time after application. For this reason, other antiseptic substances that remain on the skin or surface after the alcohol has evaporated may be added.
Some bacteria can go into a dormant phase known as a spore when conditions are unfavorable for growth and multiplication, becoming active again when conditions improve. Spores are often very resilient, and alcohols generally do not kill them, although they can temporarily prevent them from becoming active. Again, this problem can often be overcome by adding other substances that are more effective in killing spores.
Ethanol and isopropanol can have a dehydrating and irritating effect on the skin. For this reason, commercial alcohol-based hand washes usually have an emollient, or moisturizer, added to help replace moisture and oils lost from the skin.