Ethanol and isopropyl alcohol are effective antiseptics against bacteria, viruses, protozoa, and fungi. They denature proteins by breaking hydrogen bonds and coagulating essential proteins, causing cell death or inhibition of reproduction. They are commonly used in hand washing products and disinfecting surfaces, but not for sterilizing tools. They have a dehydrating effect and can interfere with cell membranes. They are fast-acting but short-lived, and may not kill spores. They can cause dryness and irritation to the skin, so moisturizers are often added to hand washes.
There are several types of alcohol, but the ones commonly used as antiseptics are ethanol, also known as grain alcohol, and isopropyl alcohol or isopropanol. Both chemicals are effective against a broad 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 to disinfect surfaces, but are not generally used to sterilize tools. Their main effect on microorganisms appears to be to coagulate essential proteins, rendering them ineffective and causing cell death or inhibition of reproduction. They can also have a dehydrating effect and can interfere with the functioning of cell membranes.
Way of acting
Proteins are common to all living things and perform many essential functions within cells. For example, a class of proteins known as enzymes enable cells to carry out the chemical reactions that keep them alive. Protein molecules have complex shapes that are crucial to how they behave, and if a protein loses its shape, it probably won’t be able to perform its function. The loss of shape in a protein is known as denaturation and occurs when the bonds that maintain its shape are broken. Proteins that have been denatured tend to coagulate, meaning the molecules clump together into a solid, shapeless mass.
Alcohols denature proteins by breaking the hydrogen bonds that connect oppositely charged hydrogen and oxygen atoms on different parts of the chain 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 a concentration of about 70%, 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, although they may be temporarily inactivated.
While 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 microbes. Ethanol appears to be slightly more effective than isopropyl alcohol against viruses, while the opposite appears to be true for bacteria. In addition to denaturing 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.
Uses
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 into the skin. Hospital staff are encouraged to wash their hands frequently with antiseptic preparations to minimize the risk of dangerous infections in vulnerable patients. Another common form is wipes, in which the chemical is soaked 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 before surgery.
While these substances may cause dryness and mild irritation to the skin, they do not harm living tissue through normal use, as they are 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 to sterilize wounds because they can cause internal tissue damage, although they have occasionally been employed for this purpose in emergency situations. Other biocides can 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 advantage of being fast-acting and very effective against most harmful microorganisms. They also evaporate quickly, so you won’t leave your skin wet for long, making drying your hands unnecessary. This can also be a disadvantage, as it means that the effect is short-lived, so that microbes can only survive on surfaces treated with this form of antiseptic a short time after application. For this reason, other antiseptic substances may be added which remain on the skin or surface after the alcohol has evaporated.
Some bacteria can enter a dormant stage known as a spore when conditions are unfavorable for growth and multiplication, becoming active again when conditions improve. The spores are often very resistant and alcohols generally do not kill them, although they may temporarily prevent them from becoming active. Again, this problem can often be overcome by adding other substances that are more effective at killing spores.
Ethanol and isopropanol can have a drying and irritating effect on the skin. For this reason, commercial alcohol-based hand washes usually have an emollient, or moisturizer, added to help replace lost moisture and oils from the skin.
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