Asking how antiseptic soaps work is a great question even though we use them daily, most people probably never think to ask what actually makes antiseptic soaps antiseptic.

To understand how antiseptic (i.e. antibacterial) soaps work, well first look at how soap functions in general. Most molecules are either water-loving (hydrophilic) or water-hating (hydrophobic). The reason water alone doesnt clean off dirt and germs is because those molecules are mostly hydrophobic. When water runs over these hydrophobic molecules, they dont get picked up by it because they do not get dissolved in water easily (think of pouring olive oil in water when youre cooking pasta the oil sticks together in a pot of water because the oil is hydrophobic).

Soap is made up of fatty acids (derived from oils or fats) and a base (like sodium hydroxide salt). The soap molecules have hydrophobic (fatty acid chain) and hydrophilic (salt) ends. This combination allows soap molecules to suspend the oils, dirt and germs that are washed away when you soap up. This happens because soap molecules act as a bridge between hydrophobic oil particles (the dirt and germs) and the hydrophilic water itself.

Soap alone is fairly good at removing dirt and germs from our skin; however, it doesnt remove or kill all of the microbes that live on us. Antibacterial soaps are increasingly popular because they are marketed as killing the bacteria on us when used. The goal of antiseptic soaps is to prevent the bacteria from reproducing, limiting the amount of bacteria present on a person's skin after using an antibacterial soap. Theoretically, less bacteria should mean less risk for infection or illness. (Side note: Its important to remember that we have microorganisms that live on us that are necessary for our own survival. These good microbes are also affected by antimicrobials.) The difference between regular soaps and antiseptic soaps are the antibacterial compounds triclosan and sometimes triclocarban. Triclosan and is also found in a lot of other goods, including cosmetics, toothpaste and plastic used in toys, kitchen and table utensils. Triclosan and triclocarban are hydrophobic, so they dont dissolve well in water, but do go through fatty substances, like the membranes of bacterial cells. Once they are in microbes cells, triclosan poisons a specific enzyme (enzymes are proteins that have particular functions, think of them as cellular machinery) that is used in making microbes cell membranes. Humans dont have this enzyme, so triclosan doesnt poison us. One molecule of triclosan permanently disables this microbial enzyme, which is why triclosan is such a strong antibiotic action even at very low concentrations. Triclocarban has a similar molecular structure, and probably kills microbes like triclosan does.

Because of the highly specific way that triclosan kills microbes and the amount in our environment, researchers concerned about its role in fostering antibiotic-resistant strains of bacteria. Researchers have demonstrated that mutations in the bacterial gene that produces the enzyme that triclosan poisons can produce triclosan-resistant bacteria. That is bad, because when people get infections with antibiotic or antimicrobial-resistant bacteria, the infections can become very difficult to treat and doctors need to find different antibiotics that the bacteria have not evolved resistance to. That is why scientists now suggest limiting the use of strong antiseptic products unless necessary. In fact, a study recently demonstrated that plain soaps are as effective as antibacterial soaps with triclosan in preventing illness and removing bacteria from the hands.

Soap molecules are both hydrophilic (likes water and goes into solution) and hydrophobic (repels water). Bacteria (and dirt and oils) get bound to the hydrophobic end of the soap molecules and the hydrophilic end helps it get washed away with water. This is just basic soap with no antibacterials in it.

Some soaps have antibiotics that kill bacteria. These antibiotics have different ways of killing bacteria. For example, the antibiotic triclosan is commonly used in soaps and targets an enzyme involved in making bacterial membranes. This enzyme is only found in bacteria and not in humans thus only killing bacterial cells.

Then there are the alcohol-based hand sanitizers (e.g. Purell). The alcohol in these sanitizers dehydrates bacteria, unfolds their proteins, and kills them.