Can someone explain how soap kills 99%* of bacterial cells on my hands without doing any damage to skin cells?
In: Biology
The short answer: bacteria have different cell components than humans, and we have found a number of compounds that can disrupt those different parts of the cell but have less (or no) impact on us. A lot of approaches like this exploit differences in cell walls/membranes.
There are a few distinctions worth considering if you want to dive deeper.
1) Removing versus killing: the predominant method by which a soap will sanitize your hand is not by killing microbes but by allowing you to mechanically remove them. Soap tends to loosen the oils on your skin and can also mobilize the cells on the very top outer-layer of your skin. When you scrub and hit them with water, that action simply moves the microbes — in this case, off of your hands and into the sink.
2) Inactivation/stopping replication versus killing: in the US, a common antibacterial used in soaps is something called triclosan. Triclosan is somewhat toxic, but its main effect is to stop the bacteria from being able to replicate… or rather, to slow down the replication a great deal. A single cell may get into your body, but it is very unlikely to make you sick, unless it replicates and makes a few billion friends to help with infecting you. A cell that is alive but is not able to replicate is called “inactivated.” Triclosan is unquestionably effective in slowing/stopping replication of bacteria and some fungi, however its utility in soaps is highly dubious… it doesn’t seem to help.
Data suggest that mechanically removing bacteria using plain soap and water is where the vast majority of effectiveness from soap lies.
3) Some, but not all, hand washes that kill bacteria actually *do* harm your skin cells: I’ve done some work out in lovely old nature where it was likely that we’d pick up something pretty gross. We used loads of precautions, but we also had a dilute bleach solution. Bleach is about as chemically nasty as it gets! It kills bacteria as well as human skin cells (and it really irritates the skin that it doesn’t kill). The reason this is okay is because your skin will repair, and the harm that it does is worth it so that you don’t accidentally take in a fecal pathogen that might really ruin your body.
1 – bacteria wash off.
2 – human skin is slightly acidic (pH 5.5), many soaps are slightly basic (my homemade is about 8.5). As a result washing may alter the bacterial environment.
Soap doesn’t necessarily kill bacteria. Instead it helps the water to remove the bacteria from your hands.
Your outer layers of skin are full of keratin filled dead cells.
If you drink large amounts of nearly pure ethyl alcohol, isopropyl, or methanol then you’ll have problems.
Likewise eat detergents like a Tide pod and you can die… get it on your skin and you can just wash it off. Alcohol not so much because it’ll absorb through your skin.
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