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I see many products, particularly hand soap and cleaning products, that claim to kill 99.9% or more of bacteria.

This makes me wonder, if the chemicals are potent enough to break down bacterial cell membranes, can they also break down human cells? If not, why not?

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salamander is right about triclosan being the active ingredient in antibacterial soap, but the reason why it doesn't kill human cells doesn't have anything to do with the skin. From salamander's same source:

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.

The enzyme in question is called Enoyl-acyl carrier protein reductase (ENR), and is used by bacteria as part of their fatty acid synthesis. Eukaryotes use a different set of enzymes for fatty acid synthesis, so we aren't effected by this activity of triclosan.

For some more specifics about the interaction of triclosan and ENR, we can turn a study about the antibacterial mechanism of triclosan that was done soon after triclosan was first crystalized in situ with its target enzyme.

From Heath, et al (1999):

Triclosan is a broad-spectrum antibacterial agent that inhibits bacterial fatty acid synthesis at the enoyl-acyl carrier protein reductase (FabI) step... The ubiquitous occurrence of type II fatty acid synthase systems in bacteria and the essential nature of the FabI reaction make this enzyme an attractive target for antibacterial drugs. Accordingly, triclosan is effective against a broad spectrum of bacteria, including multi-drug-resistant Staphylococcus aureus.

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  • $\begingroup$ Your answer is correct. Thank you for clearing that up. +1 $\endgroup$ – CDB Nov 10 '15 at 19:11
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Antibacterial soap commonly uses triclosan, which can pass through the phospholipid bilayer of bacteria and disrupt the production of essential enzymes, killing the bacteria (source). This triclosan would kill human body cells in the same way as it does bacteria, however we have a 1-1.5mm thick layer of dead skin cells(called the Stratum corneum) that exists to protect our epidermis from chemicals like antibacterial soap.

EDIT: I was absolutely wrong about triclosan killing human cells. In fact, it is impossible for triclosan to kill human cells, because the enzyme that is destroyed in bacteria by the triclosan does not exist in the human body.

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  • $\begingroup$ Can you please add a specific citation for the stuff about the Stratum corneum blocking triclosan? I can't find it anywhere $\endgroup$ – tel Nov 10 '15 at 19:05
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    $\begingroup$ Forgive my blatancy, but this is incorrect. Triclosan WOULDN'T kill human cells even if it did come into direct contact with them because it binds enoyl-acyl carrier protein reductase (ENR) used in fatty acid synthesis in bacteria. Human cells lack ENR and are thus unaffected. $\endgroup$ – CDB Nov 10 '15 at 19:06
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    $\begingroup$ @CDB No, I appreciate the frankness. According to the European Union's Scientific Committee on Consumer Safety's Opinion on triclosan you are absolutely correct. I'll edit my post to reflect this. $\endgroup$ – salamander Nov 11 '15 at 0:00
  • $\begingroup$ Excellent! Your edit is perfect. +1 for a great answer. $\endgroup$ – CDB Nov 11 '15 at 1:22

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