h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

This entry is fine as far as it goes but how does soap work in relation to bacteria?

Is it just that bacteria slides off our hands when we wash them (with our body oils) or is there something about the materials which make up soap which bacteria don't like to live in?

Have bacteria evolved to overcome the effect of soap since we've started using it?

Do we need to use disinfectants in our soap to make us clean these days?

Soaps can directly kill all bacteria and quite a few viruses. The idea that it only loosens bacteria's grip is a myth. Specialised disinfectants can be useful, but you'd be amazed how efficiently hot soapy water will kill almost anything.

A basic requirement of all cells is the ability to regulate the concentration of various ions, proteins and other gubbins inside them. Some need to be kept at levels higher than found in its environment, others need to be kept at lower levels.

They achieve this by having an outer "skin" of fat. Called a "lipid bilayer", this extremely thin layer of fat acts as a waterproof coat, stopping water and anything soluble from passing into or out of the cell except via carefully regulated channels.

Because soap allows water to interact with (hydrophobic) fats, soapy water can disrupt and destroy this coat. All of the cell's insides flow out and it's killed very quickly. Susceptible viruses are destroyed slightly differently but the explanation is longer than I have time to type right now. It's largely the same basic idea of disrupting a membrane though.

Our skin cells are protected from this by thick layers of a protein called keratin, which unfortunately ends up killing the cell as it thickesns; this is an important reason why our kin is constantly being replaced. If you get soap onto tissue not protected by keratin (inside a cut or onto mucous membranes like eyes, nasal passages etc), the cells there start suffering the same damage. That's part of why soap in cuts hurts so much.