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I was quite fascinated by the feature Should Science Pull the Trigger on Antiviral Drugs—That Can Blast the Common Cold? in this month's Wired magazine.

They explain that Penicillin is effective at killing bacteria because it interferes with the growth of bacterial cell walls.

How does Penicillin do that exactly? And why does it not dissolve human cells as well?

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The simple answer is that human cells don't have cell walls. –  Mad Scientist Mar 21 '12 at 16:15
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Bacteria have a mesh-like structure surrounding their plasma membrane called a cell wall. The cell wall is made up of peptidoglycan polymers that form a rigid crystalline structure that helps protect the osmotic pressure of the bacterial cytoplasm.

Penicillin and other β-lactams work by inhibiting the final step of peptidoglycan synthesis, which prevents transpeptidation (crosslinking) of the peptidoglycan molecules. This leads to the death of the bacterium by osmotic pressure due to the loss of the cell wall.

This drug doesn't affect human cells because they lack a cell wall surrounding their plasma membrane.

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n.b. penicillin only affects gram positive bacteria. –  Rory M Mar 21 '12 at 18:09
    
@RoryM: That is not totally correct, penicillin does affect some gram negative bacteria but it is not very effective. There are other ß-lactams that do affect gram-negative bacteria such as Carbenicillin. –  GWW Mar 21 '12 at 18:15
    
I don't think that this is necessarily correct. The beta-lactams doesn't inhibit the synthesis but rather the reorganization of the peptidoglycans. The rigidity of the now static wall then prevents cell division. –  bobthejoe Mar 27 '12 at 23:14
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@bobthejoe: I updated my answer to better explain it. Thanks. –  GWW Mar 28 '12 at 0:57
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