How Penicillin Works

Penicillin. The most famous antibiotic of all time. In The Song of the Cell, Siddhartha Mukherjee asks and answers two questions about penicillin that had never even occurred to me.

1. How does penicillin differentiate microbial cells from human cells?
2. How did it work so well against such a wide range of microbes? (Before our overuse of antibiotics undid its effectiveness)

 

A slight digression first. In my 11 yo daughter’s Biology book, there is a chapter on cells. It mentions bacteria as examples of unicellular plants. Plants, that’s right, bacteria are plants!

 

Though Mukherjee doesn’t mention it, that weird fact (bacteria are plants) is the starting point of the answer to both questions. A plant cell has a cell wall. Bacteria being plants have cell walls. To create those cell walls, they have a particular enzyme. Humans cells, being animal cells, don’t have cell walls – hence, human cells don’t need or have that enzyme.

 

Penicillin “kills” those enzymes that create cell walls. Since bacteria have it while we humans don’t, it becomes a selective killer of bacteria but not our cells. First question answered.

 

Since almost all bacteria have cell walls, therefore, penicillin becomes the proverbial magic bullet against bacterial species. Second question answered.

 

Which is why Mukherjee says:

“In this sense, every antibiotic is a “cellular medicine” – a drug that relies on the distinctions between a microbial cell and a human cell.”