Alexander Fleming

TRADUCTION FLEMING

The word « antibiotic » comes from anti and bios, which mean respectively against and life.

Therefore, an antibiotic is a natural or synthetic substance which destroys or blocks the growth of bacteria. In the first case, we talk about bactericidal antibiotic, and in the second case, we talk about  bacteriostatic antibiotic.

We must know that the antibiotics act only on bacteria. They have no effect on the viruses.

Various mechanisms of action exist to kill bacteria :

- First, Action on the wall :

Bacterial cell wall forms thanks to transpeptidase. It’s an enzyme which allows the synthesis of the peptidoglycane. The peptidoglycane is a component of the bacterial wall.

The action of antibiotics prevents the synthesis of the transpeptidase and consequently that of the peptidoglycane. So, bacteria dies because it can’t survive without wall.

- Secondly, Action on the membrane :

The existence of an intact plasmic membrane is necessary for the bacterial survival.

Its role is double.

On the one hand, it allows to keep metabolites and necessary ions inside the cytoplasm.

On the other hand, it maintains a gradient of protons between the inside and the outside of the cell thanks to the respiratory chain and Krebs cycle. This gradient feeds the ATP-synthase which creates cellular energy, the ATP.

Any disturbance of the impermeability of the membrane interrupts these exchanges and the contents of the cytoplasm run away to the extracellular environment.

Antibiotics can act on the membrane of cells as detergents which disrupt lipids, or by forming a pore, that is (c'est à dire) a hole in a membrane which is going to allow the leak of the cellular compounds and to destroy the bacteria.

- To finish, Action on the DNA:

The synthesis of nucleic acids, DNA and ARN is absolutely vital for cells. Without it, the cellular division and the production of proteins is impossible.

Some antibiotics can block in a direct or indirect way these ways of biosynthesis of nucleic acids.

They settle on the DNA to prevent the formation of the DNA polymerase. So, there won't be transcription anymore, a DNA replication anymore and thus no new bacteria.

On thousands of known antibiotics, only a little more than hundred are effective and usable for medical applications. Others are too toxic, or too unstable to humans.

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