Introducing... Benzylpenicillin - it's cracking!

Updated: Sep 4, 2020

This month I have the pleasure of introducing you to benzylpenicillin – it’s cracking!


Benzylpenicillin is an antibiotic that belongs to a class of medicines known as – you guessed it – penicillins.

Mechanism of Action

Penicillins (and a few other related antibiotics) kill bacteria by disrupting the production of the bacterial cell wall. Mammalian and bacterial cell walls are entirely different - which is fantastic news, as otherwise penicillins would be lethal for us too!

So how do mammalian and bacterial cell walls differ?

Good question. Well, as you may know, a mammal’s cell is composed of lipids (fats). However, a bacterial one is made from a substance called murein - aka peptidoglycan. Peptidoglycan is a sugar with a chain of five amino acids bolted on to it. That's probably a little difficult to imagine, so instead, think of a yo-yo...

The sugar of peptidoglycan is the body of the yo-yo, while the amino acid chain is the flailing string. As you would expect, on their own, a cluster of yo-yos do not form a particularly effective wall. However, with the help of an enzyme known as DD-transpeptidase, they get organised: the amino acids chains (the yo-yo strings) knot and matt themselves together.

OK... but what does this have to do with penicillin?

Penicillin mimics the spacial and chemical characteristics of a portion of the amino acid chain, the yo-yo string, and is highly unstable and reactive to boot. The DD-transpeptidase is tricked into binding to it and, when it does, the penicillin powerfully and irreversibly locks it in place. This means that DD-transpeptidase, the "organisation enzyme", is unavailable to bind other peptidoglycan chains together.

As a result, the bacterium is unable to produce a stable cell wall and is unable to maintain its own integrity. Essentially the cell wall breaks and so the bacterium dies.


Broadly, there are two different types of bacteria: gram positive and gram negative, which are characterised by the differences in their cell wall. Gram positive bacteria have a larger and more predominant peptidoglycan cell wall, therefore penicillins are generally more effective against them.

Responsive infections for benzylpenicillin include: skin infections (e.g. caused by staphylococcus), ear/nose/throat infections (e.g. caused by streptococcus), respiratory infections (e.g. caused by pneumococcus), syphilis (caused by spirochetes), gonorrhoea (caused by gonococcus).

As benzylpenicillin is unstable in acid it can not be taken orally (as it would be destroyed in the stomach) and therefore it is always administered via an injection.

Random fact

Penicillin was the first antibiotic ever used – it was discovered in 1929 and, strangely, not used as a chemotherapeutic agent until 1945. Although today penicillins can be found in every country in the world and are relatively cheap, back in 1945 they were unable to produce it on a large scale and therefore they were rare and exceptionally expensive.

Have a marvellous weekend!

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