2.3 What antibiotics do

Antibiotics that are used for medical treatment work through a mechanism of selective toxicity: they attack the bacteria that cause the infection but they don’t kill the patient. They often also attack bacteria that are not pathogenic.

That seems very obvious now, but let’s briefly go back in time again. Not even that long ago – let’s say about 100 years …

At that time there was some understanding about the importance of hygiene measures to prevent spread of infectious disease. However, infections were difficult to treat, because the treatments that were available not only killed the infection, but also often killed the patient.

The discovery of antibiotics was the discovery of ‘magic bullets’: a treatment was now available that only targeted microbes and therefore kept the patient alive.

Not all antibiotics attack bacteria in the same way: some attack the cell wall, some attack bacterial protein syntheses and some attack the DNA-replicating mechanism of bacteria. In all cases, they tackle structures or biochemical processes that are either not found in human or animal cells, or don’t work in the same way. Antibiotics are classified based on their chemical structure: those with a similar chemical structure tend to have similar antibacterial activity. (You will learn more about this in Introducing antimicrobial resistance.)

Some classes of antibiotics target several bacterial species: these are called broad-spectrum antibiotics. Antibiotics with a working mechanism that only attacks a small group of bacterial species are called narrow-spectrum antibiotics.