Antibiotic resistance in bacterial species

 If you ever suffered from a serious bacterial infection, your doctor probably prescribed antibiotics as a treatment. And you were probably directed that even if you feel better, you must complete the course of treatment. Why? Because if you fail to do so, you may promote antibiotic resistance. 

We do know better (1): Dosage and duration of antibiotic treatment must be customized to the bacterial species and the individual host. In order to avoid antibiotic resistance, ANTIBIOTIC TREATMENT MUST BE AS SHORT AS POSSIBLE. The explanation comes from evolutionary biology, and the distinction between target selection and collateral selection.

Target selection works like this: A malign bacterial species is targeted for antibiotic treatment. There is variation in the population. Some of the individuals are less susceptible to the antibiotic. They reproduce and develop mutants that are resistant to the antibiotic. The mutants spread through the population and kill the infected host.

But target selection occurs only in relatively few infectious diseases (e.g. tuberculosis, gonorrhoea, typhus). The predominant driver of antibiotic resistance is collateral selection. 

Collateral selection works like this: Many bacterial species live around us, or on us, or inside us. Most of the time they are benign, and many are even beneficial. When you get an infection from a malign species, you will treat them with antibiotics. This treatment exposes the malign species AND the benign species to natural selection. Because the malign species is the target of the antibiotic treatment, it will eventually be eliminated from your body. 

What stays behind are antibiotic-resistant strains of benign species. The longer the duration of the antibiotic treatment, the stronger the antibiotic resistance. These antibiotic-resistant strains are transmitted between healthy people through normal interaction. Worse, because bacteria exchange genes even between species, other benign species may acquire antibiotic resistance. 

Once in a while, a now antibiotic-resistant benign species (e.g. Escherichia coli) develops mutants that become opportunist pathogens. If you treat them with antibiotics, no effect.

NOTES AND REFERENCES

(1) M. J. Llewelyn et al. (2017), The antibiotic course has had its day. The British Medical Journal: BMJ 2017;358:j3418 doi: 10.1136/bmj.j3418.