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Understand  –  Antibiotic resistance

Mutations and selection

Mutations can result in antibiotic resistance in bacteria. Resistant bacteria survive antibiotic treatment and can increase in numbers by natural selection.


Bacteria grow and multiply fast and can reach large numbers. When bacteria multiply, one cell divides into two cells. Before the bacterium can divide, it needs to make two identical copies of the DNA in its chromosome; one for each cell. Every time the bacterium goes through this process there is a chance (or risk, depending on the end result) that errors occur; so-called mutations. These mutations are random and can be located anywhere in the DNA. Mutations can also form due to external factors like radiation or harmful chemicals.

Natural selection

While some mutations are harmful to the bacteria, others can provide an advantage given the right circumstances. Here, Darwin’s theory of natural selection comes in. If a mutation gives the bacterium an advantage in a particular environment, this bacterium will grow better than its neighbors and can increase in numbers – it is selected for.

Mutations can provide resistance to antibiotics

Mutations are one way for bacteria to become resistant to antibiotics. Some spontaneous mutations (or genes that have been acquired from other bacteria through horizontal gene transfer) may make the bacterium resistant to an antibiotic (See: Resistance mechanisms for information about how bacteria resist antibiotic action). If we were to treat the bacterial population with that specific antibiotic, only the resistant bacteria will be able to multiply; the antibiotic selects for them. These bacteria can now increase in numbers and the end result is a population of mainly resistant bacteria.

Figure 1. Natural selection of antibiotic resistant bacteria. The starting point in this example is a large bacterial population where most bacteria are susceptible to antibiotics, but a couple of bacteria are antibiotic-resistant by chance. A bactericidal antibiotic is added, which kills most of the susceptible bacteria in the population, while the resistant bacteria survive. Only the resistant bacteria will continue to proliferate in the presence of the antibiotic and increase in number over time. The end result is a population of mainly resistant bacteria.

It is important to understand that selection of antibiotic resistant bacteria can occur anywhere an antibiotic is present at a selective concentration. When we treat an infection, selection can occur at any site in the body to which the antibiotic reaches. Thus, the antibiotic can select for resistance genes and mechanisms in both pathogenic bacteria and in commensal bacteria living in the body that have nothing to do with the infection in question. By using narrow-spectrum antibiotics (when possible), the risk of selecting for antibiotic resistance in the commensal flora decreases.

Short video that explains natural selection of antibiotic resistance

Selected Resources

Resource Description
Emergence and spread of antibiotic-resistant bacteria Video. Narrated presentation from ReAct that describes how bacteria develop resistance and how bacteria can spread.
A cinematic approach to drug resistance Video that visualizes the development and spread of antibiotic resistant bacteria in real time, using a giant petri dish with different concentrations of antibiotics. With explanatory article.
The rise in antibiotic resistance Animation of selection of resistant bacteria in the body, some resistance mechanisms and how they may spread.
What causes antibiotic resistance? Video. This animated TED-Ed video gives an overview of how antibiotics function, how bacteria evolve to resist their action and how selection of resistant bacteria works (5 min). Also available in Spanish.
Antibiotic Resistance Explained Video that explains selection and spread of antibiotic resistance in an easy to understand manner.
Natural Selection and the Rock Pocket Mouse Video from HHMI BioInteractive about a real-life example of natural selection (11 min).
Mutations – selection: the bacteria resist Video about natural selection of antibiotic resistant bacteria (5 min). Also embedded in text above.
Antibiotic Resistance Explained [Internet]. [cited 2019 Nov 25]. Available from:
Pesheva E. A cinematic approach to drug resistance [Internet]. Harvard Gazette. 2016 [cited 2018 Aug 6]. Available from:
ReActTube. Emergence spread antibiotic resistance bacteria [Internet]. 2017 [cited 2017 Mar 6]. Available from:
Kishony Lab. Evolution of antibiotic resistance: predicting and manipulating the future [Internet]. Technion: Israel Institute of Technology. Available from:
Natural Selection and the Rock Pocket Mouse — HHMI BioInteractive Video [Internet]. 2014 [cited 2015 Feb 9]. Available from:
Sumanas, Inc., Sinauer Associates, Inc., W. W. Norton & Company, Inc. The Rise in Antibiotic Resistance [Internet]. Sumanas, Inc. Multimedia Development Services. 2002 [cited 2015 Jan 26]. Available from:
Mutations - selection: the bacteria resist [Internet]. 2009 [cited 2014 Dec 4]. Available from:
What causes antibiotic resistance? - Kevin Wu [Internet]. 2014 [cited 2014 Oct 2]. Available from: