Selection of antibiotic-resistant bacteria

In a population of bacteria, some bacteria may already be resistant to a particular antibiotic. This means they can survive exposure to that antibiotic, while susceptible bacteria are killed.

When the antibiotic is used, the resistant bacteria can survive and reproduce. Their offspring may also be resistant, so resistant bacteria can become a larger proportion of the population. This is selection: the antibiotic does not make each bacterium resistant; it favours bacteria that are already resistant.

Limiting the selection of resistant bacteria

Using antibiotics only when essential reduces the opportunities for resistant bacteria to be selected and spread. This helps limit the development of resistant populations, such as MRSA, a type of bacterium resistant to certain antibiotics.

Antibiotics

A drug is a substance taken into the body that affects chemical reactions in the body. Antibiotics are drugs used to treat bacterial infections by killing bacteria.

Antibiotics do not affect viruses, so they cannot treat viral infections. An infection must be caused by bacteria for an antibiotic to be useful against it.

Some bacteria are resistant to an antibiotic: the antibiotic does not kill them effectively. If resistant bacteria cause an infection, that antibiotic may be less effective at treating it.

Directional selection

Individuals in a population can vary in a heritable characteristic. If individuals towards one end of the range leave more offspring than others, that characteristic becomes more common at that end of the range over generations. This shift in the population is called directional selection.

Directional selection in antibiotic resistance

A bacterial population may contain some bacteria with an allele that makes them resistant to an antibiotic. When the population is exposed to the antibiotic, susceptible bacteria are killed or their reproduction is inhibited. A resistant bacterium may survive and reproduce, passing the resistance allele to its descendants. As a result, resistant bacteria can become a larger proportion of the population over generations.

The antibiotic selects resistant bacteria: it gives them a reproductive advantage in that environment. It does not cause a resistance mutation because the bacteria need it.

Emergence and spread of resistant bacterial strains

A mutation is a change in a bacterium’s genetic material. Mutations can produce new bacterial strains, including a strain that can survive treatment with a particular antibiotic. Such a strain is resistant to that antibiotic.

Bacteria reproduce rapidly, so many generations can arise in a short time. When a resistant bacterium reproduces, it can pass the resistance to its descendants. The descendants can then reproduce too, allowing a resistant strain to become established in the bacterial population.

A resistant strain can spread from one person to another. This is especially likely where people lack immunity to the strain, meaning their bodies are not already prepared to defend against it.

Resistance makes an infection harder to treat because the antibiotic to which the bacteria are resistant cannot readily eliminate them. If no effective antibiotic is available, the infection cannot readily be eliminated by antibiotic treatment, making its spread harder to limit.

Limiting antibiotic resistance

Antibiotics kill bacteria or stop them multiplying, but some bacteria may have variants that allow them to survive a particular antibiotic. When susceptible bacteria are removed, resistant bacteria can survive and multiply. This is selection for antibiotic resistance.

Reducing unnecessary antibiotic use gives bacteria fewer opportunities to be exposed to antibiotics and resistant populations fewer opportunities to be selected.

Using antibiotics only when needed

Antibiotics should not be prescribed for viral infections because they do not act against viruses. They should also not be prescribed for non-serious infections when they are not needed. Avoiding unnecessary prescriptions reduces selection for resistant bacteria.

Completing a prescribed course

Patients should complete their prescribed course of antibiotics as directed. The aim is to eliminate the infection so that bacteria do not survive and multiply, providing further opportunities for resistant variants to arise and be selected. Completing a course does not guarantee that every bacterium is killed.

Restricting agricultural use

Restricting antibiotic use in agriculture also reduces the exposure of bacteria to antibiotics. This limits opportunities for resistant bacteria to survive and multiply into resistant populations.

Preserving the effectiveness of existing antibiotics matters because developing new antibiotics is slow and costly. New treatments may not become available quickly enough to keep pace with the emergence of resistant strains.