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.