Evolution of antibiotic resistance in bacteria

Bacteria in a population are not all genetically identical. Some may carry an allele—a version of a gene—that makes them resistant to a particular antibiotic. This means they are more likely to survive exposure to it than bacteria without the resistance allele.

When the antibiotic is used, susceptible bacteria are more likely to die. Resistant bacteria are more likely to survive and reproduce, passing the resistance allele to their offspring. As these offspring reproduce, the allele is passed on again.

Over many generations, resistant bacteria can make up an increasing proportion of the population. This change can produce an antibiotic-resistant strain. The antibiotic does not make individual bacteria develop resistance because they need it; it favours bacteria that already carry a resistance allele.