Why does antibiotic overuse favor resistant bacteria?

Antibiotic overuse favors resistant bacteria by creating a strong selective pressure that alters the composition of bacterial populations.1

When an antibiotic is administered, it typically kills or inhibits the growth of the most vulnerable bacteria. However, if some bacteria possess natural genetic variations—acquired through mutation or the transfer of genetic material—that make them less susceptible to the drug, they are more likely to survive.231

Because the antibiotic has eliminated or reduced the number of their more vulnerable competitors, these resistant survivors face less competition for resources and can multiply rapidly. Through processes such as conjugation and transduction, these resistant bacteria can also pass their survival advantages to other members of their species. Consequently, indiscriminate or inexact use of antibiotics accelerates the proliferation of these resistant strains, often rendering infections difficult or impossible to treat with standard medications.231