Antibiotic resistance is the ability of bacteria to survive a dose of an antibiotic that would normally kill them or stop their growth. It is one of the clearest examples of natural selection happening today.Resistance evolves in these steps:1. In a large bacterial population, a random mutation may produce an allele that gives resistance, for example a gene for an enzyme that breaks down the antibiotic (penicillinase breaks down penicillin) or a change in the antibiotic's target site.2. When the antibiotic is used, it acts as a selection pressure. Non-resistant bacteria are killed.3. Resistant bacteria survive, with less competition, and reproduce rapidly by binary fission. All their offspring inherit the resistance allele. This is vertical gene transmission.4. The frequency of the resistance allele in the population increases, so the antibiotic becomes ineffective.
Resistance spreads very quickly in bacteria because they reproduce every 20 minutes or so under good conditions, and because genes can also pass between bacteria in horizontal gene transmission. Resistance genes are often carried on plasmids, which can be copied and transferred to another bacterium, even of a different species, during conjugation.Some bacteria are resistant to several antibiotics. MRSA (meticillin-resistant Staphylococcus aureus) and Clostridioides difficile are important causes of infections acquired in hospital. Infections by resistant bacteria are harder to treat, so patients stay ill for longer and are more likely to die.
Resistance spreads very quickly in bacteria because they reproduce every 20 minutes or so under good conditions, and because genes can also pass between bacteria in horizontal gene transmission. Resistance genes are often carried on plasmids, which can be copied and transferred to another bacterium, even of a different species, during conjugation.Some bacteria are resistant to several antibiotics. MRSA (meticillin-resistant Staphylococcus aureus) and Clostridioides difficile are important causes of infections acquired in hospital. Infections by resistant bacteria are harder to treat, so patients stay ill for longer and are more likely to die.