Selection pressure

• Exposure to antibiotics in the environment encourages resistance as small numbers of ‘resistant mutants’ will survive whilst susceptible organisms die off
• Particularly likely to happen in the gut of someone taking antibiotics or in bacteria in a hospital environment

How bacteria acquire resistance

• The ability to become resistant to an antibiotic is the result of a change in the bacterial DNA
• Can occur by 2 mechanisms:

Genetic mutation

• Misreading of DNA
• Bacteria reproduce rapidly - lots of scope for misreading

Transfer of bacterial DNA

• Transformation: when bacteria die and the cells break apart, ‘free-floating’ DNA released into surrounding environment may be ‘scavenged’ by other bacteria and incorporated into DNA - the DNA may contain genes for antibiotic resistance
• Conjugation: replication and transfer of plasmid DNA - plasmid DNA may contain genes for resistance
• Transduction: bacterial DNA transferred from one bacterium to the other inside a virus that infects bacteria (bacteriophages)

Mechanisms of resistance

Altered binding site

• A change in bacterial DNA can cause a change in the gene product which is the target of the antibiotic

Destruction of antibiotic

• Bacteria may possess genes which code for enzymes that chemically degrade or inactivate the antibiotic