In general, bacteria use a number of different genetic mechanisms to develop, optimize and spread the genes that give them resistance. This includes the following:
- They suddenly change their genetic information (mutation).
- They exchange plasmids. Plasmids are additional, ring-shaped deoxyribonucleic (DNA) structures in bacterial cells.
- They spread by cloning. This means that they reproduce one cell whose daughters are transferred from one person to another.
Resistance genes develop when genes in the bacterial chromosome are modified by mutation. This generally requires more than one step. Mutation normally has to take place in several genes to achieve clinically effective resistance.
The selection caused by the way antibiotics are used is contributing to the rapid pace with which resistance is increasing. Massive use of certain antibiotics can lead particularly quickly to the spread of resistant strains. Without the selection pressure exerted by antibiotics, resistance genes could develop but they would never be able to spread sufficiently to gain dominance. Individual bacteria can only become dominant if they enjoy a persistent selective advantage over the rest of the bacterial population.