Antimicrobial resistance can occur through several mechanisms. Organisms, through selective pressure, express genes when exposed to a certain antibiotic. Those that carry the gene for resistance survive when exposed to the antibiotic while organisms that do not contain the gene for resistance will die off. This selects for the more resistant organisms to continue to multiply and become the dominant strain. Another method of resistance may be through gene mutation. As the susceptible organisms are exposed to the antibiotic, most will die off, but a few may mutate, becoming resistant to the antibiotic to survive and multiply.
Still, other organisms may receive a gene encoded with the resistance mechanism through a transfer or sharing of copies of this gene from other organisms. This sharing does not always have to occur in the same genera but can often be transferred from different types of organisms. The organism with the newly acquired gene then can go on to survive and multiply in the presence of the drug.
