How does antibiotic resistance occur?
When drug resistance occurs, the bacteria has not specifically changed itself in some way to survive. Random mutations are usually responsible for drug resistance in bacteria. Fortunately (for the bacteria), these mutations incur some change in the cell?s structure or metabolism that allow it to survive in the presence of an antibiotic. When doctors try to treat an infection of these bacteria, the ones with the mutation have an advantage and will survive to pass on their new genetic information on to other generations of bacteria. Often, these mutations occur in bacteria that are harmless, but they have the ability to transfer these genes onto more virulent bacteria that are more harmful to humans. These transfers of genetic material can happen through three mechanisms1:
  In the last two situations, resistance will only occur if the genetic material is stably integrated in the host cell. More often than not, this will occur because these resistance genes are often carried on portions of DNA called transposons. These fragments of DNA have the ability to readily integrate themselves into DNA.

(These three methods are illustrated in Scientific American. Click on "picking up resistance genes" under ILLUSTRATIONS.)
 
 

Antibiotic resistance can work through several different mechanisms from altering the cell wall structure to make it impermeable to the drug to altering the genetic code of enzymes within the cell to make them less susceptible to the drug. There are three types of antibiotic resistance2:

Natural or intrinsic resistance

Mutational Resistance Extrachromosomal or acquired resistance