There are many means by which a bacterial cell can gain the genes necessary for resistance. These genes can be transmitted from one cell, which is already resistant, to another cell, which is not resistant by several different methods. As discussed earlier, many antibiotics have been isolated from nature in bacterial cells that produced these antibiotics as a defense mechanism. In many cases, these cells have developed resistance on their own. Some bacteria have been determined to have pre-existing resistance to an antibiotic found in nature and thus some isolated antibiotics are not effective.
Plasmids are small, circular, extra chromosomal loops of DNA that contain a few non-essentail genes.
Many plasmids replicate independently of the chromosome and this occurs when the bacterial cell replicates. The replication mechanisms of the plasmids also do not have checking mechanisms. The same errors can occur on them as in the bacterial chromosome itself. Therefore resistance can develop on these as well. However, plasmids do not have to wait for the bacteria to divide in order to pass on the genetic information.