Here's new information on the number of bacteria on Earth. Mutations do come into play when drug manufacturers modify an existing antibiotic to overcome resistant bugs. In that case, the bacteria already possess a gene to defeat the antibiotic, and it mutates to regain mastery over the modified antibiotic. Here's how nasty bacteria resist nice drugs. Images and caption courtesy FDA .
Bacteria acquire genes for resistance in three ways.
In spontaneous mutation, bacterial DNA may change spontaneously, as indicated by the starburst. Drug-resistant tuberculosis arises this way. In a form of microbial sex called transformation, one bacterium may take up DNA from another. Most frightening, however, is resistance acquired from a small circle of DNA called a plasmid. But bacteria do something much more clever than just mutating.