Acquired Resistance Bacteria can develop resistance to antibiotics, e.g. bacterial populations previously-sensitive to antibiotics become resistant. This type of resistance results from changes in the bacterial genome. The mutation rate for most bacterial genes is approximately 10-8. This means that if a bacterial population doubles from 108 cells to 2 x 108 cells, there is likely to be a mutant present for any given gene. Since bacteria grow to reach population densities far in excess of 109 cells, such a mutant could develop from a single generation during 15 minutes of growth.
Horizontal evolution is the acquisition of genes for resistance from another organism.
Or, more likely, Some bacterium develops genetic resistance through the process of mutation and selection and then donates these genes to some other bacterium through one of several processes for genetic exchange that exist in bacteria. During transduction, a virus transfers the genes between mating bacteria. In transformation, DNA is acquired directly from the environment, having been released from another cell. It is common for DNA to be transferred as plasmids between mating bacteria. The combined effects of fast growth rates, high concentrations of cells, genetic processes of mutation and selection, and the ability to exchange genes, account for the extraordinary rates of adaptation and evolution that can be observed in the bacteria.