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Antibiotic resistance can be achieved by horizontal acquisition of resistance genes (carried by plasmids or transposons), by recombination of foreign DNA into the chromosome, or by mutations in different chromosomal loci (15). In studies of molecular evolutionary biology, the term mutation rate is applied to estimations of the rate (per generation) of mutation per nucleotide, per locus, or, eventually, for the whole genome, and selectively favorable, unfavorable, or neutral mutations are considered. Differing with this concept, the frequency of mutation measures all the mutants present in a given population, irrespective of whether the mutation events occurred early or late during the growth of the population. In this respect, the frequency of mutants is a cross section of the bacterial population at a given time and reffects not only the mutation rate but also the history of the population before selection is applied.

In the case of …