Driving factors for beta-lactam resistance gene amplification during resistance evolution in .

Long-term exposure of to non-lethal step-wise increasing concentrations of beta-lactam antibiotics induces high levels of resistance that can be accompanied by amplification of a chromosomal fragment around the gene. We compared the amplification of the fragment in the wild type, an knockout mutant, a mutant in which the gene was replaced by a tetracycline resistance gene ), and a strain in which the has been translocated. When was removed, no amplification occurred at the original location, but DNA fragments were amplified around the genes coding for efflux pump AcrAB and the multiple antibiotic resistance operon MarRAB. When ) replaced , exposure to tetracycline induced amplification of comparable fragments, while exposure to amoxicillin induced duplication of a larger fragment elsewhere. When was translocated, a fragment around it at the new location was amplified. The importance of the presence but not of the location within the chromosome of the resistance genes for the amplification process indicates that the mechanisms are neither gene nor location specific. Without the relatively efficient resistance gene , duplication and amplification occur around and that code for amoxicillin and tetracycline resistance factors. These duplications and amplifications are prevented by amplification.