Antimicrobials as promoters of genetic variation.

The main causes of antibiotic resistance are the selection of naturally occurring resistant variants and horizontal gene transfer processes. In recent years, the implications of antibiotic contact or treatment in drug resistance acquisition by bacteria have been gradually more evident. The ultimate source of bacterial genetic alterations to face antibiotic toxicity is mutation. All evidence points to antibiotics, especially when present at sublethal concentrations, as responsible for increasing genetic variation and therefore participating in the emergence of antibiotic resistance. Antibiotics may cause genetic changes by means of different pathways involving an increase of free radicals inside the cell or oxidative stress, by inducing error-prone polymerases mediated by SOS response, misbalancing nucleotide metabolism or acting directly on DNA. In addition, the concerted action of certain environmental conditions with subinhibitory concentrations of antimicrobials may contribute to increasing the mutagenic effect of antibiotics even more. Here we review and discuss in detail the recent advances concerning these issues and their relevance in the field of antibiotic resistance.