Whereas the horizontal transfer of antibiotic resistance genes is well-established in biofilms 22, 23, 24, 25, 26, 27, 28, the emergence and evolution of genetically and vertically inheritable antibiotic resistance in biofilms is less understood. The emergence of antibiotic resistance within a tolerant biofilm population could therefore constitute an aggravating factor increasing the frequency of therapeutic failure and infection recurrence. Consequently, even when caused by non-resistant bacteria, biofilm-associated infections are difficult to eradicate and regrowth of surviving biofilm bacteria when antibiotic treatment stops is a typical cause of therapeutic failure due to bacterial infection relapse 19, 20, 21. Biofilms indeed display a characteristic high level of tolerance to a broad range of antibiotics that disappears quickly after biofilm dispersion. In some cases, this increased tolerance favoured the emergence of genetic resistance 11, 12, 13, 14.Īlthough antibiotic tolerance is relatively understudied compared to genetic resistance 15, 16, 17, it is the hallmark of surface-attached bacterial communities called biofilms 18, 19. By contrast, the use of periodic and short (3–8 h) lethal antibiotic treatments leads to mutants that have increased tolerance to antibiotics, i.e., their ability to survive but not grow under antibiotic pressure, for instance due to increased lag time or reduction of proton motive force 4, 5, 6, 7, 8, 9, 10. Exposing planktonic bacteria to sub-inhibitory or progressively increasing concentrations of antibiotics quickly leads to diverse inheritable resistance mutations 3. Adaptive laboratory evolution experiments have been used to reproduce the dynamics of emergence and selection of antibiotic resistance in various bacteria. This global increase in antibiotic resistant bacteria, particularly in pathogenic strains resistant to treatment, is a major health concern 1, 2. However, increased use of antibiotic treatments has led to the propagation of antibiotic resistance genes by horizontal gene transfer or the selection of vertically transmitted mutations. Bacterial infections are a leading cause of morbidity and mortality commonly treated with antibiotics.
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