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The emergence of resistance of bacteria to antibacterial drugs is a common phenomenon.
Emergence of resistance often reflects evolutionary processes that take place during antibacterial drug therapy.
The antibacterial treatment may select for bacterial strains with physiologically or genetically enhanced capacity to survive high doses of antibacterials.
Under certain conditions, it may result in preferential growth of resistant bacteria, while growth of susceptible bacteria is inhibited by the drug.
For example, antibacterial selection within whole bacterial populations for strains having previously acquired antibacterial-resistance genes was demonstrated in 1943 by the Luria – Delbrück experiment.
Survival of bacteria often results from an inheritable resistance.
Resistance to antibacterials also occurs through horizontal gene transfer.
Horizontal transfer is more likely to happen in locations of frequent antibiotic use.
Antibacterials such as penicillin and erythromycin, which used to have high efficacy against many bacterial species and strains, have become less effective, because of increased resistance of many bacterial strains.
Antibacterial resistance may impose a biological cost, thereby reducing fitness of resistant strains, which can limit the spread of antibacterial-resistant bacteria, for example, in the absence of antibacterial compounds.
Additional mutations, however, may compensate for this fitness cost and can aid the survival of these bacteria.

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