Easy Escape

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Science  29 Jul 2011:
Vol. 333, Issue 6042, pp. 500
DOI: 10.1126/science.333.6042.500-b

Antibiotic resistance is escalating, driven mostly by human activities, but whether low amounts of antibiotic pollution also contribute to antibiotic resistance is unclear. For several well-defined mutants of Escherichia coli and Salmonella enterica, Gullberg et al. tested how little antibiotic is needed to enrich for resistant bacteria. Using tetracycline, fluoroquinolone, and aminoglycoside antibiotics in competition experiments between susceptible and resistant bacteria, they found selection for resistance occurring at picogram to nanogram concentrations. They specify this threshold as the minimal selective concentration, which is the concentration at which the fitness cost of resistance is balanced by the selective advantage of the mutation. Twenty susceptible lineages of S. typhimurium grown in the presence of low concentrations of streptomycin contained subpopulations tolerating eight-fold greater concentrations of streptomycin by 400 generations. Thus, the surprisingly high frequencies of antibiotic-resistant bacteria found in animals from relatively pristine environments may be explained by this low-concentration enrichment effect.

PLoS Pathol. 7, e1002158 (2011).

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