Concentration-dependent selection of small phenotypic differences in TEM beta-lactamase-mediated antibiotic resistance

In this paper, the first robust experimental evidence of in vitro and in vi vo concentration-dependent selection of low-level antibiotic-resistant gene tic variants is described. The work is based on the study of an asymmetric competition assay with pairs of isogenic Escherichia coli strains, differin g only (apart from a neutral chromosomal marker) in a single amino acid rep lacement in a plasmid-mediated TEM-1 beta-lactamase enzyme, which results i n the new TEM-12 beta-lactamase, The mixture was challenged by different an tibiotic concentrations, both in vitro and in the animal model, and the sel ective process of the variant population was carefully monitored, A mathema tical model was constructed to test the hypothesis that measured growth and killing rates of the individual TEM variants at different antibiotic conce ntrations could be used to predict quantitatively the strength of selection for TEM-12 observed in competition experiments at these different concentr ations.