Knowing the full complement of
genetic changes that take place during resistance evolution
in natural and clinical environments, and addressing their
individual roles in that process, is essential to understanding
the evolution of a strain to antibiotic resistance. However,
genome sequencing of resistant clinical isolates by
itself will not provide the insight we need into the significance
of individual genotypes, and the steps involved in
reaching the resistant state. What is essential is to complement
clinical sequencing studies with experimental evolution
made under a variety of controlled conditions. In that
way, genome-sequencing data can be used to map evolutionary
trajectories, and the phenotypic significance of
individual mutations can be experimentally measured.
This type of experiment will allow reconstruction of evolutionary
histories, and deepen our understanding of the
selective forces leading to the resistant mutant phenotype.
We are already making such experiments, with various
selection pressures, including subMIC and changing antibiotic
concentrations, and where the genomes of the
resulting mutants are completely sequenced. We find that
many genetic alterations occur when bacteria are selected
at low antibiotic concentrations, and importantly, many of
these changes are not predicted from our current knowledge
of antibiotic resistance determinants. Of course some
of these experimental trajectories may represent evolutionary
dead-ends, but until we make the experiments
and compare the outcomes with clinically derived genomes
we will not even be close to fully understanding the paths
of resistance evolution