ABSTRACT Antibiotic resistance is driven by selection, but how a bacterial strain’s evolutionary history shapes drug-resistance remains an open question. Here we reconstruct the genetic and evolutionary mechanisms of carbapenem resistance in a clinical isolate of Klebsiella . A combination of short and long read sequencing, machine learning, genetic and enzymatic analyses established that this carbapenem-resistant strain carries no carbapenemase-encoding genes. Genetic reconstruction of the resistance phenotype confirmed that two distinct genetic loci are necessary for the strain to acquire carbapenem resistance. Experimental evolution of the carbapenem-resistant strains in growth conditions without the antibiotic revealed that both loci confer a significant cost, and are readily lost by de novo mutation resulting in the rapid evolution of a carbapenem-sensitive phenotype. Thus, historical contingency - a patient’s treatment history - can shape the evolution of antibiotic resistance and suggests that the strategic combinations of antibiotics could direct the evolution of low-fitness, drug-resistant genotypes.

Treatment history shapes the evolution of complex carbapenem-resistant phenotypes in Klebsiella spp.