Evolutionary drivers of divergent collateral sensitivity responses during antibiotic therapy.

With antibiotic resistance on the rise and the development of new antibiotics stagnating, novel antimicrobial strategies that slow down resistance evolution and extend the lifetime of existing drugs are urgently needed. One possible solution focuses on rationalizing antimicrobial combination and cycling therapies on the basis of the concept of collateral sensitivity, in which resistance mutations acquired against one antibiotic increase the susceptibility towards a second antibiotic. However, the clinical potential of collateral sensitivity is still uncertain as collateral responses for the same combination of antibiotics may vary from collateral sensitivity to cross-resistance, depending on stochasticity, environmental conditions and the genetic background of the pathogen. This Review therefore discusses the drivers behind this variability and proposes that they can influence collateral sensitivity either by selecting different resistance mutations with distinct collateral responses or by modulating how a given resistance mutation affects the cell, thereby altering or even inverting the collateral response. Moreover, we discuss the dynamics of collateral sensitivity in duotherapy and highlight how the selection of multi-drug resistance may contribute to the variability in treatment outcomes. To aid the translation of collateral sensitivity to a clinical setting, we finally present several strategies that could circumvent the variability in collateral sensitivity outcomes.