The Eagle Effect and Antibiotic-Induced Persistence: Two Sides of the Same Coin?

The Eagle effect describes a phenomenon in which bacteria or fungi exposed to concentrations of antibiotic higher than an optimal bactericidal concentration (OBC) have paradoxically improved levels of survival than at the OBC due to a decreased net rate of cell death. Despite extensive observational reports of this effect in different microorganisms, its underlying mode of action is not well understood. Although aspects of the Eagle effect resemble persistence, there is strong evidence that these phenomena are substantially different phenotypic responses to antibiotic treatment. We present an overview of the microorganism and antimicrobial combinations in which the Eagle effect has been observed. Proposed underlying mechanism(s) are assessed, and the Eagle effect and microbial persistence are compared and contrasted. The clinical relevance of the Eagle effect is reviewed, incorporating evidence from experimental in vitro and in vivo studies, as well as clinical reports.