Spatial drug asymmetry modulates phenotypic diversity-migration relationships under resistance evolution

At long timescales, resistant phenotypes will emerge and be selected within the bacterial population as an evolutionary response to drug exposure. This phenomenon reduces the efficacy of drug therapies and thus compromises patient health. In spatially heterogeneous drug environments, recent evidence shows that migration can either promote or decelerate the evolution of antibiotic resistance, thereby affecting the rate of resistant phenotype emergence. However, another important quantitative aspect of resistance evolution--bacterial phenotypic diversity--has often been overlooked and remains challenging to investigate in spatially extended systems, both experimentally and clinically. In order to study how diversity is reshaped by migration across space, here we designed a minimal 2-well experimental system with spatial drug asymmetry. One well contained a bacteriostatic drug (Linezolid) at the minimum inhibitory concentration, while the other well served as a sanctuary with just media. We found that the relationship between diversity and migration follows the "Intermediate Disturbance Hypothesis" (IDH), with migration as the disturbance to each well. By varying the selective drug concentrations, we observed that the diversity-migration relationship changes, and IDH can disappear. This behavior was explained by an asymmetry parameter derived from a two-phenotype growth-migration dynamic model. To further validate how different spatial drug asymmetries modulate the diversity-migration relationship through this asymmetry parameter, we applied another bactericidal drug, Ampicillin, and observed similar results. In a more complex scenario involving both Linezolid and Ampicillin, four distinct phenotypes, including cross-resistant variants, emerged. Our asymmetry parameter successfully explained the diversity-migration relationship, with unique diversity dynamics such as multiple peaks appearing from the model. The minimal generalist-specialist framework predicted these unique behaviors through global fitness advantages. Our findings provide experimental support and theoretical explanations for the emergence of phenotypic diversity in clinical settings, such as gut-lung translocation. These insights may pave the way for improved clinical strategies to manage antibiotic resistance evolution.