Abstract
Antimicrobial resistance (AMR) remains a global health crisis, yet treatment outcomes cannot be explained by resistance genes alone. Increasing evidence highlights the importance of variability at two levels: within bacterial populations and across patients. At the microbial level, cell-to-cell variability including genetic mutations, stochastic gene expression, persister cell formation, heteroresistance, and spatial heterogeneity within biofilms creates phenotypic diversity that allows subsets of bacteria to survive antimicrobial stress. At the host level, patient-to-patient variability including differences in genetic background, immune competence, comorbidities, gut microbiome composition, and pharmacokinetics shapes both susceptibility to resistant infections and the likelihood of treatment success. Together, these dimensions explain why infections with the same pathogen can lead to divergent clinical outcomes. Understanding and integrating both microbial and host variability offers a path toward more precise diagnostics, personalized therapy, and novel strategies to counter AMR.