Epigenetic Phase Variation in Bacterial Adaptation to Environmental Stress: A Narrative Review.

Epigenetic phase variation (ePV) represents a sophisticated regulatory mechanism by which bacteria rapidly adapt to environmental stressors without permanent genetic alterations. This review examines the molecular mechanisms underlying bacterial ePV mediated by DNA methylation, with particular emphasis on restriction-modification systems, orphan methyltransferases, and their roles in stress adaptation. ePV systems enable bacteria to generate phenotypically diverse populations through reversible ON/OFF switching of gene expression, facilitated by phase-variable DNA methyltransferases. These systems, termed phasevarions when controlling multiple genes, provide adaptive advantages in fluctuating environments by coordinating responses to antimicrobial stress, oxidative damage, heat shock, and nutrient limitation. The integration of genome-wide methylation patterns with transcriptional networks creates complex regulatory hierarchies that govern bacterial survival strategies. Recent advances in single-molecule real-time sequencing and methylome analysis have revealed the widespread distribution of ePV systems across diverse bacterial species, highlighting their fundamental importance in microbial ecology and pathogenesis. Understanding these epigenetic mechanisms provides important insight into bacterial adaptation and evolutionary dynamics. However, their translation into antimicrobial resistance management and therapeutic development remains largely exploratory.