Genotypic vs. phenotypic stability of antimicrobial Blue Light tolerance in Escherichia coli after long-term storage

Antimicrobial blue light (aBL) is a promising alternative for combating multidrug-resistant bacteria. While the risk of resistance development is considered low, bacteria can develop tolerance, which may be either genotypic (stable, mutation-driven) or phenotypic (transient, non-heritable). Understanding the long-term stability of aBL tolerance is crucial for evaluating its effectiveness as an antimicrobial strategy. Our research group examined the stability of aBL tolerance in E. coli K-12 populations that had undergone 10 cycles of sublethal aBL exposure and were subsequently stored at −80°C for three years. Their responses to aBL were reassessed and compared to data from 2021. At the low dose, all populations displayed similar tolerance levels. However, after longer irradiation, one population retained tolerance, another became more sensitive, and the third lost tolerance almost completely. These findings suggest that aBL tolerance can be genotypic (stable) or phenotypic (transient), with phenotypic tolerance potentially diminishing over time.