Development of aBL tolerance in gram-negative bacteria.

Antimicrobial blue light (aBL) treatment is considered low risk for the development of bacterial resistance due to its multi-target mode of action. In our previous studies [1], we assessed the development of Staphylococcus aureus, Enterococcus faecium and Streptococcus agalactiae stable tolerance to aBL. The aim of the current study [2] was to evaluate whether, using the same test protocol (as in gram-positive bacteria) [2,3], the development of tolerance also occurs in gram-negative bacteria. We evaluated the potential of tolerance/resistance development in Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa and demonstrated that representative gram-negative bacteria may develop stable tolerance to aBL. An approximately 2-3 log10 unit decrease in aBL antimicrobial effectiveness was observed starting in the fifth cycles of the consecutive treatments as compared to the control. Additionally, we performed assays involving E. coli K-12 tolC-, tolA-, umuD-, and recA-deficient mutants [4], that revealed some possible mechanisms for aBL tolerance development.