Low-dose blue light extends bacitracin's antibacterial spectrum against *E. coli* by >256-fold

Bacitracin, a well-established antimicrobial peptide, is typically effective against Gram-positive bacteria but struggles against Gram-negative bacteria like Escherichia coli due to intrinsic resistance mechanisms. The escalating global crisis of antibiotic resistance demands innovative approaches to enhance existing drug efficacy and expand their utility. Blue light has emerged as a promising non-pharmacological antimicrobial strategy, offering a potential avenue to overcome bacterial resistance without relying on new chemical entities. This research explores whether combining low-dose blue light with bacitracin can broaden its antibacterial spectrum to effectively target resistant E. coli strains.

Researchers investigated the synergistic effects of bacitracin and blue light on one laboratory strain (DH5α) and three clinical isolates of bacitracin-resistant E. coli. The antibacterial efficacy was primarily assessed using the broth microdilution method to determine the Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) of bacitracin. Bacterial cultures were exposed to various concentrations of bacitracin, either alone or in combination with sub-therapeutic blue light (wavelength 405 ± 5 nm, dose 120 J/cm²). Potentiation was quantified by comparing MIC/MBC values, and time-course viability assays were conducted to confirm bactericidal effects over several time points.

Blue light exposure alone had a minimal impact on E. coli growth across all tested strains. However, the combination of blue light with bacitracin dramatically sensitized all E. coli strains, leading to a significant reduction in bacitracin's MIC. The observed MIC reductions ranged from ≥8-fold to a remarkable ≥512-fold, depending on the specific E. coli strain. This potentiation effect was particularly pronounced in two strains, where the MICs plummeted from 256-512 μg/ml to <1 μg/ml. This represents a substantial >256-fold improvement in bacitracin's efficacy. Time-course viability assays further corroborated these findings, demonstrating a potent bactericidal effect that manifested within two hours of combined exposure. The synergy suggests a mechanism involving photodamage that likely affects bacterial cell permeability or resistance gene expression, though further investigation into the precise molecular pathways is warranted. This non-pharmacological intervention effectively reversed E. coli's intrinsic resistance to bacitracin.

For two E. coli strains, the Minimum Inhibitory Concentration (MIC) of bacitracin dropped from 256-512 μg/ml to <1 μg/ml when combined with blue light, representing a >256-fold reduction.