Antimicrobial photodynamic therapy combined with antibiotics reduces resistance and aids elimination in four resistant bacterial strains

Antimicrobial photodynamic therapy (aPDT) in combination with antibiotics leads to a notable reduction in antibiotic minimum inhibitory concentration (MIC). Four antibiotic resistant bacterial strains (E. faecalis OG1RF(+), P. aeruginosa PA01, K. pneumoniae, E. coli ETEC) were evaluated with four antibiotics (ampicillin, kanamycin, tetracycline, chloramphenicol) in a combination treatment with methylene blue aPDT. Treatment involved co-culture of antibiotics with 1.0 μ𝑀 MB followed by exposure to 0 to 14.4 ^𝐽 _𝑐𝑚² of light over 0 to 10 minutes at a rate of 30 ^𝑚𝑊 _𝑐𝑚². MIC of test groups was compared to controls to evaluate direct effects on resistance, and further aPDT controls were used to evaluate measures of synergistic effect based on fractional inhibitory concentration index (FICI). It was found that 12 of 16 strain-antibiotic combinations have a reduction in antibiotic resistance during treatment, two of which are statistically significant. The calculated FICI values for these combinations contain 6 indifferent values between 1.0 and 1.5 and 10 additive values at or below 1.0, one of which was 0.43, qualifying as synergistic treatment. Overall, this survey provides a brief exploration of aPDT as an adjunct therapy for combatting antibiotic resistant bacteria. Further targeted research on strain-antibiotic combinations of interest may reveal valuable synergistic effects.