Inactivation kinetics and lethal dose analysis of antimicrobial blue light and photodynamic therapy

Antimicrobial Photodynamic therapy (A-PDT) has been used to treat infections. Currently, microbial inactivation data is reported presenting survival fraction averages and standard errors as discrete points instead of a continuous curve of inactivation kinetics. Standardization of this approach would allow clinical protocols to be introduced globally, instead of the piecemeal situation which currently applies.

To this end, we used a power-law function to fit inactivation kinetics and directly report values of lethal doses (LD) and a tolerance factor (T) that informs if inactivation rate varies along the irradiation procedure. A deduced formula was also tested to predict LD for any given survival fraction value. We analyzed the photoantimicrobial effect caused by red light activation of methylene blue (MB-APDT) and by blue light (BL) activation of endogenous microbial pigments against 5 clinically relevant pathogens.

Following MB- APDT, Escherichia coli and Staphylococcus aureus cells become increasingly more tolerant to inactivation along the irradiation process (T < 1). Klebsiella pneumoniae presents opposite behavior, i.e., more inactivation is observed towards the end of the process (T > 1). P. aeruginosa and Candida albicans present constant inactivation rate (T˜1). In contrast, all bacterial species presented similar behavior during inactivation caused by BL, i.e., continuously becoming more sensitive to blue light exposure (T > 1).

The power-law function successfully fit all experimental data. Our proposed method precisely predicted LD and T values. We expect that these analytical models may contribute to more standardized methods for comparisons of photodynamic inactivation efficiencies.