Influence of bacterial growth modes on the susceptibility to light-activated disinfection

Aim To evaluate the efficacy of light-activated disinfection (LAD) using methylene blue (MB) and a non-coherent light source on gram-positive and gram-negative bacteria in different growth modes. The influence of different photosensitizer (PS) formulations in the MB-mediated LAD of biofilms was also evaluated. Methodology Light-activated disinfection using MB was tested on Enterococcus faecalis in a planktonic suspension, coaggregated suspension and mono-species biofilms and on Pseudomonas aeruginosa in a planktonic suspension and mono-species biofilms. Further, the difference in susceptibility of E. faecalis and P. aeruginosa biofilms to LAD with modified PS formulations was assessed by conventional culturing methods and confocal laser scanning microscopy (CLSM). Results Higher energy dose was required for LAD of bacteria in a coaggregated suspension and in biofilm compared to their planktonic counterparts. Biofilm mode of growth offered the greatest resistance to LAD in both the tested strains of pathogens (P < 0.001). Gram-positive E. faecalis was more susceptible to LAD than the gram-negative P. aeruginosa, and the use of modified PS formulations was found to enhance the efficacy of LAD to destroy the biofilm (P < 0.001). Conclusions Bacterial growth modes play a vital role in influencing the susceptibility to LAD in a dose-dependent manner. The nature of the PS formulation influences the susceptibility of biofilms to LAD.