Development of a high-level light-activated disinfectant for hard surfaces and medical devices

Bacterial spores are an important consideration in healthcare decontamination, with cross-contamination highlighted as a major route of transmission due to their persistent nature. Their containment is extremely difficult due to the toxicity and cost of first-line sporicides. Susceptibility of Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa and Escherichia coli to phenothiazinium photosensitisers and cationic surfactants under white- or red-light irradiation was assessed by determination of MICs, MBCs and time-kill assays. B. subtilis spore eradication was assessed via time-kill assays, with and without nutrient and non-nutrient germinant supplementation of photosensitiser, surfactant and photosensitiser-surfactant solutions in the presence and absence of light. Under red light irradiation, >5-log cfu/mL reduction of vegetative bacteria was achieved within 10 minutes with Toluidine Blue O (TBO) and methylene blue (MB). Cationic surfactant addition did not significantly enhance spore eradication by photosensitisers (p>0.05). However, addition of a nutrient germinant mixture to TBO achieved 6-log reductions after 20 minutes irradiation, while providing 1-2 log improvements in spore eradication for MB and Pyronin Y. Light-activated photosensitiser solutions in the presence of surfactants and germination-promoting agents provide a highly effective method to eradicate dormant and vegetative bacteria. These solutions could provide a useful alternative to traditional chemical agents used for high-level decontamination and infection control within healthcare.