Tetrahydroporphyrin-tetratosylate (THPTS)-based photodynamic inactivation of critical multi-resistant bacteria in vitro.

Abstract Background Photodynamic inactivation (PDI) provides a promising approach to treat multidrug-resistant infections. However, effectiveness of PDI is limited, especially in Gram-negative bacteria. So far, the use of photosensitizer 3,3′,3′′,3′′′-(7,8,17,18-tetrahydro-21H,23H-porphyrine-5,10,15,20-tetrayl)tetrakis[1-methyl-pyridinium]tetratosylate (THPTS) and laser light has led to very promising results. Thus, we focus on the effects of THPTS in various critical multi-resistant bacterial strains and explore the possibility of light emitting diode (LED)-based activation as a clinically more feasible alternative to laser light. Methods We further chemically characterized the photosensitizer THPTS and performed in vitro testing of photodynamic inactivation of different multi-drug resistant bacterial strains under various experimental conditions including varying drug concentration, incubation time, light source (laser and LED) and light intensity by determination of viable bacteria after treatment. Additionally, the effect of hyaluronic acid as an adjuvant for medical applications was evaluated. Results Bacterial density of all investigated bacterial strains was reduced by several orders of magnitude, irrespective of multi-drug resistance or hyaluronic acid addition. While the effect was less intense in Gram-negative strains (disinfection), it was more pronounced in Gram-positive strains (sterilization), even at reduced THPTS concentrations or decreased light treatment intensity. Controls without THPTS or without light treatment did not indicate reduced bacterial density. Conclusions PDI with THPTS and laser light is effective in all investigated bacterial strains. Gram-negative strains are less, but sufficiently susceptible to PDI. Hyaluronic acid can be added without relevant reduction of the antibacterial treatment effect. LED-based PDI is equally effective when illumination duration is increased to compensate for reduced light intensity.