Optimal effective concentration combinations (OPECCs) for binary application of membrane-targeting antiseptics and TMPyP-mediated antimicrobial photodynamic therapy

The widespread occurrence of multi-resistant bacteria is a health problem of global dimension. Infections caused by multi-resistant pathogens are difficult to treat and often associated with high mortality. Therefore, new treatment strategies are of interest, such as the use of differently acting antibacterial concepts. One of these new concepts is the use of antiseptics in combination with the antibacterial photodynamic therapy (aPDT). Currently, no method has yet been established as a standard procedure for investigating combined effects and evaluating them in a generally valid and unambiguous manner. The focus of this study was on how cationic antiseptics benzalkonium chloride (BAC) and chlorhexidine digluconate (CHX) behave in a combined application with aPDT using the photosensitizer TMPyP. For this purpose, BAC and CHX were applied in combination with the aPDT using TMPyP in non-lethal concentrations to the three bacteria Escherichia coli, Staphylococcus aureus, and Enterococcus faecalis. The results of the combination experiments with sublethal concentrations of BAC or CHX with the aPDT showed that the binary application had a lethal effect. Irrespective of the bacteria, the reduction in concentrations in OPECC, compared to individual concentrations, was more than 50% for TMPyP, 23–40% for BAC, and 18–43% for CHX. Furthermore, the optimal effective concentration combinations (OPECCs) could be determined. The latter showed that the combined application allowed the reduction of both concentrations compared to the single application.