Photosensitizers combination approach to enhance photodynamic inactivation of planktonic and biofilm bacteria

To improve bacterial photodynamic inactivation (PDI), this work analyzes the photodynamic effect caused by the combination of photosensitizers (PSs) on two bacterial models and different growth mode. Simultaneous administration of PSs from different families, zinc(II) 2,9,16,23-tetrakis[4-(N-methylpyridyloxy)]phthalocyanine (ZnPPc4+), 5,10,15,20-tetra(4-N,N,N-trimethylammonium phenyl)porphyrin (TMAP(4+)), meso-tetrakis(9-ethyl-9-methyl-3-carbazoyl)chlorin (TEMCC4+) and 5,10,15,20-tetrakis[4-(3-N,N-dimethylaminopropoxy)phenyl] chlorin (TAPC) was investigated against Staphylococcus aureus and Escherichia coli, in planktonic form, biofilm and growth curve. Various PSs combinations showed greater inactivation compared to when used separately under the same conditions but at twice the concentration. However, differences were found in the effectiveness of the PSs combinations on Gram positive and negative bacteria, as well as in planktonic or biofilm form. Likewise, the combination of three PSs completely stopped E. coli growth under optimal nutritional conditions. PSs combination allows extending the range of light absorption by agents that absorb in different areas of the visible spectrum. Therefore, PDI with combined PSs increases its antimicrobial capacity using agents' concentrations and light fluences lower than those necessary to cause the same effect as single PS. These advances represent a starting point for future research on the potentiation of PDI promoted by the combined use of PSs.