CONICET | Buscador de Institutos y Recursos Humanos

Photodynamic inactivation (PDI) combines the action of a photosensitizer with visible light, tocause microorganisms inactivation. The main advantage of PDI is its effectiveness in bacterial strains resistant to antibiotics.The 5-aminolevulinic acid (ALA) is a precursor in endogenous biosynthesis of porphyrins, some of which have photosensitizingactivity.The aim of this work was to employ ALA-based PDI to eliminate planktonic and biofilms cultures of five methicillin-sensitive (MSSA) and six -resistant (MRSA) S. aureus strains.Bacterial viability and porphyrin production were evaluated through colony count and fluorescence spectroscopy, respectively. PDI light source was an array of Tungsten lamps (90J/cm2). Viability of S. aureus planktonic cultures was reduced around 1-log after ALA-PDI treatment with 1 mM ALA. As a strategy to increase the outcome of PDI, a more lipophilic derivative of ALA, the hexyl ester ALA (H-ALA), was used. H-ALA (0.5 mM) induced bacterial viability reduction up to 4-logs in all strains tested. The biofilms of RN6390 (MSSA) and ST5-SCCmecI (MRSA, frequently involved in nosocomial infections) were less sensitive to PDI than their planktonic counterparts. In both strains, it was necessary to perform PDI with 2 mM ALA or H-ALA to reduce biofilms viability by 0.5 and 0.75 logs, respectively.To improve the effects of PDI on biofilms, we proposed a near-infrared laser treatment (NIRT) prior to performing PDI. NIRT thermal effects are capable ofinducing changes on biofilm structure. By this strategy, the viability of RN6390 and ST5-SCCmecI biofilms was reduced by 3 and 4-logs, respectively. NIRT induced increase in porphyrin synthesis from ALA or H-ALA as well as detachment of bacteria.We conclude that PDI is a promising alternative to eliminate MRSA strains of sanitary relevance. In addition, we verified the synergistic effect of two radiation-based bactericidal strategies.