Natural Photosensitizing and their Potential Application in Photodynamic Antimicrobial Chemotherapy

COMINI L., ; JELICICH N.,; NÚÑEZ S.,; PAEZ P.,; CABRERA J.L..

Santiago de Chile

Encuentro; 25th InterAmerican Photochemical Society Meeting; 2016

Comite Organizador del Evento

The photodynamic antimicrobial chemotherapy (PACT) involves the use of photosensitizers andvisible or ultraviolet light in the treatment of local infections, especially caries, periodontaldiseases, oral candidiosis as well as infected wounds. Some photosensitizers, like the psoralenderivatives, produce bacterial photoinactivation by a mechanism that allows their intercalationbetween the nucleic acid bases. Others have the same final output, but tliis is reached by means ofa photodynamic photosensitization, in which the generation of reactive oxygen species (ROS) isthe main process. Within the photodynamic mechanism, it is worth mentioning that severalanthraquinones (AQs) have been thoroughly studied in relation to their photosensitizingproperties. Thus, for instance, some of them show good antibacterial effects by producing ROS,such as superoxide anión (O2*-; photodynamic reaction Type I) and singlet molecular oxygen(´02; photodynamic reaction Type II), generating a photosensitized cellular inactivation. In thiscontext, we started the phytochemical study of the lichen Teleoschistes flavicans (SW) Nonn.(Telochistaceae), which revealed the presence of Parietin (PTN). By mean of photochemical andphotobiological studies, we have previously demonstrated that this AQ is an excellentphotosensitizer Type I and II.In this work, we study the in vitro antibacterial activity of PTN in darkness and under actinicirradiation, in order to assess the existence of a possible cellular inactivation photosensitized.The effect of PTN against four bacterial species (Staphylococcus aureus ATCC 29213,Staphylococcus epidermidis ATCC 28212, Escherichia coli ATCC 25922 y Pseudomonaaeruginosa ATCC 27853) was evaluated by the microdilution method according to CLSI.Minimum Inhibitory Concentration (MIC) and Mínimum Bactericidal Concentration (MBC) weredetermined on Mueller Hinton broth (MH, Britania). An overnight culture of each microorganismwas diluted to achieve a cell density in the range from 105 to 107 CFU/mL and incubated for 10min at 37 °C. Then, PTN were added in serial dilutions (250 - 0.125 ug/mL). Bacterial growthwas observed at 18 h of incubation. The cell suspensión (100 uL) was inoculated into each tubeto give a total volume of 200 uL. The lowest concentration of PTN that prevented bacterialgrowth was considered the MIC. Viable bacterial counts were obtained for samples withoutvisible bacterial growth by plating on MH agar, followed by aerobic incubation at 37 °C for 18 h.The PTN concentration that produces the death of 99.9% of initial inoculum was considered theMBC. This assay was simultaneously repeated under actinic irradiation.The results showed that PTN, in darkness, not exhibited antibacterial activity up to 250 ug/ mLagainst all bacterial species tested. Under irradiation, the inhibitory effect of PTN on S. aureusand S. epidermidis was observed at lower concentrations (16 and < 0.125 ug/mL respectively);valúes that are similar to the concentrations employed for known antibiotics like criprofloxacin.In addition, PTN showed bacteriostatic activity against Gram positive species (CIM/CBM: 0.25),and bactericidal activity on Gram negative species tested (CIM/CBM: 1). In strains with ability toform biofilms, as S. epidermidis and P. aeruginosa, a marked antimicrobial effect of PTN wasobserved under irradiation. These results are promising, since Parietin could be used to treatinfections associated with multi-resistant bacteria by means of PACT.