In vitro antimicrobial photodynamic therapy using tetra-cationic porphyrins against multidrug-resistant bacteria isolated from canine otitis

Antimicrobial-resistant bacteria have been frequently isolated from canine otitis. Photodynamic therapy using porphyrins as photosensitizing molecules is an alternative therapy against microorganisms in localized infections. Therefore, we evaluated the antibacterial activity of two tetra-cationic porphyrins (H2TMeP and ZnTMeP) against gram-positive and -negative bacteria isolated from canine otitis, as well as its putative action mechanism. For this, two gram-positive and two gram-negative bacteria frequently detected in cases of canine otitis (coagulase-positive and -negative staphylococci [CPS and CNS], Pseudomonas aeruginosa, and Proteus mirabilis) were used in antibacterial activity assays. Each porphyrin at a non-cytotoxic dose was incubated with a fixed concentration of each bacterium and exposed to white-light irradiation for 0, 30, 60, and 90 min. Clinical isolates of CPS and CNS were photo-inactivated after 30 min of white-light exposure by both porphyrins (p < 0.05). Gram-negative bacteria were also photo-inactivated after 30 min (p < 0.05), reaching complete inactivation after 60 and 90 min of white-light irradiation by H2TMeP and ZnTMeP, respectively. Antibacterial assays using standard bacterial strains (ATCCs) demonstrated similar results to those obtained with clinical isolates, except for P. aeruginosa, which was completely inactivated by ZnTMeP at 60 min, and the absence of a significant reduction in P. mirabilis concentration when irradiated for 30 min. Similar assays were conducted using reactive oxygen species scavengers showing that the putative mechanism for bacterial inactivation is through the production of singlet oxygen species. These results indicated that H2TMeP and ZnTMeP tetra-cationic porphyrins were effective against bacteria isolated from canine otitis.