Photoinactivation of multidrug-resistant Gram-positive bacteria isolated from dogs with otitis using tetra-cationic porphyrins bearing peripheral platinum(II) (3-PtTPyP) and palladium(II) (3-PdTPyP) complexes

The increasing prevalence of multidrug-resistant (MDR) bacteria isolated from dogs with external otitis presents a major therapeutic challenge in veterinary medicine, and antimicrobial photodynamic therapy has emerged as a promising alternative. Meso-tetra-pyridyl porphyrins bearing peripheral platinum(II) ( 3-PtTPyP ) and palladium(II) ( 3-PdTPyP ) complexes have shown activity against clinically relevant Gram-positive bacteria and may represent effective photosensitizers for otitis-associated MDR pathogens. Therefore, this study evaluated the antimicrobial activity and effects on bacterial morphology and adhesive properties of 3-PtTPyP and 3-PdTPyP on MDR isolates of Corynebacterium auriscanis , Enterococcus faecalis , Staphylococcus epidermidis , and Streptococcus canis obtained from dogs with otitis. Antibacterial activity was investigated by determining the minimum inhibitory and bactericidal concentrations (MBC) and by bacterial time-kill assay under white-light irradiation for 120 min. Alterations in bacterial morphology and nanomechanical properties were investigated using atomic force microscopy (AFM). Both porphyrins promoted a marked reduction in bacterial concentrations under white-light irradiation using MBC, although their bactericidal effects varied depending on the bacterial species and porphyrin. The 3-PtTPyP exhibited a more consistent photodynamic response against E. faecalis e C. auricanis , whereas 3-PdTPyP displayed greater effected against S. canis e S. epidermidis . AFM revealed pronounced changes in surface topography and nanomechanical properties after treatment with both porphyrins. Overall, these results demonstrate that 3-PtTPyP and 3-PdTPyP are promising photosensitizers for the inactivation of MDR Gram-positive bacteria associated with canine otitis externa, and the performance against the MDR Gram-positive bacteria was dependent of species.