Susceptibility of Cryptococcus neoformans to photodynamic inactivation is associated with cell wall integrity

Photodynamic therapy is a rapidly developing antimicrobial technology which combines a nontoxic photoactivatable dye or photosensitizer with harmless visible light of the correct wavelength to excite the dye to its reactive triplet state to generate reactive oxygen species toxic to cells. In this report we present evidence that the fungal pathogen Cryptococcus neoformans is susceptible to photodynamic inactivation by use of a polycationic conjugate of polyethyleneimine and the photosensitizer chlorin(e6). A C neoformans rom2 mutant, with a mutation involving a putative Rhol guanyl nucleotide exchange factor that is part of the protein kinase C-cell wall integrity pathway, demonstrated a compromised cell wall and less (1,3)beta-D glucan than the wild-type strain and increased accumulation of PEI-ce6 as assessed by fluorescence uptake and confocal microscopy. Interestingly, C neoformans rom2 was hypersusceptible to photodynamic inactivation and coincubation of wild-type C neoformans strain KN99 alpha with caspofungin-enhanced photoinactivation. These studies demonstrated that C neoformans is sensitive to photodynamic therapy and illustrated the significance of cell wall integrity in microbial susceptibility to antimicrobial photodynamic inactivation.