The molecular pathogenicity of Fusarium keratitis: a fungal transcriptional regulator promotes hyphal penetration of the cornea.

PURPOSE: The pathogenic mechanisms of fungal infection during human keratomycosis were investigated in an ex vivo corneal model that used strains of Fusarium oxysporum differing in the production of a fungal transcription factor.
METHODS: A pacC loss-of-function mutant and a pacC dominant-activating mutant were constructed from a wild-type isolate of F. oxysporum, and the 3 strains were characterized by in vitro growth kinetics. Twenty-seven human donor corneas maintained in tissue culture were superficially scarified and topically inoculated with the wild-type, the pacC loss-of-function mutant, or the pacC dominant-activating strains. Relative hyphal invasion into the stroma was compared histopathologically in corneal sections.
RESULTS: F. oxysporum strains demonstrated comparable exponential growth rates in vitro. Wild-type F. oxysporum invaded into the corneal tissue within 1 day and penetrated through the anterior stroma during the next 4 days. The pacC loss-of-function mutant invaded explanted corneas significantly less than the wild-type strain on day 1 (P < 0.0001) and on day 3 (P = 0.0003). The pacC dominant-activating strain adhered and penetrated explanted corneas similar to the wild-type strain.
CONCLUSIONS: The PacC pathway regulating the transcription of fungal genes allows fungal adaptation to the ocular surface and enables invasion of the injured cornea by F. oxysporum.