An Optimized Protocol for <em>Candida albicans</em> Infection in <em>Schmidtea mediterranea</em> to Study Fungal Pathogenesis and Host Defense

Candida albicans is a common opportunistic fungal pathogen that asymptomatically colonizes most humans. Although typically a benign commensal, dysbiosis caused by antibiotic use, immune dysfunction, or epithelial barrier disruption can trigger fungal overgrowth and infection, ranging from superficial mucosal disease to life-threatening systemic candidiasis. New preclinical infection models are needed to dissect C. albicans pathogenesis in vivo across distinct infection stages and with different measurable host outcomes. The planarian Schmidtea mediterranea was previously established as an invertebrate host for studying host-pathogen interactions during C. albicans infection. S. mediterranea relies entirely on conserved innate immune mechanisms capable of overcoming infection with pathogenic microorganisms, including bacteria and fungi. Planarians' remarkable regenerative capacity and accessible stem cell populations make this organism a tractable model to analyze early immune responses, tissue repair, and pathogen clearance in vivo. This model supports simultaneous analysis of fungal virulence and host transcriptional responses, providing valuable insights into infection dynamics. Here, an updated protocol with detailed modifications, standardized procedures, and optimized steps for infecting S. mediterranea with C. albicans has been presented, designed to enhance reproducibility and enable systematic studies of fungal pathogenesis and host defense.