Molecular Triggers of Yeast Pathogenicity in the Yeast–Host Interactions

<i>Candida</i> and other pathogenic yeast species, able to transition from non-invasive commensal organisms to invasive pathogens, are characterized by a high ability to adapt to stress conditions encountered in the human host, such as pH and temperature shifts, CO₂ and oxygen level variations, and nutritional limitations. Although <i>Candida albicans</i> remains the main cause of <i>Candida</i>-related infections, non-<i>albicans Candida</i> (NAC) species, including <i>C. tropicalis</i>, <i>C. parapsilosis</i>, <i>C. krusei</i>, and non-<i>Candida</i> species such as <i>Yarrowia lipolytica</i>, <i>Candidozyma auris</i>, and <i>Nakaseomyces glabratus</i>, are gaining clinical importance. These species exhibit diverse mechanisms of pathogenicity, including morphological transition, modulation of gene expression pathways (cAMP-PKA/MAPK, Hsp, calcineurin, GlcNAc-mediated signaling), cell wall remodeling, post-translational reprogramming, biofilm formation, antifungal resistance, and enzyme secretion. <i>C. albicans</i> exhibits high morphological and metabolic plasticity for survival across body niches. <i>N. glabratus</i> and <i>C. tropicalis</i> show strong azole resistance and biofilm formation, while <i>C. parapsilosis</i> and <i>C. krusei</i> pose risks through surface adhesion and treatment resistance. <i>C. auris</i> stands out for heat tolerance, multidrug resistance, and outbreak potential. <i>Y. lipolytica</i>, though rare, forms persistent filamentous biofilms in critical care settings. <i>Cryptococcus neoformans</i> remains a life-threatening pathogen capable of immune evasion and crossing the blood-brain barrier. This review compares molecular mechanisms of pathogenicity across these fungi, emphasizing environmental adaptation, conserved and species-specific responses, and potentially highlighting targets for therapeutic management.