Pathobiont fungi: virulence factors and mechanisms of infection chronicity (analytical review article)

Pathobiont fungi represent an essential component of the human microbiome, forming the so-called mycobiota, which includes hundreds of species, predominantly from the phyla Ascomycota and Basidiomycota. In the gastrointestinal tract, the most frequently detected genera are Candida, Saccharomyces, Aspergillus, Penicillium, Cladosporium, Cryptococcus, Trichosporon and Malassezia. Among them, Candida albicans is the most extensively studied species of conditionally pathogenic fungal pathobionts. Under certain conditions, C. albicans is capable of transforming from a commensal into an invasive pathogen. Key factors of its virulence include morphological plasticity, expression of adhesins, secretion of hydrolytic enzymes, and the ability to form resilient biofilms. Recognition of C. albicans by the immune system occurs through pattern recognition receptors (PRRs), which initiate the innate immune response; however, the effectiveness of this response is closely linked to the host's micronutrient status. Zinc, iron, magnesium, and calcium, while playing crucial roles in supporting immune function, simultaneously act as regulators of C. albicans pathogenicity. Their excess may stimulate morphogenesis, production of virulence factors, and biofilm robustness, whereas their sequestration limits fungal growth. In this context, adjunctive probiotic therapy is considered a promising approach for the prevention and treatment of fungal infections. Strains of Bacillus spp., Lactobacillus salivarius, Bifidobacterium spp., and Pediococcus acidilactici, included in modern probiotic formulations, demonstrate anticandidal activity through the production of antimicrobial peptides, suppression of morphogenesis, and reinforcement of the epithelial barrier. Thus, maintaining the balance between the mycobiota, micronutrients, and the immune system is crucial for preventing the transition of commensal fungi into pathogens.