Fungal spores: Initiators of colonization and infection

Abstract S5.3 Cellular pleomorphism and fungal virulence, September 22, 2022, 3:00 PM - 4:30 PM Fungi produce asexual and sexual spores for reproduction and distribution, which can be both in space and time. Distribution in space occurs, by air movement, but also, by water or other vectors such as living organisms. Filamentous fungi from the division Ascomycota that belong to the order Eurotiales produce asexual spores called conidia. Conidia are moderately stress-tolerant cells and are able to survive unfavorable conditions such as thermal stress, dehydration, osmotic pressure, oxidative stress, variations in pH, and UV. For example, conidia of the fungus Penicillium chrysogenum are isolated worldwide and must be regarded as cosmopolitan. In many cases, conidia might ‘land’ closely to the location of production, but still many spores making into the higher air layers. There is indirect evidence that spores may be able to travel large distances through the air. For example, Aspergillus sydowii conidia have been suggested to travel over thousands of kilometers from the Sahara Desert to the Caribbean reefs. Distribution in time is occurring as stress-resistant cells remain dormant at one location for an extended period, awaiting conditions that are more favorable for growth. Some ascospores (sexual spores) are extremely stress-resistant and dormant for very long periods. Other species show extended dormancy in a dried state. As microbial species are inherently variable, stress resistance varies between strains from the same species. For example, conidial heat resistance (D60) of various strains of the fungus Paecilomyces variotii ranged between 3.5 to 27.6 min. This intraspecific variation could have profound consequences on diagnostics, virulence, and antifungal treatment in clinical settings. For conidial germination in most filamentous fungi, the presence of nutrients such as inorganic salts, sugars, and amino acids is required. The swelling phase of conidia is also called isotropic growth. Swollen conidia direct the growth to one side of the cell to grow in a polarized fashion, which leads to the formation of a germ tube (polarized growth). There is a notable drop in stress resistance during isotropic and polarized growth and genes expressed during these stages might represent novel targets for fungal infection.