The fungi include a variety of general forms–yeast, mold, mushrooms, and imperfect fungi. They reproduce in a variety of ways – by a bud arising from a parent yeast cell, by the addition of a vegetative cell to chains of cells called hyphae, and by the production of both asexual and sexual spores. Although fungi form both asexual and sexual spores, they are classified based on the type of sexual spore they form.
Members of the Ascomycota form sexual spores called ascospores within a saclike structure called an ascus.
Aspergillus and Penicillium are filamentous members of this group, whereas the budding yeast Saccharomyces is a unicellular member. A bud is an outgrowth from the parent cell that pinches off, producing a daughter cell.
Aspergillus species are opportunistic. One form of pulmonary aspergillosis (fungal balls) involves colonization of the bronchial tree or tissues damaged by tuberculosis. Allergenic aspergillosis occurs in individuals who are in frequent contact with the spores and become sensitized to them.
Subsequent contact produces symptoms similar to asthma. Invasive aspergillosis is the most severe form and results in necrotizing pneumonia and may spread to other organs.
The Aspergillus fruiting body is distinctive, with chains of conidia containing asexual conidiospores arising from a swollen vesicle at the end of a stalk called a conidiophore.