Basidiospores are formed by the phylum Basidiomycota. Basidiomycetous molds are being recovered in increasing numbers, but their clinical significance is not clearly understood. The terminal cell of the resulting mycelium enlarges into a club-shaped structure called a basidium. The two nuclei within the basidium fuse to form a zygote, then undergo meiosis to produce four haploid nuclei. Four little protrusions (basidiospores) extend out from the end of the basidium, and each haploid nucleus travels into a Basidiospore. A binucleate mycelium is formed by fusion of two compatible hyphae or yeast cells, usually with the aid of clamp connections. Some Basidiomycetes, such as mushrooms, produce a protective basidocarp to lodge the basidia and basidiospores.
Clinically significant basidiomycetes are few. The only really important Basidiosmycete in medical mycology is Filobasidiella neoformans, the sexual stage of Cryptococcus neoformans. Close communication between the physician and environmental consultant may assist with determining whether the isolate is an environmental contaminant or an agent of disease.
When basidiomycetous molds are recovered in the laboratory, they typically remain sterile, complicating the identification process. One clue that a mold is a basidiomycete is the presence of clamp connections. Clamp connections occur at the septations in the vegetative hyphae and are easily visible with light microscopy. A portion of the hypha on one side of the septation grows out and connects to the hypha on the other side of the septation, thereby bypassing the septation.