Chapter 21: Fungal Infections

INTRODUCTION

The kingdom Fungi is a group of heterotrophic eukaryotic organisms composed of cells enclosed by a rigid cell wall containing ergosterol and chitin or other polysaccharides but usually not cellulose. Historically, mycology has often fallen under the discipline of botany, although fungi are more closely related to animals than to plants and are part of the eukaryotic crown group that diverged from each other roughly 1 billion years ago.¹ Fungi may be unicellular, such as yeasts, or multicellular, forming long filaments known as hyphae. Hyphae are divided into cells by septa. Various reproductive structures producing conidia (spores) develop on hyphae; the morphology of these structures provides the traditional basis for identification in the mycology laboratory and for taxonomy. Some genera have the capacity to form yeast, hyphal forms, or both, depending on environmental conditions, a characteristic known as dimorphism.

Asexual reproduction is the primary means of propagation for many fungi. Sexual reproduction involves the fusion of 2 haploid nuclei followed by meiosis. The diploid state is transient and is called the perfect state. Fungi are now classified as 1 kingdom, 1 subkingdom, 7 phyla (Chytridiomycota, Neocallimastigomycota, Blastocladiomycota, Microsporidia, Glomeromycota, Ascomycota, and Basidiomycota), 10 subphyla, 35 classes, 12 subclasses, and 129 orders.² Microsporidia, formerly considered protists, are now considered fungi on the basis of phylogenetic evidence.^3,4 The creation of the subkingdom Dikarya including the phylum Ascomycota and Basidiomycota, and the subphylum of mucoromycotina, are the most recent changes in the taxonomy. Indeed, the clades that have traditionally been classified in the phylum Zygomycota are distributed among the phylum Glomeromycota and 4 subphyla incertae sedis, including Mucoromycotina, Kickxellomycotina, Zoopagomycotina, and Entomophthoromycotina, pending resolution of ambiguous phylogenetic relationships.² The main pathogenic fungi belong to the subphylum Ascomycota (sac fungi, including Aspergillus, Candida, and Penicillium species), Basidiomycota (club fungi, including mushrooms and plant pathogens such as rusts and smuts), and mucoromycotina (bread molds, Rhizopus, Mucor). Ascomycota includes the order Onygenales, which in turn includes most of the dermatophyte genera and species including genus Arthroderma.⁵ Deuteromycota, or fungi imperfecti, no longer recognized as a formal taxon in fungal systematics, was the form phylum used to classify fungi with no known sexual reproduction; most of the pathogenic fungi were classified in this group until subsequent research demonstrated the sexual or so-called teleomorphic stage of many species, which were reclassified into other phyla.^6-10 In addition, comparison of nucleic acid sequences and nonsexual phenotypic characteristics allowed further revisions and the incorporation of asexual species, such as Penicillium, into Ascomycota. The teleomorphic reproductive propagule of the Ascomycota is the ascospore; these are produced in sacs called asci and are formed by the condensation of cytoplasm around the nucleus after meiosis.¹¹ Many of the medically important species are classified in the phylum ...