Environmental Mycology in Public Health: Fungi and Mycotoxins Risk Assessment and Management provides the most updated information on fungi, an essential element in the survival of our global ecology that can also pose a significant threat to the health of occupants when they are present in buildings.
As the exposure to fungi in homes is a significant risk factor for a number of respiratory symptoms, including allergies and hypersensitivity pneumonitis, this book presents information on fungi and their disease agents, important aspects of exposure assessment, and their impacts on health.
This book answers the hard questions, including, 'How does one detect and measure the presence of indoor fungi?' and 'What is an acceptable level of indoor fungi?' It then examines how we relate this information to human health problems.
- Provides unique new insights on fungi and their metabolites detection in the environmental and occupational settings
- Presents new information that is enriched by significant cases studies
- Multi-contributed work, edited by a proficient team in medical and environmental mycology with different individual expertise
- Guides the readers in the implementation of preventive and protective measures regarding exposure to fungi
Cellular Constitution, Water and Nutritional Needs, and Secondary Metabolites
Robert A. Samson CBS-KNAW Fungal Biodiversity Centre, Uppsalalaan, Utrecht, The Netherlands
The fungal kingdom now contains approximately 100,000 described species and our knowledge of their occurrence and properties is steadily increasing. However, estimates of the real fungal biodiversity indicate that this is only a small portion of the million taxa to be discovered. In our society, fungi can have an important impact that can be useful or harmful. It is therefore essential to understand the structures and growth conditions (nutrients, water, pH, oxygen, temperature, light) of fungi. This chapter gives a general overview of how fungi develop and produce their diverse propagules for distribution and development. In environmental mycology not only is the fungus with its mycelium and various propagules important, but also the metabolites it produces. The diversity of fungi is reflected by a great variety of metabolites, and this is particularly manifested in genera such as Aspergillus and Penicillium. Mycotoxins are important metabolites and in this chapter the significance of toxins in food and indoor environment is discussed briefly.
Fungal metabolites; Fungal structures; Growth conditions in fungi; Mycotoxins
Recent taxonomic treatments show that fungi and animals both belong to the group Opisthokonta.1
Fungi are considered the sister group of animals and part of the eukaryotic crown group that appeared about a billion years ago. Fungi share with animals the ability to export hydrolytic enzymes that break down biopolymers, which then can be absorbed for nutrition. Fungi live in their own food supply and simply grow into new food as the local environment becomes exhausted of nutrients. The organisms traditionally regarded as "fungi" belong to three unrelated groups: the true fungi in Kingdom Fungi (Eumycota), the Oomycetes, and the slime molds. Our current knowledge shows that there are approximately 100,000 described species, but a conservative estimate of the total number of fungal species thought to exist is 1.5
has indicated that until recently, estimates of numbers of fungi did not include results from large-scale environmental sequencing methods. Newer estimates based on data acquired from several molecular methods have predicted that as many as 5.1 million species of fungi may exist.6
In this chapter a summary of important fungal structures and characters is given, with emphasis on the fungi that play an important role in environmental mycology. For more detailed information on fungi the reader should consult books on introductory mycology.8
The mycelium consists of hyphae, and the type of hyphae is characteristic of specific groups of fungi. Fungi that lack cross walls (nonseptate; aseptate; coenocytic) are found, for example, in the Zygomycetes. In Ascomycetes and Basidiomycetes, species form septate hyphae, with perforations at the septa, called septal pores. These allow the movement of cytoplasm and organelles from one compartment to the next. The type and complexity of the septal pore are characteristic of specific groups of fungi. Yeasts are unicellular, although some species with yeast forms may become multicellular, in the majority of the cases through the formation of strings of connected budding cells known as pseudohyphae. Hyphae elongate almost exclusively at the tips, growing outward from the point of establishment. As a result of apical growth, hyphae are relatively uniform in diameter, and mycelium that grows in an unimpeded manner forms a circular colony on solid substrates that support fungal growth.
The asexual propagules that form inside a sporangium, which can be mostly spherical or cylindrical, through a process involving cleavage of the cytoplasm are named sporangiospores. These spores are thin walled, one celled, hyaline, or pale in color, and usually globose or ellipsoid in shape. One to 50,000 sporangiospores may be formed in a single sporangium. When mature, sporangiospores are released by breakdown of the sporangial wall, or the entire sporangium may be dispersed as a unit. Sporangiospores are produced by fungi of the Chytridiomycetes and Zygomycetes groups, as well the Oomycetes, a group of fungi that is phylogenetically unrelated to the true fungi. The sexual propagation of the fungi that produce sporangiospores occurs via the zygospore. The zygospores serve as resting and survival propagules and are found rarely in cultures of common fungi.
Conidiophores and Conidia
Many species that are relevant to environmental mycology are "anamorphic fungi." This is the current terminology for those fungi that used to be called Fungi imperfecti, Deuteromyces, Hyphomycetes, Coelomycetes, etc. These names were used for fungi of the Ascomycetes or Basidiomycetes that lack a sexual state, but phylogenetic studies have shown that within many genera, sexual and asexual species are closely related. Hence there is now a change in the nomenclature of fungi, which is based on the one fungus-one name concept.14
In some genera, such as Aspergillus
with teleomorph connections (Eurotium
etc.), the selection of the current nomenclature of the species follows the anamorphic name, for example, Aspergillus
Anamorphic fungi were also artificially grouped based on their morphological structures, such as the presence of solitary conidiophores, synnemata, or conidiophores produced within pycnidia. Phylogenetic studies have also shown that within genera or even species these different structures may occur and therefore these, sometimes distinct, morphological structures cannot be used for distinguishing genera or even species. Among the anamorphic fungi various types of conidiogenesis can be seen. The patterns of conidiogenesis are described in detail by Cole and Samson.19
How conidiogenesis takes place in a fungus is relevant to the mode of sporulation, number of conidia produced, and distribution of these propagules. A common type of conidiogenesis is through the phialide, which can produce masses of conidia in dry chains or conglomerates (e.g., Aspergillus, Penicillium
) or in so-called slimy heads (e.g., Stachybotrys, Fusarium
). Other fungi are characterized by thallic, blastic, or poroconidia (e.g., Geotrichum, Cladosporium, Alternaria
In addition to the phialide, conidia can be formed from different types of conidiogenous cells, which can be formed singly on hyphae, on the surface of aggregated hyphal structures, or within various types of fruiting bodies. Pycnidia and acervuli are fruiting bodies inside which conidia are formed. Sporodochia and synnemata are other examples of fruiting bodies on which conidia are formed. Conidium-forming fungi are primarily Ascomycetes, although they can also be found as anamorphic Basidiomycete species. A good example is Wallemia sebi,
which belongs to a separate family, Wallemiomycetes,
and is very common in indoor environments and on low water activity food. For many years it was assumed that the spores/conidia and perhaps large mycelial fragments were the source of exposure to fungi21
and that spore counting could be used for exposure assessment. However, it has been demonstrated that fragments significantly smaller than spores (down to 0.1
µm) are released from the mycelia of infested materials.22
These fragments can be liberated in numbers hundreds of times higher than the number of spores, with no correlation between the numbers of released fragments and spores.23
It is important to consider exposure to the small fungal fragments when assessing exposure to fungal allergens.
Ascomata and Basidiomata
The ascoma (plural: ascomata) is the fruiting body of an Ascomycete and mostly consists of very tightly interwoven hyphae and may contain asci, each of which typically contains four, eight, or more ascospores. These fruiting bodies are most commonly bowl-shaped (apothecia), spherical (cleistothecia), or flask-like (perithecia), closed or with an opening. Genera such as Byssochlamys
are characterized by naked asci, which lack an ascoma wall. A basidioma (plural: basidiomata) is the fruiting body of a Basidiomycete and consists of a multicellular structure that bears the spore-producing hymenium. Basidiomata are characteristic of the hymenomycetes; rusts and smuts do not produce such structures. Epigeous (aboveground) basidiomata that are visible to the naked eye are commonly referred to as mushrooms, while hypogeous (underground) basidiomata are usually called false truffles.
Chlamydospores are survival structures formed from an existing hyphal cell or a conidium that develops a thickened wall and cytoplasm packed with lipid reserves. The thickened cell walls may be pigmented or hyaline, and chlamydospores develop singly or in clusters, depending upon the fungus. Chlamydospores are passively dispersed, in most instances when the mycelium breaks down. Chlamydospores are formed by many different groups of fungi and are often found...