Biomedical Engineering Reference
In-Depth Information
are not listed in the schema of pathogenic organisms (below) because there
are no known chytrids that cause human infection. However, chytrids, a rare
aquatic fungus (most fungi grow in soil), are currently ravaging amphibian
populations. The chytrid Batrachochytrium dendrobatidis seems capable of
infecting thousands of different amphibian species and threatens many of
these species with extinction.
Non-aquatic fungi (i.e. all classes of fungi other than the chytrids) lack
flagella, presumably lost when these fungi adjusted to life in soil, and no
longer needed a flagellum to propel themselves through water. For most of
the fungi that are pathogenic in humans, individual fungal organisms grow
on a surface or as a mycelial mass in the soil. Propagation occurs when asex-
ual or sexual spores are expelled into the air and wafted to another location.
Aside from the ancestral single posterior flagellum, that establishes a
close relationship between fungi and animals, there is also the presence of
chitin. Chitin is a long-chain polymer built from units of N-acetylglucosa-
mine, and found in the cell walls of every fungus. It is analogous to cellu-
lose, which is built from units of glucose. Importantly, chitin is never found
in plants, and cellulose is never found in fungi. Aside from its presence in
fungi, chitin is found in some member of Class Protoctista and in some
members of Class Animalia (particularly arthropods). Chitin is the primary
constituent of the exoskeleton of insects. The important structural role of chi-
tin in fungi and animals should have been a clue to the close relationship
between these two classes. It happens that chitin was not discovered until
1930 (by Albert Hoffmann); well before that time, Class Fungi had been
incorrectly assigned to the plant kingdom.
Lastly, fungi and animals are heterotrophic, acquiring energy by metabol-
izing organic compounds obtained from the environment. Plants, unlike ani-
mals and fungi, are phototropic autotrophs, producing organic compounds
from light, water, and carbon dioxide.
Interactions between fungi and humans vary, following one or more of
the following scenarios, listed in order of increasing clinical consequence:
1. The fungus grows in the external environment, usually in soil or on
plants, never interacting in any way with humans.
2. Spores and asexual reproductive forms are emitted into the air. In warm
and tropical locations, fungal elements are the predominant particulate
matter found in air samples. Humans are exposed constantly to a wide
variety of fungi just by breathing (spores and conidia), by ingestion
(fungi grow on the plants we eat), and by direct skin contact with fungal
colonies in soil and airborne organisms.
3. After exposure, fungi may leave, without colonizing (e.g. you inhale
them, and then you exhale them, and they're gone).
4. After exposure, fungi may transiently colonize a mucosal surface, such
as the oral cavity, the nose, the gastrointestinal tract, the respiratory
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