Biomedical Engineering Reference
In-Depth Information
There are four classes of fungi that contain pathogenic organisms:
Class Zygomycota (Chapter 34), Class Basidiomycota (Chapter 35), Class
Ascomycota (Chapter 36), and Class Microsporidia (Chapter 37). Class
Ascomycota contains the greatest number of fungal organisms infectious in
humans, and it contains most of the fungi that regularly cause clinically life-
threatening disease in otherwise healthy individuals. Class Ascomycota,
along with Class Basidiomycota, comprise the dikaryotic fungi
(see
Chapter 35 for full discussion).
All members of Class Ascomycota that reproduce sexually produce an ascus
(from the Greek “askos,” meaning sac), containing spores. Unfortunately for taxo-
nomists, many members of Class Ascomycota simply do not reproduce sexually;
hence, they do not produce the ascus that characterizes their taxonomic class.
Taxonomists invented a temporary class of organisms known as the deutero-
mycotes (or imperfect fungi) to hold these asexual species. Thanks to molecular
analyses, many of these ascus-impaired species have been sorted into proper
subclasses within Class Ascomycota. Currently, three major classes account for
all of the pathogenic members of Class Ascomycota: Saccharomycotina,
Taphrinomycotina, and Pezizomycotina. Class Saccharomycotina are yeasts;
round, unicellular fungi that reproduce by budding. This class contains a single
genus that is pathogenic in humans: Candida. Class Taphrinomycotina contains a
single species that is pathogenic in humans: Pneumocystis jiroveci. All of the
remaining Ascomycotes, and there are many, belong to Class Pezizomycotina.
Ascomycota
Saccharomycotina
Saccharomycetes
Saccharomycetales
Saccharomycetaceae
*Candida
Candida, the sole pathogenic genus in Class Saccharomycotina, is a nor-
mal inhabitant of humans, and various species are found on the skin, respira-
tory tract, gut, and female genital tract of healthy individuals. An ecological
balance exists between Candida species and various bacterial commensals.
When this balance is disrupted by the use of antibiotics, overgrowth
of Candida species may occur. In addition, as with virtually all of the
pathogenic fungi, overt diseases may occur in immune-deficient individuals.
Patients undergoing intense chemotherapy are at particular risk for life-
threatening candidal infections.
The least worrisome of the candidiases are superficial infections confined
to mucosal surfaces. These are common in the mouth and GI tract and are
characterized by thick colonies of yeast that form a white surface crust.
So-called invasive candidiasis involves the penetration of organisms through
Search WWH ::




Custom Search