Clinical classification of the Fungi
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p707-709
2025-11-10
136
The botanic taxonomic schema for grouping the fungi has little value in a clinical microbiology laboratory. Table 1 is a simplified taxonomic schema illustrating the major groups of fungi. For clinicians, dividing the fungi into four categories of mycoses, according to the type of infection, is much more useful. The fungi are categorized as follows:
• Superficial (cutaneous) mycoses
• Subcutaneous mycoses
• Systemic mycoses
• Opportunistic mycoses
Table1. Phylogenetic Position of Medically Significant Fungi
The superficial, or cutaneous, mycoses are fungal infections that involve the hair, skin, or nails without direct invasion of the deeper tissue. The fungi in this category include the dermatophytes (agents of ring worm, athlete’s foot) and agents of infections such as tinea, tinea nigra, and piedra. All of these infect keratinized tissues.
Some fungi cause infections that are confined to the subcutaneous tissue without dissemination to distant sites. Examples of subcutaneous infections include chromoblastomycosis, mycetoma, and phaeohyphomycotic cysts.
As traditionally defined, agents of systemic fungal infections include the genera Blastomyces, Coccidioides, Histoplasma, and Paracoccidioides. Infections caused by these organisms primarily involve the lungs but also may become widely disseminated and involve any organ system. P. marneffei, a geographically limited cause of systemic mycosis in a select patient population, may also be considered a part of this group.
Any of the fungi could be considered an opportunistic pathogen in the appropriate clinical setting. The list of uncommon fungi found to cause disease in humans expands every year. Fungi previously thought to be non pathogenic may be the cause of infections. The infections these organisms cause occur primarily in patients with some type of compromise of the immune system. This may occur secondary to an underlying disease process, such as diabetes mellitus, or it may be caused by an immunosuppressive agent. Although any fungus potentially can cause disease in these patients, the most commonly encountered genera in this group are Aspergillus, Candida, and Cryptococcus, among others. All of these organisms may cause disseminated (systemic) disease. Some of the dematiaceous fungi may cause deeply invasive phaeohyphomycoses (i.e., produce brown-pigmented structures) in this patient population.
Classification by type of infection allows the clinician to attempt to categorize organisms in a logical fashion into groups having clinical relevance. Table 2 presents an example of a clinical classification of infections and their etiologic agents that is useful to clinicians.
Table2. General Clinical Classification of Pathogenic Fungi
PRACTICAL WORKING SCHEMA
To assist individuals working in clinical microbiology laboratories with the identification of clinically important fungi, Koneman and Roberts have suggested a practical working scheme designed to do the following:
• Assist with the recognition of fungi most commonly encountered in clinical specimens
• Assist with the recognition of fungi recovered on culture media that are strictly pathogenic fungi
• Provide a pathway that allows an identification to be made based on a few colonial and microscopic features
Table 3 presents these features. However, the table includes only organisms commonly seen in the clinical laboratory. With practice, most laboratorians should be able to recognize these on a day-to-day basis. For other, less commonly encountered fungi, the microbiologist must use a variety of texts that have photomicrographs, which can aid identification.
Table3. Most Commonly Encountered Fungi of Clinical Laboratory Importance: a Practical Working Schema
Use of the identification scheme just described requires examination of the fungal culture for the presence, absence, and number of septa. If the hyphae appear to be broad and predominantly nonseptate (i.e., cells are not separated by a septum or wall), zygomycetes should be considered. If the hyphae are septate, they must be examined further for the presence or absence of pigmentation. If a dark pigment is present in the hyphae, the organism is considered to be dematiaceous, and the conidia are then examined for their morphologic features and their arrangement on the hyphae. If the hyphae are nonpigmented, they are considered to be hyaline. The fungi are then examined for the type and the arrangement of the conidia produced. The molds are identified by recognition of their characteristic microscopic features (see Table 3). Murray has developed an expanded morphologic classification of medically important fungi based on general microscopic features and colonial morphology. The color pigmentation of colonies is presented as a useful diagnostic feature.
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