General considerations for the identification of Molds
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p721-722
2025-11-15
61
Filamentous fungi are also identified by a combination of tests (Figure 1). Molds are identified using a com bination of the following:
• Growth rate
• Colonial morphologic features
• Microscopic morphologic features
Fig1. Identification of filamentous fungi from clinical specimens.
In most cases the microscopic morphologic features provide the most definitive means of identification. Determination of the growth rate can be most helpful when a mold culture is examined. However, this may have limited value, because the growth rate of certain fungi varies, depending on the amount of inoculum present in a clinical specimen. Slow-growers form mature colonies in 11 to 21 days, and intermediate-growers form mature colonies in 6 to 10 days. Rapid-growers form mature colonies in 5 days or less.
The growth of C. immitis is often rapid and is hazardous to microbiologists. In general, the growth rate for the dimorphic fungi, B. dermatitidis, H. capsulatum, and P. brasiliensis, is slow; 1 to 4 weeks usually are required before colonies become visible. In some instances, cultures of B. dermatitidis and H. capsulatum may be detected within 3 to 5 days. This is a somewhat uncommon circumstance, encountered only when large numbers of the organism are present in the specimen. Colonies of Mucorales may appear within 24 hours, whereas the other hyaline and dematiaceous (melanized) fungi often exhibit growth in 1 to 5 days. The growth rate of an organism, therefore, is important, but it must be used in combination with other features before a definitive identification can be made.
The colonial morphologic features may have limited value for identifying molds because of natural variation among isolates and colonies grown on different culture media. Although common organisms recovered repeatedly in the laboratory may be more easily recognized, colonial morphology is an unreliable criterion that should be used only to supplement the microscopic morphologic features of the organism.
The color of the colony can be important. The examiner must be sure to notice both the front and reverse sides of the culture. The colony topography describes the various elevations of the colony on the agar plate. Topography can be described as verrucose (furrowed or con voluted), umbonate (slightly raised in the center), and rugose (furrows radiate out from the center).
The colony’s texture should also be noted. Various textures can be seen, such as cottony (loose, high aerial mycelium), velvety (low aerial mycelium resembling a velvet cloth), glabrous (smooth surface with no aerial mycelium), granular (dense, powdery, resembling sugar granules), and wooly (high aerial mycelium that appears slightly matted down).
Incubation conditions and culture media must also be considered. For example, H. capsulatum, which appears as a white-to-tan fluffy mold on brain-heart infusion agar, may have a yeastlike appearance when grown on the same medium containing blood enrichment.
In general, the microscopic morphologic features of the molds are stable and show minimal variation. Definitive identification is based on the characteristic shape, method of reproduction, and arrangement of spores; however, the size of the hyphae also provides helpful information. The large, ribbonlike, pauciseptate hyphae of the mucorales are easily recognized; small hyphae, approximately 2 µm in diameter, may suggest the presence of one of the dimorphic fungi or a dermatophyte.
The fungi may be prepared for microscopic observation using several techniques. The procedure tradition ally used by most laboratories is the cellophane (Scotch) tape preparation (see Procedure 59-2 on the Evolve site; Figure 59-7). It can be done easily and quickly and often is sufficient to make the identification for most fungi. However, some laboratories prefer the wet mount (see Procedure 59-3 on the Evolve site; Figure 59-8) or tease mount (see Procedure 59-4 on the Evolve site). A microslide culture method (see Procedure 59-5 on the Evolve site; Figure 59-9) may be used when greater detail of the morphologic features is required.
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