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Hgure 50-3 Pseudohyphae consisting of elongated cells (arrow) with constrictions where attached (430x).

Figure 50-5 Hyaline hyphae that have rare or no discernible septations (aseptate or pauciseptate) (430x).

Hgure 50-3 Pseudohyphae consisting of elongated cells (arrow) with constrictions where attached (430x).

Figure 50-5 Hyaline hyphae that have rare or no discernible septations (aseptate or pauciseptate) (430x).

contrast to specialized reproductive hyphae. The nutrient-absorbing and water-exchanging portion of the fungus is called a vegetative mycelium. The portion extending above the substrate surface is known as aerial mycelium; aerial mycelia often give rise to fruiting bodies from which asexual spores are borne.

Three types of hyphae exist in the medically important fungi: the coenocytic (sparsely septate) hyphae of the Zygomycetes (Figure 50-5), the dark and pigmented septate hyphae of the dematiaceous fungi (Figure 50-6), and the septate, nonpigmented hyphae of the hyaline molds (Figure 50-7). The terms

Figure 50-6 Oematiaceous hyphae showing pigmentation and septations (arrows) (430x).

Figure 50-7 Hyaline hyphae showing septations (arrow) (430x).

dematiaceous and hyaline describe the presence or absence, respectively, of pigmentation within the hyphae of molds. Hyphal pigmentation is a useful feature to differentiate some fungi and is discussed later in this chapter.

The hyphae of Zygomycetes are wider in diameter compared with those of the fungi producing septate hyaline or dematiaceous hyphae. The branching of the Zygomycetes often occurs at angles greater than 45 degrees and up to 90 degrees, in contrast to the acutely branching hyphae of the septate dematiaceous and hyaline molds. A limited sampling of hyphae of the Zygomycetes often appear nonseptate or aseptate; however, a few septa are usually present on close inspection and vary depending on the organism and the age of the culture. These septa are often located near specialized reproductive hyphae. In these pauciseptate hyphae, nuclei and cytoplasm are free to flow throughout the length of the hyphal element. This has important implications when a commonly employed technique of tissue grinding is used to process all clinical specimens. When such processing is used, the hyphae of Zygomycetes may be destroyed, because the pauciseptate hyphae are not as compartmentalized by septa as they are with the hyaline and dematiaceous septate molds. Any disruption of any area of the hyphal strand in the Zygomycetes causes cytoplasmic leakage and eventually death of the organism. Conversely, the septate hyphae of dematiaceous and hyaline molds will survive such tissue processing because of the excessive compart-mentalization of the hyphae by septa and the viability of individual vegetative fungal cells.

The vegetative hyphae of the dematiaceous and hyaline molds are morphologically similar, apart from the differentiating pigmentation. Both have true septate hyphae that branch at acute angles. The cell walls of the dematiaceous fungi contain melanin-related compounds, which make the hyphae darkly pigmented when observed by light microscopy. In histopathologic sections, these pigments may be accentuated by the use of the special stains (e.g., Fontana-Masson stain).

taxonomy of the fungi

Fungi are composed of a vast array of organisms that are unique compared with plants and animals. Included among these are the mushrooms, rusts and smuU, molds and mildews, and the yeasts. Despite the great variation in morphologic features of the fungi, most or all share the following characteristics:

  • The presence of chitin in the cell wall
  • The presence of ergosterol in the cell membrane
  • Reproduction by means of spores, produced either asexually or sexually
  • The lack of chlorophyll
  • Lack of susceptibility to antibacterial antibiotics

Figure 50-8 A cieistothedum of Pseudallescheria boydii that has opened and is releasing numerous ascospores <750x).

Their heterotopic (derive nutrition from organic materials) nature

Traditionally, the fungi have been categorized into four well-established phyla: the Zygomycota, Ascomy-cota, Basidiomycota, and Deuteromycota. The phylum â– Zygomycota includes those organisms that produce sparsely septate hyphae and exhibit asexual reproduction by sporangiospores and sexual reproduction by the production of zygospores. Some of the clinically important genera in this phylum include Rhizopus, Mucor, Rhizomucor, Absidia, Cunninghamella, and Saksenaea

The Ascomycota include many fungi that reproduce asexually by the formation of conidia (asexual spores) and sexually by the production of ascospores. The filamentous ascomycetes are ubiquitous in nature, and all produce true septate hyphae. All exhibit a sexual form (teleomorph) but also exist in an asexual form (anamorph). In general, the anamorphic form correlates well with the teleomorphic classification. However, different anamoiphic forms may have the same teleomorphic form. For example, Pseudallescheria boydii (Figure 50-8), in addition to having the Scedosporium tyiospermum anamorph (Figure 50-9) may exhibit a Graphium anamorph (Figure 50-10). This latter anamorph may be seen with several other fungi. Examples of other clinically important fungi that belong to the Phylum Ascomycota include Histoplasma capsulation and Blastomyces dermatitidis, which have a teleomorph designed as Ajellomyces. Some species of Aspergillus have a

Iarrow]) (430x).
Figure 50-10 Graphium anamorph of P. boydii (500x).

teleomorph, Eurotium. Additionally, numerous yeast spedes belong to the Ascomycota and include Saccha-romyces and some species of Candida.

The phylum Basidiomycota includes those fungi that reproduce sexually by the formation of basidios-

pores on a specialized structure called the basidia* The basidiomycetes are generally plant pathogens or environmental organisms that rarely cause disease in humans. Included are the smuts, rusts, mushrooms, and the Gyptococcus neoformarts. The teleomorphic form of Cryptococcus neoformarts is Filobasidiella neoformarts.

The phylum Deuteromycota includes those fungi that lack a sexual reproductive cycle and are characterized by their asexual reproductive structures, primarily conidia. It is possible that the sexual forms of organisms in this group exist but have not yet been described.

practical classification of the fungi

The botanic taxonomic schema for grouping the fungi has little value in a clinical microbiology laboratory. Table 50-1 is a simplified taxonomic schema illustrating the major groups of fungi; these have been previously described within this chapter.

Clinicians find value in categorizing the fungi into four categories of mycoses;

  • Superficial or cutaneous mycoses
  • Subcutaneous mycoses
  • Systemic mycoses
  • Opportunistic mycoses

The superficial, or cutaneous, mycoses are fungal infections that involve the hair, skim or nails without direct invasion of the deeper tissue. The fungi in this category include the dermatophytes (agents of ringworm, athlete's foot, and so on) and agents of infections such as tinea, tinea nigra, and piedra. All of these infect keratinized tissues.

Some fungi produce infections that are confined to the subcutaneous tissue without dissemination to distant sites. Examples of subcutaneous infections include chromoblastomycosis, mycetoma, and the phaeohyph-omycotic cysts, which are discussed under subcutaneous mycoses later in this chapter.

As traditionally defined, agents of systemic fungal infections contain the genera Blastomyces, Cocddioides, Histoplasma, and Paracoccidioides. Infections caused by these organisms primarily involve the lungs but also may become widely disseminated and involve any organ system. Penicillium mameffei, 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 nonpathogenic may be the cause of infections. The infections these organisms cause occur primarily in patients with some type of compromise to their immune system. This may be secondary to an underlying disease process, such as diabetes mellitus, or due to an immuno suppressive agent. Although any fungus could potentially cause disease in these patients, the most cammonljr encountered organisms in this group include Aspergillus. Zygomycetes, Candida, and Cryptococcus, among others. All of these organisms may cause disseminated (systemic) disease. Some of the dematiaceous fungi may cause deeply invasive phaeohyphomycoses in this patient population.

This type of classification allows the clinician to attempt to categorize organisms in a logical fashion into groups having clinical relevance. Table 50-2 presents an example of a clinical classification of infections and their etiologic agents useful to clinicians.

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