Figure 50-41 In this preparation of a Trichophyton species, the numerous small, spherical microconidia (A) are contrasted with a large, elongated macroconidium (B) (430x).

termed a "penicillus," in which each branch terminates in secondary branches (metulae), and phialides, from which chains of conidia are borne (Figure 50-40). Spedes of Penicillium and Paecilomyces are representative of this type of sporulation. In other instances, fungi may produce conidia of two sizes: microconidia that are small, unicellular, round, elliptical or pyriform in shape or macroconidia that are large, usually multiseptate, and club- or spindle-shaped (Figure 50-41). Microconidia may be borne directly on the side of a hyphal strand or at the end of a conidiophore. Macroconidia are usually borne on a short to long conidiophore and may be smooth or rough-walled. Microconidia and macroconidia are seen in some fungal spedes and are not specific, except as they are used to differentiate a limited number of genera.

The hyphae of the Zygomycetes are sparsely septate. Sporulation takes place by progressive deavage

Figure 50-43 Rhizopus spp. showing sporangium (A) on long sporangiophore (B) arising from pauciseptate hyphae. Note presence of characteristic rhizoids (C) at the base of the sporangiophore (250x).

Figure 50-42 Large saclike sporangia that contain sporangiospores (arrow) characteristic of the zygomycetes (250x).

Figure 50-43 Rhizopus spp. showing sporangium (A) on long sporangiophore (B) arising from pauciseptate hyphae. Note presence of characteristic rhizoids (C) at the base of the sporangiophore (250x).

during maturation within the sporangium, a saclike structure produced at the tip of a long stalk (sporangiophore). Sporangiospores, spores produced within the sporangium, are produced and released by the rupture of the sporangial wall (Egure 50-42). Rarely, some isolates may produce zygospores, rough-walled spores produced by the union of two matching types of a zygomycete; this is an example of sexual reproduction.

identification of molds hyaline, pauciseptate molds: the zygomycetes

Genera and Species to Be Considered—Zygomycetes (Rhizopus, Mucor, Absidia)

general characteristics

The Zygomycetes characteristically produce large, ribbonlike hyphae that are irregular in diameter and contain occasional septa. The septa may not be apparent in some preparations, which has resulted in the characterization of this group as aseptate. The specific identification of these organisms is confirmed by observing the characteristic saclike fruiting structures (sporangia), which produce internally spherical, yellow or brown spores (sporangiospores) (Figure 50-43), Each sporangium is formed at the tip of a supporting structure (sporangiophore). During maturation, the sporangium becomes fractured and sporangiospores are released into the environment. Sporangiophores are usually connected to one another by occasionally septate hyphae called stolons, which attach at contact points where rootlike structures (rhizoids) may appear and anchor the organism to the agar surface. The identification the three most common Zygomycetes—Mucor, Rhizopus, and Absidia—is, in part, based on the presence or absence of rhizoids and the position of the rhizoids in relation to the sporangiophores.

epidemiology and pathogenesis

Although the Zygomycetes (Rhizopus, Mucor, Absidia, Syncephalastrum, Cunninghamella, and others) are a less common cause of infection compared with the asper-i^ gilli, they are an important cause of morbidity and mortality in immunocompromised patients, particular]^ patients with diabetes mellitus. The organisms involved have a worldwide distribution and are commonly found on decaying vegetable matter or old bread (they are a common bread mold), or in soil. The infection is gene? rally acquired by inhalation of spores followed by subsequent development of infection. Once established, infection is rapidly progressive, particularly in patients with diabetes mellitus who have infections that involve the sinuses.

spectrum of disease

Immunocompromised patients, particularly those whd have uncontrolled diabetes mellitus and those who are receiving prolonged corticosteroid, antibiotic, or cyto^ toxic therapy, are at greatest risk.71 The organisms involved in causing zygomycosis (infection caused by zygomycete) have a marked propensity for vascula| invasion and rapidly produce thrombosis and necrosis*^ tissue. One of the most common presentations is the^ rhinocerebral form wherein the nasal mucosa, palate!

sinuses, orbit, face, and brain are involved; each shows passive necrosis with vascular invasion and infarction, perineural invasion also occurs in zygomycoses and is a potential means of retroorbital spread (i.e., invasion into die brain). Other types of infection involve the lungs and gastrointestinal tract; some patients develop disseminated infection. The Zygomycetes have also caused skin infections of patients with severe burns and in infections of subcutaneous tissue of patients who have jjndergone surgery.

laboratory diagnosis

Specimen Collection and Transport

See General Considerations for the Laboratory Diagnosis of Fungal Infections.

Specimen Processing

See General Considerations for the Laboratory Diagnosis of Fungal Infections.

Direct Detection Methods

Stains. The rapid diagnosis of zygomycosis may be made by examination of tissue specimens or exudate from infected lesions in a calcofluor white or potassium hydroxide preparation. Branching, broad-diameter, predominantly nonseptate hyphae are observed (see Figure 50-15). It is important that the laboratory notify the clinician of these findings because Zygomycetes grow rapidly and vascular invasion occurs at a rapid rate.

Antigen-Protein. Antigen-protein-based assays are not used for the diagnosis of zygomycosis.

Nucleic Acid Amplification Nucleic acid testing is not routinely used for the diagnosis of zygomycosis. These assays maybe available in research settings.

Cultivation. The colonial morphologic features of the Zygomycetes allow one to immediately suspect organisms belonging to this group. Colonies characteristically produce a fluffy, white to gray or brown hyphal growth that diffusely covers the surface of the agar within 24 to 96 hours (Figure 50-44). The hyphae appear to be coarse and fills the entire culture dish or tube rapidly with loose, grayish hyphae dotted with brown or black sporangia. It is impossible to distinguish between the different genera and species of Zygomycetes based on their colonial morphologic features, because most are identical in appearance.

Approach to Identification

Rhizopus has unbranched sporangiophores with rhizoids that appear at the point where the stolon arises, at the

Figure 50-44 Rhizopus colony.

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