The Mucorales
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p732-734
2025-11-27
59
GENERAL CHARACTERISTICS
Previous Name Zygomycota The mucorales (zygomycetes) characteristically produce large, ribbonlike hyphae that are irregular in diameter and contain occasional septa. Because the septa may not be apparent in some preparations, this group sometimes has been characterized 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 1). 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. Identification of the mucorales (Mucor, Rhizopus, Lichtheimia, and Absidia spp.) is partly based on the presence or absence of rhizoids and the position of the rhizoids in relation to the sporangiophores.
Fig1. Rhizopus spp. showing sporangium (A) on long sporangiophore (B) arising from pauciseptate hyphae. Note the characteristic rhizoids (C) at the base of the sporangiophore (×250).
EPIDEMIOLOGY AND PATHOGENESIS
Although the mucorales (Rhizopus, Mucor, Lichtheimia, Syncephalastrum, Cunninghamella spp., and others) are a less common cause of infection than the aspergilli, they are an important cause of morbidity and mortality in immunocompromised patients, particularly 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 organism is generally acquired by inhalation or ingestion of spores or through percutaneous routes, followed by subsequent development of infection. Once established, the infection is rapidly progressive, particularly in patients with diabetes mellitus who have infections that involve the sinuses.
SPECTRUM OF DISEASE
Immunocompromised patients are at greatest risk, particularly those who have uncontrolled diabetes mellitus and those who are undergoing prolonged corticosteroid, antibiotic, or cytotoxic therapy. The organisms that cause mucormycosis (an infection caused by mucorales) have a marked propensity for vascular invasion and rapidly produce thrombosis and necrosis of tissue. One of the most common presentations is the rhinocerebral form, in which the nasal mucosa, palate, sinuses, orbit, face, and brain are involved; each shows massive necrosis with vascular invasion and infarction. Perineural invasion also occurs in mucormycoses and is a potential means of retroorbital spread (i.e., invasion into the brain). Other types of infection involve the lungs and gastrointestinal tract; some patients develop disseminated infection. The mucorales have also caused skin infections in patients with severe burns and infections of subcutaneous tissue in patients who have undergone surgery.
LABORATORY DIAGNOSIS
Direct Detection Methods
Stains. A mucormycosis may be diagnosed rapidly by examining tissue specimens or exudate from infected lesions in a calcofluor white or potassium hydroxide preparation. Branching, broaddiameter, predominantly nonseptate hyphae are observed (Figure 2). It is important that the laboratory notify the clinician of these findings, because mucorales grow rapidly, and vascular invasion occurs at a rapid rate.
Fig2. Phase-contrast microscopy of a potassium hydroxide preparation of sputum. Note the fragmented portions (arrows) of broad, predominantly nonseptate hyphae of Rhizopus spp.
Antigen-Protein. Antigen-protein–based assays are not used for the diagnosis of mucormycosis.
Nucleic Acid Amplification. Nucleic acid testing is not routinely used for the diagnosis of mucormycosis. These assays may be available in research settings.
Cultivation. The colonial morphologic features of the mucorales allow immediate suspicion that an organism belongs to this group. Colonies characteristically produce a fluffy, white to gray or brown hyphal growth that resembles cotton candy and that diffusely covers the surface of the agar within 24 to 96 hours (Figure 3). The hyphae can grow very fast and may lift the lid of the agar plate (also known as a “lid lifter”). The hyphae appear to be coarse. The entire culture dish or tube rapidly fills with loose, grayish hyphae dotted with brown or black sporangia. The different genera and species of mucorales cannot be differentiated by colonial morphologic features, because most are identical.
Fig3. Rhizopus colony.
Approach to Identification
Rhizopus spp. have unbranched sporangiophores with rhizoids that appear opposite the point where the stolon arises, at the base of the sporangiophore (see Figure 1). In contrast, Mucor spp. are characterized by sporangiophores that are singularly produced or branched and have a round sporangium at the tip filled with sporangiospores. They do not have rhizoids or stolons, which distinguishes this genus from the other genera of the mucorales (Figure 4). Lichtheimia spp. and Absidia spp. are characterized by the presence of rhizoids that originate between sporangiophores (Figure 5). The sporangia of Lichtheimia spp. are pyriform and have a funnel-shaped area (apophysis) at the junction of the sporangium and the sporangiophore. Usually a septum is formed in the sporangiophore just below the sporangium. Other genera of Glomeromycota that are encountered much less frequently in the clinical laboratory are Rhizomucor, Saksenaea, Cunninghamella, Apophysomyces, Conidiobolus, and Basidiobolus spp.
Fig4. Mucor spp. showing numerous sporangia without rhi zoids (×430).
Fig5. Lichtheimia spp. (A) showing sporangia on long sporangiophores arising from pauciseptate hyphae (B). Note that rhizoids are produced between sporangiophores and not at their bases (×250).
Serodiagnosis
Serology is not useful for diagnosing zygomycosis.
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