Medically Important Fungi 1: Alternaria sp. and Ascospore

Alternaria sp. – Pathogenicity: Commonly considered a saprophytic contaminant but occasionally causes phaeophyphomycosis, most commonly in subcutaneous tissue. There have also been a few reports of infection of nails, eye, and nasal sinuses.
Rate of Growth: Rapid; mature within 5 days.
Colony Morphology: Surface is at first grayish white and woolly and later becomes greenish black or brown with a light border. May eventually become covered by short, grayish, aerial hyphae. Reverse is black.
Microscopic Morphology: Hyphae are septate and dark. Conidiophores are septate and of variable length and sometimes have a zigzag appearance. Conidia are large (usually 8-16 X 23-50 um) and brown, have both transverse and longitudinal septations, sometimes produce germ tubes, and are found singly or in chains; they are usually round at the end nearest the conidiophores while narrowing at the apex, producing a clublike shape.

Ascospore – A sexual spore produced in a sac-like structure known as an ascus. Characteristic of the class Ascomycetes.

Medically Important Fungi 2: Aspergillus sp.

Pathogenicity: Members of the genus Aspergillus cause a group of diseases known as aspergillosis; the disease may be in the form of invasive infection, colonization, toxicoses, or allergy. The organisms are opportunistic invaders, the most common molds to infect various sites in individuals with lowered resistance due to neutropenia and/or treatment with high-dose corticosteroids or cytotoxic drugs. Aspergillus spp. are widespread in the environment and are commonly found as contaminants in cultures. Approximately 175 species of Aspergillus are known, but only about 20 have been found to cause disease.
Rate of Growth: Usually rapid; mature within 3 days; some species are slower growing.
Colony Morphology: Surface is at first white and then any shade of green, yellow, orange, brown, or black, depending on species. Texture is velvety or cottony. Reverse is usually white, goldish, or brown.
Microscopic Morphology: Hyphae are septate (2.5-8.0 um in diameter); an unbranched conidiophore arises from a specialized foot cell. The conidiophores is enlarged at the tip, forming a swollen vesicle. Vesicles are completely or partially covered with flask-shaped phialides (formerly referred to as sterigmata), which may develop directly on the vesicle (uniseriate form) or be supported by a cell known as a metula (biseriate form). The phialides produce chains of mostly round, sometimes rough conidia (2-5 um in diameter).

 

Medically Important Fungi 3: Penicillium spp.

Pathogenicity: Commonly considered contaminants, but found in a variety of diseases in which their etiologic significance is uncertain. They have been known to cause corneal, cutaneous, external ear, respiratory, and urinary tract infections, as well as endocarditis after insertion of valve prostheses. Disseminated disease has been reported in severely immunocompromised patients. Many strains produce toxins.
Rate of Growth: Rapid; mature within 4 days. Usually no or poor growth at 37°C.
Colony Morphology: Surface at first is white, then becomes very powdery and bluish green with a white border. Some less common species differ in color and texture. Reverse is usually white, but may be red or brown.
If the isolate produces a red reverse and diffuse pigment in the agar, P. marneffei must be considered, and if the organism should be tested for thermal dimorphism; this is especially relevant if the patient has recently visited Southeast Asia.
Microscopic Morphology: Hyphae are septate (1.5-5 um in diameter) with branched or unbranched conidiophores that have secondary branches known as metulae. On the metulae, arranged in whorls, are flask-shaped phialides that bear unbranched chains of smooth or rough, round conidia (2.5-5 um in diameter). The entire structure forms the characteristic “penicillus” or “brush” appearance.

 

Medically Important Fungi 4: Basidiospore and Bipolaris spp.

 


Basidiospore – A sexual spore formed on a structure known as a basidium. Characteristic of the class Basidiomycetes.

Bipolaris spp.
Pathogenicity: Most commonly cause allergic sinusitis and, in immunocompromised patients, may progress to invade bone and cause lesions in the brain. Occasionally infect a variety of other sites, including the eye, skin, aorta, lung, and central nervous system. Also known to be present as contaminants in clinical specimens.
Rate of Growth: Rapid; mature within 5 days.
Colony Morphology: Surface is at first grayish brown, becoming black with a matted center and raised grayish periphery. Reverse is dark brown to black.
Microscopic Morphology: Dark septate hyphae. Conidiophores elongate and bend at the point where each conidium is formed (sympodial geniculate growth); this produces a knobby zigzag appearance. The conidia are brown, oblong to cylindrical (6-12 X 16-35um), appear thick walled, and have 3-5 septations and a slightly protruding hilum.
Drechslera spp.– a plant pathogen with an extremely similar appearance to Bipolaris that has never been reported as a human pathogen

Medically Important Fungi 5: Chaetomium sp.

Pathogenicity: Commonly considered a contaminant; occasionally implicated in systemic and cutaneous phaeohyphomycosis.
Rate of Growth: Rapid; mature within 5 days.
Colony Morphology: Surface is cottony, spreading, usually white, becoming tannish gray or grayish olive with age. Reverse is usually orange-tan tinted with red but may be brown to black.
Microscopic Morphology: Hyphae are septate with large (90-170 X 110-250), round, oval, or flask-shaped perithecia (best seen on potato dextrose agar) that are olive to brown and fragile and have wavy and/or straight filamentous appendages. Asci are stalked and club shaped, contain 4-8 spores, and usually dissolve soon after release from the ostiole (opening) of the perithecium. Ascospores, readily observed, are oval or lemon-shaped, single celled, and usually olive-brown but may occur in a variety of shapes, sizes, and colors.

 

Medically Important Fungi 6: Cladosporium spp.

Pathogenicity: Commonly considered saprophytic contaminants. They have only occasionally been implicated in infections.
Rate of Growth: Moderately rapid; mature within 7 days at 25°C. Most strains do not grow at 37°C, but some do.
Colony Morphology: Surface is greenish brown or black with grayish velvety nap, becoming heaped and slightly folded. Reverse is black.
Microscopic Morphology: Hyphae are septate and dark; conidiophores are dark and branched, vary in length, and usually produce 2 or more conidial chains. Conidia are brown, round to oval (3-6 X 4-12 um), and usually smooth; they form branching treelike chains and are easily dislodged, showing dark spots (hila) at the point where they were attached to the conidiophores or other conidia.The cells bearing the conidial chains are large and sometimes septate, resemble shields, and may be mistaken for macroconidia when seen alone.

 

Medically Important Fungi 7: Curvularia spp.

Pathgenicity: Etiologic agents of opportunistic infections, most commonly of the cornea and sinuses; also cause mycetoma and phaephyphomycosis at various sites, including nails, subcutaneous tissue, and systemic organs. Dissemination to the brain is known to occur occasionally. They are also encountered as contaminants.
Rate of Growth: Rapid; mature within 5 days.
Colony Morphology: Colony is dark olive-green to brown or black with a pinkish gray, wooly surface. Reverse is dark.
Microscopic Morphology: Hyphae are septate and dark. Conidiophores are simple or branched and bent or knobby at points of conidium formation (sympodial geniculate growth). Conidia are large (8-14 X 21-35um), usually contain 4 cells, and eventually appear curved due to swelling of a central cell. Conidia differ from those of Bipolaris spp. by having a central cell that is darker than the end cells, a thinner cell wall, narrower septations between cells, and a distinct curve that develops with age.

 

Medically Important Fungi 8: Epicoccum sp.

Pathogenicity: Commonly known as a contaminant; not known to cause disease.
Rate of Growth: Moderately rapid; mature within 7 days.
Colony Morphology: Colonies are irregularly cottony and usually yellow to orange at first, becoming brown to black where the dark mature conidia eventually form. Reverse is sometimes red or may be dark brown or grayish. A diffusible pigment may color the medium yellow, orange, red, or brown.
Microscopic Morphology: Clusters of short conidiophores form on hyphae by repeated branching to form a dense mass from which conidia arise. Young conidia are round to pear shaped, pale, smooth, and nonseptate. Mature conidia (15-30 um in diameter) are almost round, multiseptate both longitudinally and transversely, dark brown or black, and often rough and warty. Characteristically, all stages of conidia will be present simultaneously in the clusters.

 

Medically Important Fungi 9: Fusarium spp.

Pathogenicity: Frequent agents of mycotic eye infections, most commonly affecting the cornea. They are also occasionally involved in a variety of infections, including mycetoma, sinusitis, septic arthritis, and nail infections. Fusarium spp. are increasingly the cause of disseminated systemic infections in severely neutropenic hosts; in these cases, the organism can often be cultured from skin lesions and blood specimens. Additionally, disease has been reported in individuals who ingested food prepared from grain that had been overgrown by toxin-producing species. Fusarium is also encountered as a contaminant.
Rate of Growth: Rapid; mature within 4 days.
Colony Morphology: At first white and cottony, but often quickly develops a pink or violet center with a lighter periphery. Some species remain white or become tan or orangey. F. solani is unique in becoming blue-green or bluish brown where clusters of conidiogenous cells develop. Reverse is usually light, but may be deeply colored.
Microscopic Morphology: Septate hyphae. There are two types of conidiation: (i) unbranched or branched conidiophores with phialides that produce large (2-6 X 14-80 um) sickle- or canoe-shaped macroconidia (with 3-5 septa) and (ii) long or short simple conidiophores bearing small (2-4 X 4-8 um), oval, 1- or 2-celled conidia singly or in clusters resembling those of Acremonium spp.

 

Medically Important Fungi 10: Memnoniella, Myxomycetes, Pithomyces sp.

Pathogenicity: Commonly considered a contaminant, but very occasionally has been implicated as an etiologic agent in immunocompromised hosts. Causes facial eczema in sheep.
Rate of Growth: Rapid; mature within 5 days.
Colony Morphology: Surface is light to dark brownish black, cottony. Reverse is dark.
Microscopic Morphology: Septate hyphae, pale or light brown. Conidiophores are short, simple, and peg-like. Conidia are single, oval (10-20 X 20-30 um), yellow to dark brown, and usually rough, with transverse and longitudinal septations.

 

Medically Important Fungi 11: Stachybotrys chartarum (S. alternans, S. atra)

Medically Important FungiPathogenicity: Commonly considered a contaminant. It produces several mycotoxins that appear to have the ability to affect humans and animals after ingestion, inhalation, or percutaneous absorption. The fungus has been associated with pulmonary hemorrhage and hemosiderosis in infants. It has also been implicated in illnesses (with a variety of symptoms) in occupants (of all ages) of water-damaged homes and other buildings. Additional studies are needed to establish a firm causal relationship.
Rate of Growth: Moderately rapid; usually mature within 7 days but can be rather fastidious on routine laboratory media; prefers medium with high cellulose content.
Colony Morphology: Surface is at first white, becoming dark gray to black with age; powdery to cottony, spreading. Reverse is at first light and then dark.
Microscopic Morphology: Hyphae are septate and colorless to dark. Conidiophores are simple or branched, may become pigmented and rough with age, and bear clusters of 3-10 phialides. The phialides are colorless or pigmented, non-septate, and cylindrical, with a swollen upper portion. Conidia are dark, oval (average, 4.5 X 9 um), single celled, and smooth or rough walled and usually form in clusters on the phialides.

 

Medically Important Fungi 12: Stemphylium sp.

Pathogenicity: Commonly considered a contaminant.
Rate of Growth: Rapid; mature within 5 days.
Colony Morphology: Surface is brown to black, cottony. Reverse is black.
Microscopic Morphology: Septate hyphae, light to dark brown. Conidiophores are simple or occasionally branched, with a dark swollen terminus bearing individual conidia; the conidiophore develops a nodular or knobby appearance as it ages and produces more conidia. Conidia (12-20 X 15-30 um) are dark, smooth or rough, and round or oval and have transverse and longitudinal septations, often with marked constriction at the central septum.

 

Medically Important Fungi 13: Torula, Trichoderma spp.

Pathogenicity: Commonly considered a contaminant, but there have been occasional reports of infection in immunocompromised patients and several cases of peritonitis in patients undergoing peritoneal dialysis.
Rate of Growth: Rapid; mature within 5 days.
Colony Morphology: White fluff covers agar in a few days and then becomes more compact and woolly. Green patheces are eventually produced due to formation of conidia (initially at the center of the colony and spread to the margin). Reverse is colorless or light orangey tan to yellow.
Microscopic Morphology: Hyphae are septate. Conidiophores are short and often branched at wide angles; phialides are flask shaped and form at wide angles to the conidiophores. Conidia are round (3-4 um in diameter) or slightly oval (2-3 X 2.5-5.0 um), single celled, and clustered together at the end of each phialide. Clusters are easily disrupted unless microscopic preparations are handled with exceptional care.

 

Medically Important Fungi 14: Ulocladium sp.

Pathogenicity: Commonly considered a contaminant; very rarely involved in phaeohyphomycosis.
Rate of Growth: Rapid; mature within 5 days.
Colony Morphology: Surface is dark brown to black, cottony. Reverse is black.
Microscopic Morphology: Septate hyphae, light to dark brown. Conidiophores are simple or branched and bent at points of conidial production, giving a zigzag appearance. Conidia are brown to black, smooth or rough, and round to oval or slightly egg shaped (7-12 X 18-24 um), with transverse and longitudinal septations.

Unless otherwise stated, all information is from Medically Important Fungi: A Guide to Identification 4th ed. Davise H. Larone ASM Press

 

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