DESCRIPTIONS OF MEDICAL FUNGI

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Descriptions of Medical Fungi
Fusarium Link ex Fries
Most Fusarium species are soil fungi and have a worldwide distribution. Some are plant
pathogens, causing root and stem rot, vascular wilt or fruit rot. Several species have
emerged as important opportunistic pathogens in humans causing hyalohyphomycosis
(especially in burn victims and bone marrow transplant patients), mycotic keratitis and
onychomycosis (Guarro 2013). Other species cause storage rot and are important
mycotoxin producers.
Multi-locus sequence analysis of EF-1α, β-tubulin, calmodulin, and RPB2 have revealed
the presence of multiple cryptic species within each “morphospecies” of medically
important fusaria (Balajee et al. 2009). For instance, Fusarium solani represents a
complex (i.e. F. solani complex) of over 45 phylogenetically distinct species of which
at least 20 are associated with human infections. Similarly, members of the Fusarium
oxysporum complex are phylogenetically diverse, as are members of the Fusarium
incarnatum-equiseti complex and Fusarium chlamydosporum complex (Balajee et al.
2009, Tortorano et al. 2014, Salah et al. 2015).
Currently the genus Fusarium comprises at least 300 phylogenetically distinct
species, 20 species complexes and nine monotypic lineages (Balajee et al. 2009,
O’Donnell et al. 2015). Most of the identified opportunistic Fusarium pathogens
belong to the F. solani complex, F. oxysporum complex and F. fujikuroi complex. Less
frequently encountered are members of the F. incarnatum-equiseti, F. dimerum and F.
chlamydosporum complexes, or species such as F. sporotrichioides (O’Donnell et al.
2015, van Diepeningen et al. 2015).
Morphological Description: Colonies are usually fast growing, pale or brightcoloured
(depending on the species) with or without a cottony aerial mycelium. The
colour of the thallus varies from whitish to yellow, pink, red or purple shades. Species
of Fusarium typically produce both macro- and microconidia from slender phialides.
Macroconidia are hyaline, two to several-celled, fusiform to sickle-shaped, mostly with
an elongated apical cell and pedicellate basal cell. Microconidia are one or two-celled,
hyaline, smaller than macroconidia, pyriform, fusiform to ovoid, straight or curved.
Chlamydospores may be present or absent.
Identification of Fusarium species is often difficult due to the variability between isolates
(e.g. in shape and size of conidia and colony colour) and because not all features
required are always well developed (e.g. the absence of macroconidia in some isolates
after subculture). Note: Sporulation may need to be induced in some isolates and a
good slide culture is essential. The important characters used in the identification of
Fusarium species are as follows.
1. Colony growth diameters on potato dextrose agar and/or potato sucrose agar after
incubation in the dark for four days at 25 O C.
2. Culture pigmentation on potato dextrose agar and/or potato sucrose agar after
incubation for 10-14 days with daily exposure to light.
3. Microscopic morphology including shape of the macroconidia; presence or
absence of microconidia; shape and mode of formation of microconidia; nature
of the conidiogenous cell bearing microconidia; and presence or absence of
chlamydospores.