Introduction to Fungi, Third Edition

672 BASIDIOMYCETE YEASTS
Fig 24.9 Malassezia pachydermatis.
Freeze fracture image of a budding cell
showing the inner surface of the wall.
The spiral ridges are not seen in the collar
region. From David et al. (2003), by copyright
permission of Scripta Media,Brno.Original
image kindly provided by M. David, M.Gabriel
and M. Kopecka¤.
showing a predilection for dogs and cats where it
can cause skin infections (Guillot & Bond, 1999).
Most healthy humans carry Malassezia spp., but
the species composition and density of colonization
are altered in patients affected by dermatitis
(Sugita et al., 2001; Crespo Erchiga & Delgado
Florencio, 2002). The pattern of colonization is
dependent upon several factors, e.g. the degree of
sweating and the amount of lipid produced.
Malassezia spp. are often most abundant on
people in early adulthood because at that age
the lipid-producing sebaceous glands at the base
of the hair shaft are most active. A clear role
in pathogenesis has been demonstrated only
for M. globosa which causes a superficial mycosis
known as pityriasis versicolor in adults below
middle-age (Gupta et al., 2002). The disease is
more common in the tropics than in cooler
climates. Infected skin areas differ in pigmentation
from the normal skin. Infections are
associated mainly with the hyphal growth form
of M. globosa whereas commensal growth is yeastlike
(Crespo Erchiga & Delgado Florencio, 2002).
Occasionally, Malassezia spp. also cause contaminations
of catheters.
The association of Malassezia spp. with
dandruff has been suggested but is not yet
proven (Piérard-Franchimond et al., 2000). Evidence
in favour of the argument is that dandruff,
like pityriasis versicolor and other superficial
skin infections caused by Malassezia spp., disappears
upon treatment with shampoos or lotions
containing selenium sulphide or other topical
antifungal agents (Kwon-Chung & Bennett,
1992). Further, some Malassezia infections are
more common in immunocompromised patients
than in healthy subjects, and there is evidence
of both humoral and cell-mediated immune
responses against Malassezia in immunocompetent
humans (Ashbee & Evans, 2002).