that can withstand the host’s cellular defenses is
a potential threat to health. But these infections
are normally restricted to immunocompromised or
immunosuppressed individuals, including patients
with advanced diabetes or cancers. A notable feature
of many of these infections is that they are geo-
graphically confined to specific regions – in other
words, they are endemic. For example, Coccidioides
immitis(a fungus known to have killed a laboratory
worker) is largely confined to the arid desert regions
of southwestern USA and northern Mexico.
4 A few fungi with melanized hyphal walls, such as
species of Phialophora, Cladosporium, and Sporothrix,
can invade traumatized tissues such as deep wounds
and cause tissue-degrading lesions. Although they can
be important causes of infection, especially among
agricultural workers in poorer countries, they are
essentially pathogens of wounds or traumatized
tissues. They will not be considered further in
this chapter. Similarly, some common members
of the Zygomycota cause traumatic infections of
people who suffer from diabetes and ketoacidosis, as
discussed in Chapter 2.
5 A specialized group of primitive fungus-like
organisms, now classified as different species of
Pneumocystis, are found in the lungs of humans and
a wide range of mammalian hosts. They seem to be
host-specific because DNA sequence comparisons of
Pneumocystis strains from different hosts show
wide genetic diversity, but close similarity between
strains isolated from any one type of host. The fun-
gus that infects humans, Pneumocystis jirovici(formerlyknown as P. carinii), seems to be particularly com-
mon in young children, because often more than 80%
of children show an antibody response to the pneu-
mocystis antigen in skin tests. But the fungus seems
to disappear as children age, and is found again in
AIDS patients, where it causes a virulent pneumonia
that can be transmitted from patient to patient by
airborne cells. Pneumonia caused by P. jiroviciin
HIV patients is often regarded as one of the first
“AIDS-defining” illnesses.Dermatophytic fungiThe dermatophytes are a clearly defined group of
about 40 species, traditionally assigned to three genera
of mitosporic fungi – Trichophyton, Microsporum, and
Epidermophyton, based on features of their conidial
stages (Figs 16.1, 16.2a). Some species of Trichophyton
(and similarly of Microsporum) have been found to
produce a sexual stage when strains of different mat-
ing types are paired in laboratory culture. The sexual
stage, named Arthroderma, is a cleistothecium contain-
ing asci and ascospores. However its role, if any, in
epidemiology is not known, and so most medical
mycologists use the familiar “asexual” names because
these are associated with specific diseases and geo-
graphical distributions.
The characteristic feature of dermatophytes is their
ability to grow in the dead, keratinized tissues of
the skin, nails, and hair (Figs 16.2b, 16.3), where their
metabolic products can induce an inflammatory324 CHAPTER 16
Fig. 16.1Spores of the three common genera of dermatophytic fungi. (a) Macroconidia (about 50μm long) and
microconidia (about 4μm) of Trichophytonspp. (b) Macroconidia of Epidermophytonspp, which do not produce micro-
conidia. (c) Spindle-shaped macroconidia, and microconidia of Microsporumspp.FB4eC16 04/20/2005 02:53PM Page 324