Page 409 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART V Mycology
398
1
Molds grow as long filaments (hyphae) and form a mat
(mycelium). Some hyphae form transverse walls (septate
by fragmentation of the ends of hyphae and are the mode
hyphae), whereas others do not (nonseptate hyphae).
of transmission of Coccidioides immitis; (2) chlamydo-
spores, which are rounded, thick-walled, and quite resis-
Nonseptate hyphae are multinucleated (coenocytic). The
growth of hyphae occurs by extension of the tip of the
tant (the terminal chlamydospores of C. albicans aid in its
hypha, not by cell division all along the filament.
budding process by which yeasts reproduce asexually
Several medically important fungi are thermally dimor-
(some yeasts, e.g., C. albicans, can form multiple buds that
phic (i.e., they form different structures at different tem-
do not detach, thus producing sausagelike chains called
peratures). They exist as molds in the environment at identification); (3) blastospores, which are formed by the
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pseudohyphae, which can be used for identification); and
ambient temperature and as yeasts (or other structures) in
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(4) sporangiospores, which are formed within a sac (spo-
human tissues at body temperature.
Most fungi are obligate aerobes; some are facultative
rangium) on a stalk by molds such as Rhizopus and Mucor.
Although this book focuses on the fungi that are human
anaerobes; but none are obligate anaerobes. All fungi
require a preformed organic source of carbon—hence their
frequent association with decaying matter. The natural
the production of important foods (e.g., bread, cheese,
wine, and beer). Fungi are also responsible for the spoilage
habitat of most fungi is, therefore, the environment. An
important exception is Candida albicans, which is part of
of certain foods. Because molds can grow in a drier, more
acidic, and higher osmotic pressure environment than bac-
the normal human flora.
Some fungi reproduce sexually by mating and forming
grains, vegetables, and jams.
sexual spores (e.g., zygospores, ascospores, and basidio-
spores). Zygospores are single large spores with thick walls;
ascospores are formed in a sac called ascus; and basidio- teria, they tend to be involved in the spoilage of fruits,
PATHOGENESIS
spores are formed externally on the tip of a pedestal called
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a basidium. The classification of these fungi is based on
The response to infection with many fungi is the formation of
their sexual spores. Fungi that do not form sexual spores
granulomas. Granulomas are produced in the major systemic
are termed “imperfect” and are classified as fungi
imperfecti.
blastomycosis, as well as several others). The cell-mediated
Most fungi of medical interest propagate asexually by
immune response is involved in granuloma formation. Acute
forming conidia (asexual spores) from the sides or ends of
suppuration, characterized by the presence of neutrophils in the
specialized structures (Figure 47–1). The shape, color, and
exudate, also occurs in certain fungal diseases such as aspergil-
arrangement of conidia aid in the identification of fungi.
losis and sporotrichosis. Fungi do not have endotoxin in their
cell walls and do not produce bacterial-type exotoxins.
Activation of the cell-mediated immune system results
in a delayed hypersensitivity skin test response to certain
fungal antigens injected intradermally. A positive skin test
indicates exposure to the fungal antigen. It does not imply
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current infection, because the exposure may have occurred
in the past. A negative skin test makes the diagnosis
Because most people carry Candida as part of the normal
B
A
flora, skin testing with Candida antigens can be used to
determine whether cell-mediated immunity is normal.
The transmission and geographic locations of some
important fungi are described in Table 47–2.
Intact skin is an effective host defense against certain
fungi (e.g., Candida, dermatophytes), but if the skin is dam-
aged, organisms can become established. Fatty acids in the
skin inhibit dermatophyte growth, and hormone-associ-
ated skin changes at puberty limit ringworm of the scalp
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F
E
D
caused by Trichophyton. The normal flora of the skin and
FIGURE 47–1
mucous membranes suppress fungi. When the normal flora
Asexual spores. A: Blastoconidia and pseudohy-
is inhibited (e.g., by antibiotics), overgrowth of fungi such
phae (Candida). B: Chlamydospores (Candida). C: Arthrospores
(Coccidioides). D: Sporangia and sporangiospores (Mucor). E: Microco-
nidia (Aspergillus). F: Microconidia and macroconidia (Microsporum).
(Reproduced with permission from Conant NF et al. Manual of Clinical Mycology.
1
The term spores can be replaced with conidia (e.g., arthroconidia).
3rd ed. Saunders; 1971.)
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