Pathogenic fungi can
exist as yeasts or as hyphae. A mass of hyphae is called mycelia.
Yeasts are
unicellular organisms and mycelia are multicellular filamentous structures,
constituted by
tubular cells with cell wall. The yeasts reproduce by budding. The mycelia
forms branch and the
pattern of branching is an aid to the morphological identification. If
the mycelia do not
have SEPTA, they are called coenocytic (nonseptate). The terms
“hypha” and “mycelium”
are frequently used interchangeably. Some fungi occur in both
the yeast and
mycelial forms. These are called dimorphic fungi.
Dimorphic Fungi
The dimorphic fungi
have two forms
1. YEAST- (Parasitic or
Pathogenic form). This sis the form usually seen in tissue,
in exudates, or
cultured in an incubator at 370C .
2. MYCELIUM-
(Saprophytic form). The form observed in nature or when cultured
at 260C.
Conversion to the yeast form appears to be essential for pathogenicity. In
the dimorphic fungi,
fungi are identified by several morphological or biochemical
characteristics,
including the appearance of their fruiting bodies. The asexual
spores may be large
(Macroconidia, Chlamydospores), or small (microconidia,
blastospores,
arthroconidia).
There are four types
of mycotic diseases:
1. Hypersensitivity-
an allergic reaction to molds and spores
2. Mycotoxicoses-
poisoning of man and animals by feeds and food products
contaminated by fungi
which produce toxin from the grain substrate.
3. Mycetismus- the
ingestion of toxin (mushroom poisoning)
4. Infection
We shall be concerned
mainly with the last type; pathogenic fungi that cause infections.
Most common
pathogenic fungi do not produce toxins but they do show physiologic
modifications during
a parasitic infection ( e.g. increased metabolic rate, modified
metabolic pathways
and modified cell wall structure). The mechanisms that cause these
modifications as well
as their significance as a pathogenic mechanism are just being
described. Most
pathogenic fungi are also thermo tolerant, and can resist the effects of
the active oxygen
radicals released during the respiratory burst of phagocytes. Thus,
fungi are able to
withstand many host defenses. Fungi are ubiquitous in nature and most
people are exposed to
them. The establishment of mycotic infection usually depends on
the size of the
inoculums and on the resistance of the host. The severity of the infection
seems to depend mostly
on the immunologic status of the host. Thus, the demonstration
of fungi, for
example, in blood drawn from an intravenous catheter can correspond to
colonization of the
catheter, to transient fungemia (i.e. dissemination of fungi through
the blood stream), or
to a true infection. The physician must decide which is the clinical
status of the patient
based on clinical parameters, general status of the patient, laboratory
results, etc. the
decision is not trivial, since treatment of systemic fungal infections
requires the
aggressive use of drugs with considerable toxicity. Most mycotic agents are
soil saprophytes and
mycotic diseases are generally not communicable from person-to24
person (occasional
exceptions: candida and some dematophytes). Outbreaks of disease
may occur, but these
are due to a common environmental exposure, not
communicability. Most
of the fungi which cause systemic infections have a peculiar
characteristic
ecologic niche in nature. This habitat is specific for several fungi which
will be discussed
later. In this environment, the normally saprophytic organism
proliferate and
develop. This habitat is also the source of fungal elements and/or spores,
where man and
animals, incidental hosts, are exposed to the infectious particles. It is
important to be aware
of these associations to diagnose mycotic diseases. The physicians
must be able to
elicit a complete history from the patient including occupation, vocation
and travel history.
This information is frequently required to raise, or confirm, your
differential
diagnosis. The incidence of mycotic infections is currently increasing (in
man) dramatically,
due to an immunosuppressive therapy, and the use of more invasive
diagnostic and
surgical procedures (prosthetic implants). Fungal diseases are noncontagious
and non-reportable
diseases in the national public health statistics.
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