Environmental Engineering Reference
In-Depth Information
for initial infestation and subsequent growth. Though water is required for
growth, fungal species have a broad tolerance range for its availability. The
term
) is used to describe the moisture content of substrates.
Water activity is the relative humidity of the substrate expressed as a decimal
fraction (e.g., 95% = 0.95 a
water activity
(a
w
). The range of water activities that individual
species grow under varies from approximately 0.55 to 1.0 a
w
. Species that
w
require high water activities (>0.95 a
) are described as
hydrophilic
; those that
w
can tolerate lower water activities are
.
Fungi grow over a range of temperature conditions (<40 to 140°F, <5 to
60°C). Mesophilic fungi, the largest group, have an optimum temperature
range of 68 to 86°F (20 to 30°C). Thermophilic fungi that are human patho-
gens grow well at temperatures of 95 to 104°F (35 to 40°C); true thermophilic
fungi grow at temperatures of 113 to 140°F (45 to 60°C), e.g., compost heaps.
Cryophilic fungi can grow at relatively low temperatures (<40°F, <5°C), e.g.,
on materials in cold storage and in regions having cold climates.
xerophilic
E.
Classification
Up until the last several decades, fungi were classified as members of the
Plant Kingdom because some contain cellulose in their cell walls. In today's
classification, the approximately 70,000 identified species are placed in either
the Kingdom Protista or Kingdom Fungi. The slime molds, which have
affinities to both fungi and animals (division Myxomycota), and the water
molds (division Oomycota), which produce motile spores, are classified in
the Kingdom Protista. The Kingdom Fungi includes five major fungal types
or divisions. These are the Chytridiomycota, Zygomycota, Ascomycota,
Basidiomycota, and Deuteromycetes. The Chytridiomycota are a primitive
group of fungi which do not produce airborne spores and thus pose little
risk of human exposure and health effects.
Fungal species in the Zygomycota produce thick-walled sexual spores
(zygospores) and asexual spores called sporangiospores. The mycelium is
characterized by the absence of cross walls (i.e., it is nonseptate). Zygomycota
species are saprobic and can grow on a variety of substrates, particularly in
areas with high relative humidity. Two major genera in this group (
Rhizopus
and
) are commonly isolated from indoor environment samples. In
culture, they rapidly overgrow colonies of other genera. Genera in this group
colonize soil and house dust.
Mucor
Rhizopus stolonifera
colonizes bread, and thus
is referred to as bread mold.
The Ascomycota represent a large and widely distributed group of
organisms characterized by the production of sexual spores in a sac-like
structure called an
. Members of this group are, for historical reasons,
called ascomycetes. They range from the single-celled yeasts to species that
produce mushroom-like fruiting bodies (morels). Many members of the
Ascomycota reproduce asexually by producing conidia (spores) in large
masses on specially differentiated hyphal structures. The asexual spores
produced by Ascomycota are common indoor air contaminants (e.g., those
ascus
