common in self-heating composts (Chapter 11) and can
also be isolated from materials such as birds’ nests and
sun-heated tropical soils. The fungus with the highest
recorded growth temperature (62°C) is Thermomyces
lanuginosus(Fig. 8.3), which is very common in com-
posts. However, far fewer than 100 thermophilic fungi
have been described – an astonishingly low number,
given the large number of habitats from which they
can be isolated across the globe. This raises the ques-
tion of whether different sampling approaches than
those used to date might reveal new thermophilic
species of biotechnological significance.
Aspergillus fumigatusis an extremely common fungus,
found in a wide range of environments, but it is
regarded as thermotolerantrather than thermophilic,
because it can grow at temperatures as low as 12°C
and its temperature optimum is less than 40°C. This
fungus is remarkable for its very wide temperature
range, 12–52°C, and its equally wide range of habitats.
It grows commonly in composts, on mouldy grain, and
on other decaying organic matter. It can also grow on
the hydrocarbons in aviation kerosene. In addition to
this, it is an opportunistic invader of the respiratory
tract of birds and humans, where it can form per-
sistent colonies termed aspergillomasin the lungs
(Fig. 8.4). In recent years A. fumigatushas become a
significant problem in surgical wards, especially in
transplant units because it can colonize wounds and
grow within the tissues of transplant patients. It is, how-
ever, an essentially saprotrophic species. We deal with
the medical significance of this important fungus in
Chapter 16, because it is unique in the sense that
it can readily colonize humans as an opportunistic
invader but it seems to lack any specific pathogenicity
factors.
Most fungi are mesophilic, commonly growing
within the range 10 – 40°C, though with different toler-
ances within this range. For routine purposes these fungi
can usually be grown at room temperature (22–25°C).
Two important examples shown in Fig. 8.2 are
Aspergillus flavus, which produces the potent aflatoxins
in stored grain products, such as peanuts and cotton-
seed meal (Chapter 7), and Penicillium chrysogenum,
used for the commercial production of penicillins
(Chapter 7).
Psychrophilicfungi are defined as having optimum
growth at no more than 16°C and maximum growth
of about 20°C. In many cases they would be expected
to grow down to 4°C or lower, whilst psychrotrophic
fungi would be those that can grow at low temperat-
ures but also above 20°C. There are many environments
that could suit these organisms, including the polar
and alpine regions, and the great oceans which have
a stable temperature of about 5°C. Several yeasts have
been found in polar and subpolar regions, including
a basidiomycetous yeast isolated from clothing of theENVIRONMENTAL CONDITIONS 143
grown in sterilized soiland inoculated with the take-
all fungus (Gaeumannomyces graminis), an aggressive
root pathogen (see Fig. 9.11). The fungus caused
progressively more disease as the soil temperature
was raised from 13 to 23 or 27°C (its optimum for
growth in pure culture). But in natural, unsterile soil
the amount of disease declined as the temperature
was raised above 18°. The main reason is that
higher temperatures favor other microorganisms
more than they favor the take-all fungus. Several
antibiotic-producing fluorescent Pseudomonasspp.
antagonize the take-all fungus, and they provide the
basis for current attempts to develop an effective
biological control strategy against the take-all disease
(Chapter 12).All these points caution against a simple approach to
interpreting the effects of individual environmental
factors on fungal activities.
Temperature and fungal growth
Microorganisms are often grouped into four broad
categories in terms of their temperature ranges for
growth: psychrophiles (cold-loving), mesophiles
(which grow at moderate temperatures), thermophiles
(heat-loving), and hyperthermophiles. A possible
fifth category consists of psychrotolerantorganisms,
which can grow at low temperatures (at or below 5°C)
but prefer more moderate temperatures.
However, these categories mean different things
when applied to different types of microorganism. For
example, most fungi are mesophiles and relatively few
can grow at or above 37°C (human body temperature)
or even above 30°C, whereas many bacteria can grow
at this temperature. The upper limit for growth of any
fungus (or any eukaryote) is about 62°C. By contrast,
some bacteria thrive at 70 – 80°C, and some archaea can
grow at over 100°C, the current record being for the
archaeon Pyrolobus fumarii, which is found around
natural thermal vents and has a temperature optimum
of 106°C and a maximumof 113°C in culture.
The temperature ranges for growth of some repre-
sentative fungi are shown in Fig. 8.2. By convention,
thermophilic fungiare defined as having a minimum
growth temperature of 20°C or above, a maximum
growth temperature of 50°C or above, and an optimum
in the range of about 40 –50°C. Cooney & Emerson, in
1964, described all the thermophilic fungi that were
known at that time. Haheshwari et al.(2000) provide
updated information on the taxonomy and physio-
logical features of the thermophilic fungi. The tem-
perature ranges of three of these species – Thermomyces
lanuginosus, Chaetomium thermophile, andThermomucor
pusillus. – are shown in Fig. 8.2. All three are very