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In Search of Useful Fungi
Technician Eileen Bayer examines a newly discovered Penicillium species.
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A treasure trove of microbes known as the ARS Culture Collection has now
become richer, thanks to the teamwork of ARS scientists and others who found them by
relishing science and its problem-solving potential.
The recent additions, including many Penicillium species, came from
far and wide. And several came from Peoria, Illinoishome of the
collection located at the National Center for Agricultural Utilization
Research.
The upsurge in fungal accessions started with NCAUR microbiologist Donald T.
Wicklow's longstanding interest in the fungus Aspergillus flavus. A
journey of the mind then led him and his colleagues to discover antifungal
compounds in surprising places.
A. flavus is fungus non grata because it produces aflatoxin, a
hazardous substance that poses a health risk if it gets into food or livestock
feed. In corn, cotton, and peanut fields infested with A. flavus, the
fungus survives extensive droughts or freezing temperatures by forming a
hardened protective structure called a sclerotium. In the late 1980s, Wicklow
teamed up with University of Georgia plant pathologist David M. Wilson to
research the long-term survival of A. flavus in the soils of Illinois
and Georgia crop fields. Wilson is stationed at the Coastal Plains Research
Station in Dawson, Georgia. They buried, for later retrieval, tea strainers
containing soil with added laboratory-raised sclerotia.
"Though 3 years later many of the sclerotia still survived, we were
pleased to discover that these hardy life forms were under attack by numerous
mycoparasitic fungi, including Humicola fuscoatra," says Wicklow.
The researchers went on to identify antifungal chemicals that H.
fuscoatra used to kill A. flavus.
Cultures of mycoparasites isolated form wood-decay fungi and Aspergillus
sclerotia will be used in laboratory studies to identify antifungal
chemicals.
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Noting that long-lived fruiting bodies of some wood-decay fungi are
structurally similar to the sclerotia, the scientists came upon an idea.
Inasmuch as the coffee-ground-size black sclerotia were dispersed as widely as
just one per pound of soil, wouldn't it be easier to collect the long-lived
fungal bodies on decaying wood and attempt to recover parasitic fungi? Wicklow
shared the idea with Harry Thiers, a mushroom expert who had been his academic
advisor at San Francisco State College in the 1960s. Just retired, Thiers had
moved to Peoria, his wife's hometown. He became involved as an ARS scientific
volunteer, an endeavor that included collecting mushrooms.
Thiers camped in one state park after another across New Mexico, Colorado,
and several midwestern States. Within 2 summers, he had collected about 200
specimens of wood-decay fungi, including several from a woody ravine behind his
house.
Though not specimens that most collectors would seek, these old wood-decay
fungi held a rich array of Wicklow's fancies. He and undergraduate assistant
Eileen Bayer isolated about 600 different microscopic fungi from these
specimens.
The NCAUR team is working with natural product chemist James B. Gloer of the
University of Iowa at Iowa City, who has support for the project from the
National Science Foundation to discover novel chemicals that inhibit the growth
of A. flavus and Fusarium verticillioides (formerly known as
F. moniliforme).
These are two of the most important mycotoxin-producing fungi that infect
corn kernels. Thus far, the scientists have extracted, from the microscopic
wood-decay fungi, dozens of chemicals with novel structures that work against
the grain-infecting fungi.
With additional support from the Biotechnology Research and Development
Corporation in Peoria, Wicklow's team has produced thousands of extracts from
these 600 fungal cultures and others and has shared those extracts with BRDC
member companies. The companies have been screening them for production of
chemicals that might serve as agricultural pesticides, animal health products,
and antifungal antibiotics.
Brand-New Penicilliums!
While isolating and examining the fungi Thiers had collected, Wicklow found
that 91 strains fit into the genus Penicillium. He supplied the
isolates to NCAUR microbiologist Stephen W. Peterson for further examination.
Microbiologists Stephen Peterson (left) and Donald Wicklow examine the
persistent, blackened fruiting bodies of the wood-decay fungus Hypoxylon
found growing on a dead hardwood branch.
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Using an automated DNA sequencer, Peterson compared sequences of particular
genes in the isolates to those of analogous genes in archived
Penicillium cultures in the ARS Culture Collection.
"I was surprised to find that many of the isolates Thiers had obtained
didn't correspond to known species," Peterson says. "We then
determined additional gene sequences that helped us verify the unique positions
of these isolates in the phylogenetic tree of Penicillium species.
Phylogenetic trees are charted depictions of evolutionary relatedness."
"When we found so many Penicillium fungi taken from fruiting
bodies of wood-decaying fungi, we realized we had come upon an untapped
resource," says Wicklow. Until then, scientists had mainly focused on
Penicillium from moldy food, moldy livestock feed, and soil samples.
From the 91 strains taken from the unusual setting, Peterson found 39 species
that were new to science.
The researchers are developing detailed descriptions of these new fungi,
integrating physiological data and observations from scanning electron
microscopy and light microscopy with DNA sequence analysis.
By using modern microbiological tools, the NCAUR scientists, in one fell
swoop, ran into the largest discovery of new Penicillium species
described by any person or group since the genus was first described in H.F.
Link's dissertation in 1809. Peterson added the new species to the ARS Culture
Collection's 102 previously known Penicillium species, including those
that scientists at NCAUR used to help launch the antibiotics industry a half
century ago.
"The addition of so many new species will greatly add to our knowledge
of the genus from which penicillin was obtained," says Peterson.
"Integrating what we know into a new, comprehensive identification
system may help us to find more useful Penicillium isolates from
nature."By Ben
Hardin, Agricultural Research Service Information Staff.
This research is part of two ARS National Programs: Plant, Microbial,
and Insect Genetic Resources, Genomics, and Genetic Improvement (#301),
described on the World Wide Web at
http://www.nps.ars.usda.gov/programs/cppvs.htm,
and Food Safety (#108), described at
http://www.nps.ars.usda.gov/programs/appvs.htm.
Donald T. Wicklow and
Stephen W. Peterson are at the
USDA-ARS National Center for Agricultural
Utilization Research, 1815 N. University St., Peoria, IL 61604; phone (309)
681-6243 [Wicklow] and (309) 681-6384 [Peterson], fax (309) 681-6686.
"In Search of Useful Fungi " was published in the
November 1999 issue of
Agricultural Research magazine.
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