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Introduction
Microorganisms have simultaneously served and assaulted man throughout
history. Man is totally dependent on some microbes for life processes, while
remaining subject to the destructive capacities of others in diseases not yet
conquered.
The study of microorganisms and microbial processes has provided a vari-
ety of benefits. For instance:
· World health has been improved through the discovery of the microbial
causes of most human, animal, and plant diseases, leading to the development
of vaccines, antibiotics, and chemical agents to combat many of these dis-
eases.
· Foods have been improved in quality and protected from spoilage to
enable wide distribution and storage against times of need.
· Sewage treatment methods have been developed to break the chain of
disease transfer through waterborne pathogens. Microorganisms also enhance
the water quality of rivers and lakes by degrading naturally occurring organic
matter.
· Farming practices have been improved through recognizing and capital-
izing on the role of soil microorganisms; microbes have been used to break
down nonedible crop residues for reuse by new crops. Nitrogen-fixing micro-
organisms have been used to inoculate legumes.
· Microbial fermentation processes have provided foods, beverages, medi-
cines, and chemicals for human use.
Microbes, as organized systems of enzymes, can often perform these func-
tions more efficiently than purely chemical processes, and current environ-
mental and economic constraints make the potential contribution of
microbes increasingly attractive.
From these examples it is clear that microbes can be marshaled to aid in
solving many important global problems including food shortages, resource
recovery and reuse, energy shortages, and pollution Microbiology is particu-
larly suited to make important contributions to human needs in developing
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2
MICROBIAL PROCESSES
countries, yet it has received comparatively little attention. The range of
possible applications covers uses by individuals and industries in rural settings,
villages, and cities.
This report covers examples of microbial processes that may be useful in
developing countries. Although many of these processes may not have a
direct and immediate use, their scope and diversity should serve to indicate
the strong potential for microbial applications.
Above all, the report highlights the pervasiveness and importance of mi-
crobes, along with the increasing need to train microbiologists and to support
their research and development activities. A group of well-trained micro-
biologists with adequate support can make valuable contributions to social
welfare.
Organisms Involved in Microbial Processes
The organisms responsible for the microbial processes discussed in this
report are an integral, all-pervasive part of the biological world. Although
they are rarely seen (the larger fungi, mushrooms, are perhaps the most
visible), it is estimated that microorganisms make up about one-quarter of the
biomass—the total weight of living organisms in the world—with animals
and plants accounting for the remainder. Microorganisms occur everywhere,
and extraordinary aseptic measures are required to exclude them from
places where their presence would be harmful, such as the operating room
of a hospital or a food-processing plant. Even then, these measures are not
always successful.
The bulk of microorganisms reside in the soil, where they are responsible
for the predominant biological activity. Others are located in the upper layers
of the oceans and in fresh and brackish waters, as well as on the surfaces
above ground, in the air, and of course inside larger organisms, both plant and
.
ammo .
A number of microorganisms are harmful, or pathogenic, to humans and
animals. Although the terms microbe or germ initially were used to describe
any minute microorganism, they tend to be used especially to connote harm-
ful organisms. Yet most microorganisms are either hatless or essential for
the maintenance of the biological cycles on which all life depends.
Microorganisms comprise the following classes of organisms:
· Bacteria
· Fungi (yeasts and molds)
· Algae
· Protozoa
Viruses.
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INTRODUCI ION
3
Their classification, characteristics, and harmful and beneficial effects are
shown in Table 1.
Physicochemical Factors Affecting Microbial Growth
A number of physical factors affect the growth or retardation of micro-
organisms, including temperature, osmotic pressure, acidity or alkalinity, the
presence of oxygen or lint, and the degree of agitation. Although no species
of microbes can survive over the complete range of conditions found in
nature, there are varieties that thrive in hot springs, polar wastes, acidic bogs,
and highly saline waters like the Dead Sea.
Temperature
Most microorganisms grow within a temperature range of 30°C. Individual
species have well-defined upper and lower temperature limits and optimum
temperatures for growth.
Microorganisms are usually divided into three groups with respect to their
most favored temperature range. Psychrophiles grow best between about
0°C and 30°C. These organisms occur in cold areas and are frequently associ-
ated with refrigerated food spoilage. Mesophiles grow best between about
20°C and 50°C. Most disease-causing bacteria are in this group. Thermophiles
grow best from 40°C to 70°C. This division into three groups is convenient
but somewhat arbitrary, since the dividing lines are not sharp. Further, not
every organism can grow over the entire range indicated for its group.
Acidity and Alkalinity
Taken as a whole, microorganism species can tolerate a wide range of
acidity and alkalinity. Some thrive under highly acidic conditions (pH 1-3)
and others in alkaline environments (pH9-10~. However, most microorgan-
isms grow best at neutral pH (pH 7~.
Oxygen
Microorganisms can be divided into three major groups with respect to
their oxygen requirements. Obligate aerobes have a requirement for oxygen
and grow best when oxygen is continuously available. Obligate anaerobes
grow in the absence of free oxygen. The requirement for oxygen reflects the
metabolic pathways the organisms use to obtain energy. Aerobes break down
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OCR for page 5
INTRODUCTION
as
their nutrients by a sequence of enzyme reactions that require oxygen. An-
aerobes utilize a pathway for metabolism that does not require free oxygen,
and in fact they may be inhibited by it.
The third group of organisms are the facultative anaerobes. These can use
either metabolic pathway, depending on the presence or absence of oxygen.
Osmotic Pressure
The osmotic pressure across a cell wall depends on the relative concentra-
tion of dissolved substances within the cell and outside it. For example, most
bacteria can grow over a broad range of salinity because their cells are capable
of maintaining a relatively constant internal salt concentration. But if salt
concentrations outside the cell become too high, water is lost from the cell
and growth is inhibited. This is the basis for food preservation by salt. Sugars
and other substances also influence osmotic relationships between cells
and their environment.
/
Nutritional Requ irements for Microbial Growth
All microorganisms require water to grow and water can be considered the
single most important component in their growth.
Microorganisms can be divided into two groups based on the source of
carbon they convert into their cell components. Heterotrophic organisms
utilize organic compounds as a source of carbon for both synthesis and en-
ergy. Autotrophic organisms utilize carbon dioxide as their major source of
carbon for synthesis and obtain energy either from the sun (through photo-
synthesis) or by metabolizing inorganic compounds. The inorganic com-
pounds that can be used by various autotrophic organisms include ammonia,
hydrogen, reduced iron, manganese and other minerals, and hydrogen sulfide.
Heterotrophs can utilize a wide variety of organic materials as sources of
carbon. In fact, there are probably no biologically generated materials in the
environment that cannot be degraded by some species of microorganism.
In addition to carbon, all organisms require sources of the other elements
found in cell components. These include nitrogen, sulfur, phosphorus, and
potassium. Both heterotrophs and autotrophs require certain inorganic salts
for optimum growth and reproduction.
Most microorganisms cannot utilize (fix) atmospheric nitrogen and require
nitrogen in the form of an ammonium or nitrate salt or in an organic form.
Sulfur is usually obtained from sulfate salt and phosphorus from salt of
phosphoric acid.
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6
Raw Materials for Microbial Processes
MICROBIAL PROCESSES
A variety of materials have been used in microbial processes in industrial-
ized countries. For less-developed nations, however, it is not necessary to
restrict usage to these substances; indigenous raw materials, for example
agricultural residues, may be much more appropriate.
Food and Animal Feed
Microorganisms have long been used to produce certain foods, beverages,
condiments, and animal feeds. Recently, several new commercial microbial
processes have been developed. These include the production of single-cell
protein to supplement animal feeds; mushrooms for human food from agri-
cultural wastes; microbial rennet for cheese making; enzymes such as glucose
isomerase; meat-like flavorings using the Chinese soy sauce and Japanese miso
processes; xanthan and amino-, hydroxy-, and keto-acids, and vitamins,
among other products.
There are many potential ways for utilizing microorganisms in food pro-
duction, from the household and village level to full-scale commercial opera-
tions. The need continues for better food preservation and methods to reduce
postharvest food spoilage.
Soil Microbes in Plant Health and Nutrition
The region where the roots of plants make contact with the soil is called
the rhizosphere. This is a complex biological area in which the microbial
population is considerably hiker and its activity greater than in root-free soil.
Growth of microorganisms in the rhizosphere is undoubtedly enhanced by
nutritional substances released from the roots, and growth of plants is in-
fluenced by microbial metabolic products released into the soil.
Of great significance are certain fungi that infect roots and form mycor-
rhizae. These fungi can absorb and translocate phosphate and other essential
nutrients and make them available to plants. With a greater need for food for
an ever-growing population, increased attention should be given to the effects
of the rhizosphere on plant nutrition.
Nitrogen Fixation
As demands for fertilizer increase, and as the energy crisis becomes more
acute, greater attention must be given to microbial fixation of atmospheric
nitrogen. The emphasis should be on applying known technology, of which
legume inoculation to increase crop production is a good example. Basic
OCR for page 7
INTRODUCTION
7
research on culture and ecology of both symbiotic and nonsymbiotic nitro-
gen-f~xing microorganisms could lead to an increase in the world's supply of
edible protein. This would be of even greater significance if microorganisms
that fix nitrogen, or their nitrogen-f~xing genes, could be transferred to micro-
organisms that can be established in nonleguminous crops, such as rice and
other cereals, so they could utilize nitrogen from the air. Cultivation of
free-living nitrogen-fixing blue-green algae that grow in nitrogen-deficient sub-
strates is another goal. The potential for development in these areas is great.
Microbial Insect Control Agents
In the search for safe, alternative methods of controlling insect pests, the
use of microorganisms that cause disease in insects offers distinct possibilities.
Insects, like humans, animals, and plants, are susceptible to microbial dis-
eases. Microbes that produce diseases in insects are termed entomopathogens.
In many cases they can significantly reduce natural populations of insects.
Safety, specificity, effectiveness, and cost are the decisive considerations in
the development of any insecticide. A number of entomopathogens fulfill
these criteria and are therefore potentially useful bioinsecticides. Some are
already being produced commercially, and more are in development.
Fuel and Energy
Most nations today are facing shortages of fuel and energy. Yet if develop-
ment is to proceed, increasing amounts of energy will be required. To meet
these growing requirements, attention must be directed to the development
of unconventional and renewable energy systems.
Microbial processes already help provide energy. In the countries of South
and Southeast Asia and in the People's Republic of China, for example, many
small farms and villages are using methane generators that utilize fermented
animal manure, human wastes, and other waste substances to produce "bio-
gas" for household cooking, lifting, and power. In some countries alcohol
produced by microbial fermentation is added to petroleum products to
supplement scarce fuel supplies. These processes that depend upon the solar-
produced biomass may hold unique promise for supplying some of the energy
requirements of less-developed nations. The microbiological conversion of
plant matter into fuel circumvents the millions of years required for plant
material to become fossil fuel through natural processes.
Waste Treatment and Utilization
A number of water and wastewater purification processes utilize microbes.
Many opportunities exist for waste utilization and recycling, including refeed-
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8
MICROBIAL PROCESSES
ing of animal wastes; algal farming for fish culture and as a source of animal
feed and fermentable substrates; and the upgrading of cellulose wastes by
protein enrichment for use as fodder.
Cellulose Conversion
Cellulose, a renewable resource from agricultural and forestry products, is
a major component of many solid wastes and residues. Usually, cellulose is
bound to lignin. The lignocellulose complex is a substrate that must be chem-
ically degraded before the cellulose can be used in some commercial
processes.
Cellulose can be degraded by chemical or enzymatic hydrolysis to soluble
sugars. These sugars can then be used by microbes to form ethanol, butanol,
acetone, single-cell protein, methane, or other products of fermentation. In
some cases, cellulose can be converted directly into these products by fer-
mentation. The technology for refined cellulose degradation is readily avail-
able for recycling paper, cardboard, etc.
Biomass agriculture and forestry may hold great economic potential for
certain less-developed countries, particularly in tropical and subtropical re-
gions.
Antibiotics and Vaccines
Although approximately 4,000 antibiotics are known, most have no prac-
tical value because of their toxicity to human beings, lack of efficacy, or him
production cost. There are only about 50 widely used antibiotics. Extensive
use of antibiotics in medicine began in 1945 with penicillin. Currently, anti-
biotics are widely used in human and veterinary medicine, and to a lesser
extent in agriculture, where they are used to increase the weight of livestock
and poultry, to control plant diseases, and as insecticides. New antibiotics are
being sought and old ones are being modified to improve their properties.
Killed, attenuated, or living microorganisms, or their products, have been
used for many years to produce immunity against certain human diseases
such as smallpox, cholera, yellow fever, tetanus, and diphtheria. Additional
research is needed to improve these vaccines and to produce new ones. Spe-
cial emphasis should be placed on effective programs and delivery systems for
. ~ . .
existing vaccines.
Pure Cultures for Microbial Processes
Microorganisms are an extremely important natural resource. Because of
the present and potential usefulness of beneficial microorganisms, it is essen-
OCR for page 9
INTRODUCTION
9
tial that their germ plasm be preserved, just as plant germ plasm is preserved
in seed banks and endangered animal life is protected in various ways.
Several outstanding culture collections of microorganisms exist today, and
they are essential to research and teaching in microbiology as well as com-
mercial microbial production.
Representative terms from entire chapter:
osmotic pressure
