Click for next page ( 2


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 1
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

OCR for page 1
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 biomassthe total weight of living organisms in the worldwith 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.

OCR for page 1
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 30C. 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 0C and 30C. These organisms occur in cold areas and are frequently associ- ated with refrigerated food spoilage. Mesophiles grow best between about 20C and 50C. Most disease-causing bacteria are in this group. Thermophiles grow best from 40C to 70C. 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

OCR for page 1
a: In In ~9 o Id so: ct In ~9 o Id In ~ .O ~ In . - .~ at ct ct ct en v v In ~ - ~: ct oo If .~ .= be EN O ~ . ~ A ~ ~ 5= -, it, 3~, ~ . Or ~ c~3 o ~ ~ ~= = ~ ~ =0 0 ~ c O ~ E s: ~ ,~e ~ ~ ~ ~ - 3 ~ 3 s ~ . . ~ 1 ~ , 2- ~ - == ~ , . ~ ~ 3 ~ , . ~ ~ cam 0 c-, ~ v: 0 ~ ~ , ~ .= ct c' 0 ~ ~ ~ .c) .. c ~ ~ 0 of ~ O c ~ 4 'A 3 ~ :> o Ct

OCR for page 1
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.

OCR for page 1
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 1
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-

OCR for page 1
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 1
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.