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Microorganisms

MICROORGANISMS

MICROORGANISMS. Microorganisms are organisms (forms of life) requiring magnification to see and resolve their structures. "Microorganism" is a general term that becomes more understandable if it is divided into its principal typesbacteria, yeasts, molds, protozoa, algae, and rickettsiapredominantly unicellular microbes. Viruses are also included, although they cannot live or reproduce on their own. They are particles, not cells; they consist of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), but not both. Viruses invade living cellsbacteria, algae, fungi, protozoa, plants, and animals (including humans)and use their hosts' metabolic and genetic machinery to produce thousands of new virus particles. Some viruses can transform normal cells to cancer cells. Rickettsias and chlamydiae are very small cells that can grow and multiply only inside other living cells. Although bacteria, actinomycetes, yeasts, and molds are cells that must be magnified in order to see them, when cultured on solid media that allow their growth and multiplication, they form visible colonies consisting of millions of cells.

Many people think of microorganisms mainly in terms of "germs" causing diseases, but some "germs" are beneficial to humans and the environment. Diseasecausing (pathogenic) microorganisms need to be controlled, and in many cases, beneficial microorganisms are also controlled in plant and food production.

For thousands of years, people had no concept or knowledge of organisms invisible to the naked eye. In fact, it is only within the last several hundred years that magnification systems (lenses, magnifiers, microscopes) were developed that enabled scientists to observe microorganisms. In 1673 Antoni van Leeuwenhoek, a linen merchant in Delft in the Netherlands, was the first to observe and study microorganisms, using single lenses that magnified objects fifty to three hundred times. The role played by microorganisms was not clarified until the 1830s, when Theodor Schwann in Germany demonstrated that yeasts were responsible for alcohol production in beer and wine fermentations.

In 1854, Louis Pasteur in France found that spoilage of wines was due to microorganisms (bacteria) that convert sugars to lactic acid, rather than the alcohol produced by yeasts. He developed the process of "pasteurization," in which the temperature of food materials is raised to about 140 to 158°F (60 to 70°C), thereby killing many spoilage organisms. Pasteur also discovered that certain bacteria are responsible for the souring of milk. Today, milk is generally pasteurized to reduce its content of microorganisms, to extend its keeping quality, and to protect against pathogenic microorganisms that may be present.

Pasteur also discovered that each type of fermentation, as defined by the end products, is caused by specific microorganisms and requires certain conditions of acidity or alkalinity. He discovered further that some microorganisms, the aerobes, require oxygen and others, the anaerobes, grow only in the absence of oxygen. The latter probably developed in the earliest days of the earth when there was no oxygen in the atmosphere.

Microorganisms are present in high populations in soil, and in varying numbers in the air we breathe, the water we drink, and the food we eat; they are on our skin and in our noses, throats, mouths, intestinal tracts, and other bodily cavities. They are everywhere in our environment.

Evolution of Microorganisms

Microorganisms came into being on earth over a period of about 1.2 to 1.5 billion years. Fossil microbes have been found in rocks 3.3 to 3.5 billion years old. Since then, microorganisms have had the principal task of recycling organic matter in the environment. As such they are absolutely essential to the health of the earth. Without them, the earth would be a gigantic, permanent waste dump.

Microorganisms are responsible for recycling the huge masses of organic matter synthesized by plants as life on earth evolved. Furthermore, microorganismsthe cyanobacteria or their DNA in the chloroplasts in plant cellswere the source of most of the free oxygen in the early atmosphere. They also oxidize ammonia (the universal end product of protein metabolism) to nitrate, which is the only nitrogen source used by plants and is therefore essential for production of our plant foods. Microorganisms also are responsible for cellulose hydrolysis in the rumens (first stomach compartments) of cattle, facilitating the production of animal protein for human consumption. And, in recent times, microorganisms have been the sources of antibiotics that have enabled the cure of numerous diseases.

Blue-green algae (cyanobacteria) are prokaryotes (that is, their cells have no distinct nucleus). They are very independent nutritionally since they can perform photosynthesis using chlorophyll a. Thus they can synthesize sugars for energy from carbon dioxide using the sun's radiation. They also release oxygen. They can respire aerobically and can fix nitrogen, generating amino acids and protein. They require only water, nitrogen gas, oxygen, carbon dioxide, some minerals, and sunlight. The evidence is that they were on earth 3.2 billion years ago. The cyanobacteria are among the earliest microorganisms and very important even today.

Green algae are eukaryotes (that is, their cells have a distinct nucleus). They evolved about one billion years ago. They contain chlorophylls a and b, which enable them to convert carbon dioxide, through sunlight radiation, to sugars, and to polymerize sugars to starches, hemicelluloses, and cellulosessome of our most important sources of food energy.

Green algae are still major sources of food in the oceans. Green algae were likely the life forms that evolved into plants, which first lived primarily in the oceans but moved to the land about 450 million years ago, about the same time as the amphibians and first land animals evolved. It is believed that the first mammals evolved about 150 million years later, along with insects and reptiles, which were dominant. Another 150 million years later, dinosaurs and the first birds evolved, along with the first flowering plants. During the entire period from 3.6 billion years ago, microorganisms were consuming and recycling the organic matter from themselves and other forms of life as they lived and died. For several billion years, bacteria, algae, and other microorganisms served as food for other microbes and for higher animals as they evolved. When plants evolved in the oceans and then subsequently moved to land, they became the major sources of food for other forms of life, including microorganisms, animals, and eventually humans.

Evolution of Plants: The Basis for Human Foods and Animal Feeds

For at least 400 million years before humans appeared on earth, plants were producing food consisting of leaves, stems, seeds, nuts, berries, fruits, tubers, etc., that made life possible for humans and animals when they evolved. Early plant evolution was essential not only for food but also for producing an oxygen environment necessary for animal and human survival. Plants introduced a very effective way of using the sun's radiation to transform carbon dioxide into food materials, such as sugars, starches, and cellulose, through the green pigment chlorophyll and the organelle that serves as the site for photosynthesis, the chloroplast.

Both plants and animals evolved in a microbial environment, where the microbes were ready and able to recycle organic matter. Plants and animals had to develop ways of resisting microbial invasion. Plants did this in part by developing a lignocellulosic body resistant to microbial breakdown. Humans also evolved in a sea of microorganisms and have a tough skin over their bodies resistant to microbial invasion. They had to develop internal immune systems against invasion by microorganisms. Human blood contains phagocytes similar to and probably derived from free-living amoebas, which search out and consume invading bacteria. Then as now, some microorganisms could invade the live animal or human, causing disease.

Microbes enter our bodies in the air we breathe into our noses and lungs, into our mouths and throats, stomachs, and intestinal tracts via the water and foods we swallow, through our eye sockets, through our skin via abrasions and punctures, and through our genitals and other mucous membranes. This intimate contact with microbes begins at birth and continues through life. Some microorganisms become regular inhabitants, parasites of our bodies; they become what can be described as our normal flora. Some microorganisms are virulent, invading our bodies and upsetting our metabolic activities and causing disease; these are the pathogenic microbes. Other microbes are normal microbial flora or pathogens on plants. Still other microbes are continuously invading plant food materials and recycling the organic matter. If this activity is controlled and stopped at the proper levels, these become our fermented foods, which include alcoholic foods and beverages; vinegars; lactic-acidfermented cabbage and other vegetables (that is, sauerkraut and pickles); lactic-acid-fermented milks and cheeses; sourdough breads; Indian idli (from rice); Ethiopian enjera (a bread made from teff, an indigenous cereal grass); textured-vegetable-protein meat-substitutes, such as Indonesian tempeh (from soybeans or, sometimes, peanuts) and ontjom (from peanuts or, sometimes, soy fiber); high-salt meat-flavored amino acid/peptide soy sauces and pastes; African alkaline-fermented foods such as dawadawa, soumbara, and iru (all from locust beans [Parkia biglobosa] or soybeans); Indian kenima, Japanese natto, and Thai thua-nao (all from soybeans); and leavened yeast breads.

Microorganisms Causing Food Poisoning

Three species of bacteria cause food poisoning via preformed toxin: Clostridium botulinum, Staphylococcus aureus, and Bacillus cereus.

Clostridium botulinum is a bacterium that grows in the absence of oxygen and produces one of the most toxic, deadly chemicals known to humans. It was first isolated from sausages, but later was responsible for death in persons consuming home-canned vegetables. The symptoms are flaccid paralysis eighteen to thirty-six hours after ingestion, with respiratory paralysis and death if untreated. There are antitoxins against botulinum toxin, if the type is identified and the antitoxin is injected in time. Botulinum toxin can be inactivated by heating the food to boiling for five minutes. Interestingly enough, botulinum toxin, in spite of its great toxicity is finding a use in eliminating lines and wrinkles from human skin by preventing activity of muscles directly involving those areas of the skin that have wrinkles or expressions. This is partially a response to the fact that very toxic substances in minute quantities can become stimulants.

A second serious type of food poisoning is caused by the ingestion of staphylococcal toxin produced by Staphylococcus aureus in foods such as cream puffs, mayonnaise, ice cream, or other nutritious foods that become infected with staphylococci, often carried in the nasal secretions of food handlers. Staphylococcal toxin causes a rather violent nausea and vomiting thirty minutes to six hours after consuming food contaminated with the toxin. Staphylococcus toxin is not inactivated by boiling. It generally is not fatal.

Bacillus cereus also produces a food-poisoning toxin. Steamed rice held overnight at room temperature has been a typical food causing Bacillus cereus poisoning. There are two toxins involvedone causing nausea and vomiting, the other causing diarrhea. The toxins are not inactivated by boiling.

Microorganisms Producing Food Poisoning by Toxins Formed in the Intestinal Tract

Clostridium perfringens, an anaerobic microorganism that can cause gangrene in wounds, can also cause food poisoning if it overgrows food materials, such as gravies and meats, which are then consumed. It produces its toxin in the intestinal tract of the consumer and causes diarrhea.

Vibrio cholerae is a major cause of cholera in man; it is spread via contaminated water and food. The symptoms are profuse diarrhea, which, if not treated to replace fluids in the body, will lead to death. Vibrio parahemolyticus, found in contaminated shellfish, also leads to profuse diarrhea and requires fluid replacement and antibiotics.

Shiga toxinproducing Escherichia coli (STEC), found in contaminated water and meats such as hamburger, is a serious food pathogen leading to hemorrhagic colitis (diarrhea with blood). Bovine products are a major source, but lettuce, alfalfa sprouts, and apple cider have also been implicated.

Enterotoxigenic E. coli (ETEC) is frequently found in developing countries in contaminated water and food and is associated with travelers' diarrhea (diarrhea without blood).

Food-Borne Bacteria Invading Intestinal Epithelial Cells

Common causes of food-borne illness are salmonella bacteria. Salmonella typhi and Salmonella paratyphi, gram negative bacilli that invade the intestinal epithelial cells, cause typhoid and paratyphoid fever, respectively. They are generally found in water or food contaminated with fecal material from carriers of salmonella. Other salmonellae are carried by infected poultry meats and eggs.

Campylobacter spp. are now recognized as one of the most common causes of food gastroenteritis. Main vehicles are raw meats (especially poultry), milk, and water. Fever (sometimes high), headache, and myalgia (muscle pain) precede nausea, vomiting, and diarrhea. Yersinia spp., carried chiefly in undercooked pork but sometimes also in milk, is another serious food-borne infection.

Listeria monocytogenes is the cause of a food-borne disease that is frightening because of its high mortality (fatality is over twenty percent). Among incriminated foods are milk, cheese, raw vegetables, and undercooked meat, including frankfurters.

Viral food-borne pathogens include hepatitis A, hepatitis E, rotavirus, and Norwalk virus. Although these viruses do not reproduce in food or water, they are spread by contaminated human carriers and food handlers through such media.

Fermented Foods

Seeds for plants germinate in the soil surrounded and covered with microorganisms. A pinch of dirt can contain a billion microorganisms of many types. The plants destined as foods for humans and animals grow in soil surrounded and covered with microorganisms ready to invade any organic matter and recycle it (essentially consume the organic matter and return it to compost utilizable by new seeds and plants). When the plant materialsseeds, nuts, leaves, tubers, stems, rootsare harvested, they are contaminated or infected with the types of microbes present in the soil; the microbes immediately start to grow on any susceptible organic matter that is available, as long as there is sufficient moisture to allow growth. Dry seeds and leaves are resistant to overgrowth by microorganisms, but as soon as they absorb enough moisture, they become susceptible to microbial growth. If the products of the microbial growth have desirable or attractive aromas and flavors and if they are nontoxic and do not cause disease when consumed, they can be described as "fermented foods" and can become an accepted food in the diet. If they have unpleasant aromas or bad flavors or if they cause food poisoning or death when consumed, they are considered to be spoiled and become garbage on their way to compost or soil. From the earliest times, our food supply has been strongly affected by fermentation.

Alcoholic beverages. The earliest sweet food on earth was likely honey, produced by honeybees and stored for their future use. Humans, in competition with animals such as bears, have always striven to collect honey for their own consumption. Honey is very resistant to spoilage in its concentrated form (about eighty percent sugars), but if it is collected and stored in a container and becomes diluted by rain water, yeasts present in the environment ferment the sugar in the honey to ethyl alcohol (ethanol). The products are called mead or honey wine, one of the earliest alcoholic beverages known to humans and still consumed today.

Similarly when humans started collecting sweet fruits and berries in containers, the juices as well as the fruits and berries themselves were quickly invaded by yeasts on the surfaces of the fruits that ferment the sugars to alcohol (actually a step in recycling), producing a primitive wine. For better or worse, humans have prized alcoholic beverages and they are still consumed in large quantities throughout the world except in those populations that avoid alcohol because of religious restrictions. In some religions, wines are a component of the religious services. Humans discovered ways of producing other alcoholic beverages. For example, early man probably discovered that chewed corn when mixed with water and stored in a container produces an alcoholic beverage. The process occurs because saliva contains an enzyme, diastase, that converts starch in the corn to sugars; then yeasts in the environment ferment the sugars to alcohol. The beverage thus produced is called chicha in the Andes region of South America. In ancient times, an emperor in that region could hold office only as long as he delivered sufficient chicha to the citizens to keep them happy. Even today, among families in the Andes region, husbands will get drunk one weekend and wives will get drunk the next, ensuring that at least one parent is sober and able to look after the children.

Juices from palm trees are collected by cutting the flowers and allowing the sap to flow through bamboo tubes into a container. As the juices flow through the tubes, they become infected with yeasts and other microorganisms. The sugars are fermented to alcohol and the product, palm wine, is produced in large quantities in the tropics. It is very rich in vitamins valuable to the consumer.

When cereal grains such as rice, barley, wheat, and corn are collected and soaked, or if they become wet from rain, they start to germinate, and starch in the seeds is changed to fermentable sugars that are fermented by yeasts in the environment, yielding an alcoholic beer. It has been suggested by anthropologists that this process was an early cause of fundamental social change. To ensure the continuity of supply of fermentable sugars, people settled in permanent locations. Agriculture, in turn, was a way of ensuring the regularity of production of fermentable cereal grains.

Alcoholic beverages are major fermented foods in the diet of humans. The yeast fermentation not only leads to a highly accepted beverage, it is a safe method of preserving fruit and berry juices until they can be consumed. The yeasts also enrich the beverages with B-vitamins.

As long as the wine or beer is kept anaerobic (air is excluded), it is preserved, but if there is access to air, there is a second fermentation by bacteria (Acetobacter) in the environment that transforms the alcohol to acetic acid (vinegar), which is even more preservative than ethyl alcohol. Many primitive wines and beers contain both alcohol and acetic acid. The vinegar fermentation is an ancient process that is still very important today. Vinegar is used to preserve cucumbers and other vegetables as pickles, which make an important contribution to the food supply of people around the world.

Milk products. As soon as humans started milking cows, they found that milk held a few hours at room temperature became sour. They did not know why, but it was, in fact, the streptococci and lactobacilli in the environment that produce lactic acid from lactose in the milk. This is the basis for yogurts, and the souring process as practiced in the early days also led to the development of cheeses.

The principal early milks were those from sheep or goats. Milk was often collected and stored in animal stomachs or hides, which allowed for the souring process to occur, the butter to be removed, and the milk curds to accumulate. The skin of a sheep or goat was carefully removed undamaged. The openings of the limbs and neck and the natural openings were tied. The hair was removed and the skin bag was used to collect the milk. During souring, the curds separate from the whey. The curds gradually lose moisture through the porous container, and further microbial activity and chemical changes lead to a primitive cheese. Today there are more than three hundred types of milk cheeses available. They have a wide range of flavors and textures and add variety and high-quality nutrition to the diets of consumers.

In addition to the bacterial cheeses, fungal cheeses involving growth of Penicillium roqueforti (Roquefort cheese and blue cheese) and Penicillium camemberti (Camembert cheese) on or in the cheese curd led to new flavors and textures for this class of fermented foods.

The Chinese developed a cheese from soybean milk, called sufu. Soybeans are soaked, ground with water, filtered to obtain the fluid milk, and heated to near boiling, and the curd is precipitated with calcium or magnesium salts. The filtered and pressed curd is then inoculated and becomes overgrown with Mucor spp. mold, after which it is aged in a salt and alcoholic brine. It is then ready for consumption.

Lactic-acid fermentations. An ancient food-fermentation technique is found in the South Pacific, where islanders centuries ago discovered that foods such as cassava, plantains, and bananas could be preserved for long times by piercing them and packing them in pits that were sealed against oxygen entry. Lactobacilli, leuconostocs, and streptococci ferment sugars in the stored food materials to lactic acid, acidifying them and preserving the food against spoilage as long as the pits remain sealed. Pits opened after one hundred years of storage have revealed edible productsthe result of bacterial fermentation.

In Ethiopia, pulp of the false banana, a starchy paste, is similarly stored in pits and undergoes lactic-acid fermentation, preserving the starch, which serves as a base for bread. Lactic-acid fermentations of cabbagefor example, sauerkraut and Korean kimchi (which is based upon Chinese cabbage, radishes, and red pepper)are important processes around the world. Sauerkraut and kimchi are particularly interesting applications of bacterial fermentation. The cabbage is shredded and two to three percent common salt is added. The salt extracts nutrients from the cabbage and a series of bacterial species (Leuconostoc, Lactobacillus, Pediococcus) overgrow the cabbage, producing lactic acid and carbon dioxide that preserve the cabbage; and as long as the product is kept anaerobic, it remains preserved.

Soybeans, with a content of about twenty percent fat and forty percent protein, are a very nutritious food source, first cultivated in Asia. They are harvested dry and have an excellent keeping quality. However, if they are moistened or soaked in water, they become susceptible to overgrowth by bacteria that first acidify them. Then they may be boiled, as in preparation for eating. After this, they become susceptible to overgrowth by molds. In Korea and northern China, the average temperature is cool, below 86°F (30°C), and the moistened soybeans become overgrown by Aspergillus oryzae, a mold that is present in the environment, particularly on the soybean straw. If such soybeans are stored under the roof, as is commonly practiced, the soybeans first become white from the mold mycelium (a mass of filamentous growth). Then they become green from the mold spores. During this time, the mold is producing many kinds of digestive enzymes. If such mold-covered soybeans are then mixed with water and salt to form a paste, it will be found that the paste has a meatlike flavor because of the amino acids and peptides released by the mold as it digests the soybean proteins. The end product of this process, called miso in Japan and chiang (soybean paste) in China, is used extensively as an ingredient for soup. If the mold-covered soybeans are placed in salt water, especially concentrated salt brine, it is found that the soybeans, which are initially bland in flavor, become meat-flavored, as in the miso process. The product, when filtered, is soy sauce. Today, soy sauces are used to season and marinate foods, not only in Asia but around the world.

Soybeans are also used in Southeast Asiain Indonesia, Malaysia, and Vietnam. However, the average temperature is generally higher, about 90 to 100°F (32 to 38°C). Aspergillus molds grow optimally at about 77 to 86°F (25 to 38°C), so they tend to invade the soybeans in North Asia. In Southeast Asia, other molds such as Rhizopus oryzae and Mucor spp. grow faster and better at the higher temperature. Thus the environment becomes infected with spores of these molds. When soybeans are soaked or moistened in Southeast Asia and are then cooked and cooled, they become overgrown with molds of the Rhizopus or Mucor types. If allowed to digest as a paste or in salt brine, they also can lead to a soy-sauce or miso flavor, but the Indonesians and Malaysians allow the mold-covered soybeans to become knitted into a cake that can be sliced and deep-fat fried or used in soups as a substitute for meat, which is generally in short supply in the diet. The product is called tempeh kedelee when made from soybeans. The Indonesians have developed other products using peanut and coconut press cakes (from the production of oil) as substrates. The pulverized, soaked press cakes are reformed into cakes and steamed. They then become overgrown with Rhizopus or Neurospora molds to produce foods called ontjom (peanut) and bongkrek (coconut) that like tempeh have a texture that allows them to be sliced and used as a substitute for meat in soups.

Fermented foods have been consumed by humans for centuries and are generally safe, but it should be cautioned that some molds produce toxic, even carcinogenic, products (for example, aflatoxins) and should not be consumed.

There are numerous other fermented foods that utilize edible microorganisms in their production and add variety and nutritive value to our diets.

The Role of Microorganisms in Soil

Plant life, our basic food supply, is dependent upon the trillions and trillions of microbes that exist in the soil, degrading organic matter, recycling nitrogen and carbon, and producing new soil in forms plants can use directly. Thus, good soil, far from being dead, should be described as "living soil," because of its content of living microorganisms. In fact, the rhizosphere, the area surrounding the roots of most plants, contains a wide variety of microorganisms that help the plant to absorb minerals and other plant nutrients. Some plants, such as legumes, have nodules on their roots that contain nitrogen-fixing bacteria, which take nitrogen from the air and produce nitrogen compounds the plants use in the synthesis of amino acids and protein; these are an important protein source in the human diet.

Microorganisms as Food

Blue-green algae of the genus Spirulina have been harvested from ponds and eaten for centuries by the ancient Aztecs in Mexico and Africans in the region of Lake Chad.

Mushrooms, the fruiting bodies of microorganisms that live on decaying lignocellulosic compounds in soil, are highly prized as food by nearly all human societies, as well as by many animals, including insects.

Fermentation plays several roles: (1) enrichment of the human diet through development of a wide diversity of flavors, aromas, and textures in food; (2) preservation of substantial amounts of food through lactic acid, alcoholic, acetic acid, and alkaline fermentations; (3) enrichment of food substrates biologically with protein, essential amino acids, essential fatty acids, and vitamins. Protein content is often increased, as for example in Malaysian tape ketan and tape ketella by utilization of the carbohydrates, lowering their percentage and raising the percentage of protein in the food. Protein quality is also increased by the synthesis of essential amino acids such as lysine, first limiting amino acid in rice. In the Malaysian tape fermentation the content of lysine is raised, improving its protein quality. In the Indian idli fementation, it has been reported that methionine, the first limiting amino acid in many legumes, is increased from 10.6 to 60 percent. Highly polished rice is deficient in thiamine (vitamin B1), and consumption can lead to beriberi, a disease characterized by muscular weakness. In the Malaysian tape fermentation, thiamine content is raised to that of the original unpolished rice. In the Indonesian tempeh fermentation the content of riboflavin doubles, niacin increases seven-fold, and vitamin B12, which generally absent in vegetarian foods, is synthesized. In the African kafir beer fermentation, riboflavin doubles and niacin/nicotinic acid concentration nearly doubles. Mexican pulque, the oldest alcoholic beverage on the American continent, contains thiamine, riboflavin, niacin, pantothenic acid, pyridoxine, and biotin that are of particular importance to the low income children of Mexico.

There is much hunger, starvation, and malnutrition in parts of the world today, and the world population is predicted to reach eight to twelve billion by the year 2050. As world population increases, the supply of meat and other animal products available per person is likely to decrease. A large, capable research institute in England has developed a process in which edible mold mycelium is grown and used to provide protein and texture for meat analogues (substitutes) for the human diet. Microbial protein can also be extracted from cells, and then concentrated, isolated, and spun or extruded to make meat substitutes.

Although this would appear to be very advanced technology, the Indonesians for centuries have overgrown soaked, partially cooked soybean cotyledons with the mold Rhizopus oligosporus (as mentioned above), which knits the soybean cotyledons into a firm cake that can be sliced and deep-fat fried or used in chunks as a substitute for meat in soups. The protein content rivals that of meat and the cost is very low, within the means of the average Indonesian. Also, the microorganisms involved enrich the food with vitamin B12, increase niacin by a factor of seven, and double the riboflavin content.

Among plants, the grasses are the most efficient fixers and utilizers of carbon dioxide, producing sugars, starches, and cellulose; they are also synthesizers of protein, using nitrogen from the soil. Grasses can double their cell-mass in two to three weeks. A 1000 kg harvest of grass can be repeated every two to three weeks. However, yeasts are much more efficient in this regard. A yeast (1000 kg) grown in tanks on limited land space can produce 168,000 kg of cells containing 84,000 kg of protein every two weeks.

Bacteria are even more efficient: whereas yeasts can double their cell mass in about two hours, some bacteria can double their cell mass in twenty minutes. Still, 1000 kg of yeast growing in a suitable fermentor can produce 1000 kg of new cells for harvesting every two hours, with a daily production of 12,000 kg of cells containing 50 percent or 6000 kg of protein. (Molds generally grow more slowly, doubling their cell mass in four to six hours.) Since the protein content of bacterial cells may reach 80 percent (compared with 40 to 45 percent in soybeans, for example), there is no method of producing protein that can compete with microbial cells. Except for algae, microbes require energy sources such as sugars, starches, cellulose, or hydrocarbonsall derived originally from the sun's radiation. But they can utilize energy sources that humans cannot digest, such as cellulose and lignocellulose found in straw. As described earlier, mushrooms are a good example of such microorganisms: they produce delicious, edible food directly from straw and sugarcane bagasse.

Only about twenty-five species of more than two thousand edible fungi are widely accepted as human food. The four most important mushrooms are the commonly cultivated white mushroom or button mushroom (Agaricus campestris), the black forest mushroom shiitake (Lentinus edodes), the straw mushroom (Volvariella volvacea), and the oyster mushroom (Pleurotus ostreatus).

Mushrooms can be grown on a wide variety of inexpensive, inedible substrates such as cereal straws, sugarcane bagasse, banana leaves, sawdust, cotton wastes, and animal manure. World production of straw is estimated to be about two billion tons. One kg of dry straw compost material can yield one kg of fresh mushrooms. Thus, straw, if all were used for production of mushrooms worldwide, could provide eight billion consumers with 250 grams of fresh mushrooms daily. Mushrooms are high in essential amino acids and nutritional value. They also appeal to almost all consumers for their flavors and flavor-enhancing capabilities. Mushrooms, a microbial product, are thus likely to play an important role in feeding the world in the future. And straw, after serving as a substrate for mushroom production, is nutritionally superior to raw straw for feeding cattle. The straw has been partially recycled and made more digestible in the process.

As world population rises in the twenty-first century, microbes may be used to a much greater extent to feed mankind, or at least feed animals that, in turn, will yield meat for the human diet. Humans, plants, and animals have been intimately involved with microorganisms ever since they evolved. While some of the microorganisms cause serious diseases, there are also many that provide foods and feeds and are beneficial to other life on earth.

See also Cheese ; Fungi ; Microbiology ; Packaging and Canning ; Pasteur, Louis ; Safety, Food ; Southeast Asia ; Soy .

BIBLIOGRAPHY

Jay, James M. Modern Food Microbiology. 6th ed. Westport, Conn.: AVI Publishing, 2000.

Kosikowski, Frank V. Cheese and Fermented Milk Foods. 2d ed. Brooktondale, N.Y.: F. V. Kosikowski, 1982.

Readers Digest Association, Ltd. The Last Two Million Years. London: Readers Digest, 1974.

Schopf, J. W., and B. M. Packer. "Early Archean (3.3 billion-to 3.5 billion-year-old) Microfossils from the Warrawoona Group, Australia." Science 237 (1987): 7073.

Singleton, Paul. Bacteria in Biology, Biotechnology, and Medicine. 5th ed. New York: Wiley, 1999.

Steinkraus, Keith H. "Bio-Enrichment: Production of Vitamins in Fermented Foods." In Microbiology of Fermented Foods, edited by B. J. B. Wood, pp. 603621. London: Blackie Academic and Professional, 1998.

Steinkraus, Keith H. "Classification of Fermented Foods: Worldwide Review of Household Fermentation Techniques." Food Control 8, no. 56 (1997): 311317.

Steinkraus, Keith H., ed. Handbook of Indigenous Fermented Foods. 2d ed. New York: M. Dekker, 1996.

Steinkraus, Keith H., ed. Industrialization of Indigenous Fermented Foods. New York: M. Dekker, 1989.

Steinkraus, Keith H. "Nutritional Significance of Fermented Foods." Food Research International 27 (1994): 259267.

Tortora, Gerard J., Berdell R. Funke, and Christine L. Case. Microbiology: An Introduction. 7th ed. San Francisco: Benjamin Cummings, 2001.

Toussaint-Samat, Marguellonne. Trans. Anthea Bell. History of Food. Cambridge, Mass.: Blackwell, 1993.

Wilson, Edward O., et al. Life on Earth. 2d ed. Sunderland, Mass.: Sinauer, 1978.

Keith H. Steinkraus

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microorganisms

microorganisms These are microscopic forms of life which are ubiquitous in the environment and on the human body. They were first detected in 1675 by a Dutch draper, Anthony van Leeuwenhoek, who noticed tiny ‘animalcules’ in droplets of rainwater under his microscope. He went on to discover that they were present in dental plaque, faeces, and many other substances.

Most microorganisms are harmless or even beneficial to man. Only a minority cause disease in healthy humans (although many more may do so in patients with damaged immune systems). Thus it may not be so surprising that it was another 112 years after van Leeuwenhoek's discovery before it was shown that these minute creatures were also under certain circumstances the agents of disease: this was first demonstrated by Agostino Bassi in 1835 for a bacterial infection of silkworms. The German Robert Koch was the first to prove that a bacterium could cause a human disease, namely anthrax, in 1876. Naturally this discovery aroused huge scientific and public interest, although of course there were those in both the lay and the scientific communities who were opposed to the new theory of infection. This is the subject of Ibsen's play An Enemy of the People, in which the town doctor learns that the presence of bacteria in the water supply might transmit diseases such as typhoid and cholera. He campaigns to have the water cleaned, but the whole idea of tiny invisible ‘animals’ is met with ridicule by the community and his career is ruined.

The study of microorganisms is known as microbiology and not surprisingly much of the study is directed at those organisms which do cause human disease. It is now realized that not only are microorganisms responsible for what are conventionally considered ‘infectious diseases’ but they may also contribute to such diverse illnesses as peptic ulcers, angina, and cervical cancer. There is no doubt that in the future microorganisms will be found to be involved in many more ‘non-infectious’ diseases. However, they are also essential to human life. Every square inch of our body surface is colonized by many thousands of organisms which help to protect the body from invasion by other potentially harmful organisms. If this normal ‘flora’ is damaged, for instance by a course of antibiotics, it leaves the way open for the harmful organisms to get a foothold and establish themselves instead. Microorganisms are also employed in a wide variety of home and industrial processes. For example, yeasts are essential for making bread rise and for the process of alcohol fermentation; genetically engineered bacteria are used to make insulin and in the production of genetically modified foods.

Microorganisms are classified into bacteria, viruses, fungi, and parasites. Prions, which are thought to be infective protein particles rather than live organisms, are included also in the field of microbiology. These groups are totally unrelated to one another, the only common factor being that they are all microscopic.

Bacteria

These are single-celled organisms, usually either rod-shaped or roughly spherical. They are classified according to their reaction to Gram's stain: those that go blue with this stain are termed Gram positive, those staining red are Gram negative. This reflects a fundamental difference in the structure of their cell walls. They are responsible for diseases ranging from typhoid, plague, cholera, meningococcal meningitis, tuberculosis, tetanus, gonorrhoea, and syphilis, to the more mundane urinary tract infections, boils, and acne. They are killed by antiseptics and by boiling, although they may produce toxins which are not destroyed. Many were originally sensitive to antibiotics such as penicillins, but overuse of these drugs has resulted in many multi-resistant bacteria. The most notorious is multi-drug resistant tuberculosis. In terms of numbers, however, the greatest problem in the UK has been methicillin resistant Staphylococcus aureus, or MRSA. Although often it merely colonizes the skin and causes no harm, it can cause a wide range of infections and is difficult to treat. Great attempts have been made to limit its spread in hospitals, for instance by placing patients carrying the organism in isolation rooms.

Viruses

These are even tinier than bacteria and cannot be seen through a light microscope: electron microscopy is required. Their name comes from the Latin for ‘toxin’. Their existence was first suspected when it was found that some infectious agents could pass through a fine filter, unlike bacteria, which were too large. They are very simple life forms, often consisting simply of a DNA or RNA core and a protein coat. They cannot reproduce except inside another living cell, which they must hijack in order to do so. They are susceptible to heat and to some antiseptics.

The most high-profile virus of recent years has been the human immunodeficiency virus — HIV — which causes AIDS. Other viral illnesses include influenza, the common cold, Lassa fever, and ebola. There have been few anti-viral drugs available, and the main line of defence has been vaccination, which has eradicated smallpox and may do the same for polio in the near future. Unfortunately, many viruses, including those causing HIV and the common cold, have a very high rate of genetic change so that they can evade the immune system, which makes the development of vaccines extremely difficult. Recently there have been great advances in the treatment of HIV with the development of ‘triple therapy’, or ‘highly active anti-retroviral therapy’ (HAART). This consists of the use of three or sometimes more drugs together to combat the infection, and has been highly successful in slowing the progression of the disease to full-blown AIDS. The use of the combination of drugs prevents the virus from becoming drug-resistant so rapidly.

Fungi

Like bacteria, microscopic fungi are everywhere. They may be yeasts or moulds. Yeasts have been used for centuries by peoples worldwide to ferment sugar to alcohol; the drug penicillin was found in a mould. They have a very resilient cell wall, which makes them difficult to treat. The commonest fungal infections are vaginal thrush, which often occurs after a course of antibiotics has destroyed the normal vaginal bacteria, and nail and skin infections such as ‘ringworm’.

In the UK more serious fungal infections are generally only found in seriously immune-deficient patients, such as leukaemia or AIDS patients. However, in other parts of the world there are fungi which can cause severe disease in healthy individuals.

Parasites

These are organisms which live in or on the body of another known as the ‘host’. The host may provide a source of nutrients or a safe haven in which to reproduce. They range in size from single cells, like the malaria parasite, to tapeworms which may reach thirty feet in length and hence are not microscopic at all! Parasites are mainly a problem of developing countries, although some, such as thread worm, are also common in the UK.

Prions

These are the simplest infectious particles known, now widely accepted as the cause of a group of degenerative diseases of the brain that includes Creutzfeld–Jacob disease (CJD) in humans, bovine spongiform encephalopathy (BSE) in cattle and scrapie in sheep. Since the BSE epidemic in Britain in the 1980s there has been concern that a new variant of CJD may be transmitted to humans who eat the meat (specifically nervous tissue) of infected cattle. Prions are thought to consist merely of a protein with no DNA or RNA so in this sense are not ‘live’. They are nonetheless infectious because when the abnormal prion protein comes into contact with a similar but normal protein it causes a change in the way the normal protein molecule is folded up. This turns the normal protein into another abnormal protein, which alters other nearby proteins in turn, and so on until disease is evident. No treatment is available for prion diseases; also prions can withstand heat, desiccaion and conventional antiseptics, making safe disposal of infected material very difficult.

In the 1960s and 70s the rapid expansion in the number of antibiotics being developed led man to believe that he had won the battle against infection with microorganisms, but it is now evident that this is far from being the case. The emergence of new viruses and prion diseases, together with ever more antibiotic-resistant organisms, makes the study of microorganisms as urgent as ever.

Angharad Puw Davies


See also epidemics; immunization; infectious diseases; prions.

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Microorganisms

Microorganisms

Microorganisms are minute organisms of microscopic dimensions, too small to be seen by the eye alone. To be viewed, microorganisms must be magnified by an optical or electron microscope . The most common types of microorganisms are viruses , bacteria , blue-green bacteria, some algae, some fungi , yeasts, and protozoans.

Viruses, bacteria, and blue-green bacteria are all prokaryotes, meaning that they do not have an organized cell nucleus separated from the protoplasm by a membrane-like envelope. Viruses are the simplest of the prokaryotic life forms. They are little more than simple genetic material, either DNA (deoxyribonucleic acid ) or RNA (ribonucleic acid ), plus associated proteins of the viral shell (called a capsid) that together comprise an infectious agent of cells. Viruses are not capable of independent reproduction. They reproduce by penetrating a host cell and diverting much of its metabolic and reproductive physiology to the reproduction of copies of the virus.

The largest kingdom of prokaryotes is the Monera. In this group, the genetic material is organized as a single strand of DNA, neither meiosis nor mitosis occurs, and reproduction is by asexual cellular division. Bacteria (a major division of the Monera) are characterized by rigid or semi-rigid cell walls, propagation by binary division of the cell, and a lack of mitosis. Blue-green bacteria or cyanobacteria (also in the Monera) use chlorophyll dispersed within the cytoplasm as the primary light-capturing pigment for their photosynthesis .

Many microorganisms are eukaryotic organisms, having their nuclear material organized within a nucleus bound by an envelope. Eukaryotes also have paired chromosomes of DNA, which can be seen microscopically during mitosis and meiosis. They also have a number of other discrete cellular organelles.

Protists are a major kingdom of eukaryotes that includes microscopic protozoans, some fungi, and some algae. Protists have flagellated spores, and mitochondria and plastids are often, but not always, present. Protozoans are single-celled microorganisms that reproduce by binary fission and are often motile, usually using cilia or flagellae for propulsion; some protozoans are colonial.

Fungi are heterotrophic organisms with chitinous cell walls, and they lack flagella. Some fungi are unicellular microorganisms, but others are larger and have thread-like hyphae that form a more complex mycelium , which take the form of mushrooms in the most highly developed species. Yeasts are a group of single-celled fungi that reproduce by budding or by cellular fission.

Algae are photosynthetic, non-vascular organisms, many of which are unicellular, or are found in colonies of several cells; these kinds of algae are microscopic.

In summary, microorganisms comprise a wide range of diverse but unrelated groups of tiny organisms, characterized only by their size. As a group, microorganisms are extremely important ecologically as primary producers, and as agents of decay of dead organisms and recycling of the nutrients contained in their biomass. Some species of microorganisms are also important as parasites and as other disease-causing agents in humans and other organisms.

See also Bacteria and bacterial infection; Genetic identification of microorganisms; Viruses and responses to viral infection; Microbial flora of the skin; Microbial genetics; Microbial symbiosis; Microbial taxonomy; Microscope and microscopy

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