Micro-organisms Micro-organisms (or microbes) are literally microscopic organisms, which can only be seen properly with the aid of a microscope. These include bacteria, microscopic fungi (moulds) and protoctists. Although viruses, which are even smaller than bacteria, are generally considered to be non- living entities, they might also be included here as they are important disease-causing agents. Micro-organisms are the most numerous organisms in any ecosystem. There are about 159,000 known species, although this is thought to be less than 5% of the total in existence. There is vast genetic diversity among micro-organisms, which is not surprising as they began evolving over a billion years before land plants. This, coupled with their small size and reproduction, helps explain why micro-organisms, particularly bacteria, are the most widely distributed forms of life on the planet. While many are cosmopolitan species, others exist in habitats totally inhospitable to larger organisms. There are species of bacteria able to grow in hot springs up to 90° C, others live below freezing point in Antarctica, in soda lakes, anaerobic situations, and sites with high concentrations of metals, sulphur and other normally toxic compounds.
and people Micro-organisms are of immense importance to the environment, to human health and to our economy. Some have profound beneficial effects without which we could not exist. Others are seriously harmful, and our battle to overcome their effects tests our understanding and ingenuity to the limit. However, certain micro-organisms can be beneficial or harmful depending on what we want from them: saprophytic decomposers play an important role in breaking down dead organic matter in ecosystems, but these same micro- organisms can be responsible for food spoilage (rotting, going bad, going off) and subsequent illness.
micro-organisms Disease and decay are not inherent properties of organic objects, nor do they result from physical damage or being eaten by insects, it is micro- organisms that bring about these changes. We are surrounded by bacteria, viruses, protoctists and fungi. Many cause disease in farm animals and commercial crops, many others are capable of invading our bodies and causing human disease.
Examples of familiar human diseases include:
Bacteria: salmonella, tetanus, typhoid, cholera, gangrene, bacterial dysentery, diphtheria, tuberculosis, bubonic plague, meningococcal meningitis, pneumococcal pneumonia
Viruses: rabies, influenza (flu), measles, mumps, polio, rubella (german measles), chicken pox, colds, warts, cold sores
Protoctists: malaria, amoebic dysentery
Fungi: athlete's foot, ringworm
These disease-causing organisms are called pathogens and we often refer to them in everyday, non-scientific terms as ‘germs’ or bugs. Each disease has a specific pathogen, i.e. different diseases are caused by different kinds of germ. If the disease organism can be transmitted from one person to another it is said to be infectious. Non-infectious diseases, such as allergies, cancer, vitamin deficiency, and mental illness may develop when the body is not functioning properly.
Common infectious diseases can be spread (or be caught) by consuming food or water containing pathogens or their toxic products (e.g. salmonella, typhoid, cholera ); by ‘droplet infection’, which is inhaling or ingesting droplets of moisture which have been breathed, coughed or sneezed out by an infected person (e.g. colds, flu); by entry through a wound or sore (e.g. tetanus); by direct contact with an infected person (e.g. athletes foot, ringworm). Some pathogens are carried by vectors from one organism to another. For example: mosquitoes carry the malaria protoctistan; rat fleas carried the bacterium that caused the Black Death; houseflies can spread micro-organisms from faeces to our food. The vectors should not be confused with the pathogenic organisms that they are carrying.
We are usually able to develop immunity to infections by virtue of our immune system. Our blood produces specific antibodies in response to the presence of specific foreign bodies called antigens. These antibodies gradually proceed to destroy the invading organisms. However, over 40% of all deaths in developing countries, including the annual deaths of 14 million children, are caused by infectious diseases. In developed countries, where there are good medical services, people seldom die from infectious diseases. Diseases can be prevented or cured. Prevention is principally through improved standards of hygiene, personal health and the development of vaccinations. Vaccines contain killed or non-virulent (less pathogenic) strains of bacteria and viruses, and when these are injected into the blood, or swallowed, the body has a mild form of the disease, and is able to manufacture sufficient antibodies to acquire immunity. This is the process of immunisation, and vaccinations are an effective way of stimulating the body's defence against such diseases as diphtheria, polio, measles, mumps, german measles, tetanus, tuberculosis and hepatitis B. Vaccinations do exist for flu, but these have to be continually developed, because flu virus antigens are frequently changing, producing new strains of virus to which people are not immune. New strains can result in a flu epidemic.
Most bacterial infections can be treated with antibiotics which are chemicals extracted from fungi or other bacteria. Penicillin was the first antibiotic drug. It was discovered by Alexander Fleming (1881 - 1955), isolated from the Penicillium mould, and commercially produced using biotechnology. They can be swallowed or injected to kill internal bacteria or prevent them from multiplying, although this is not an instantaneous process. However, as we use more and more antibiotics, some bacteria are becoming resistant to them. One strain of Staphylococcus aureus is resistant to all known antibiotics except one, but this drug can have dangerous side-effects. Contributing factors to this resistance include the over-prescribing of antibiotics for people and for farm animals, and patients not finishing their course of the drugs. Antibiotics cannot treat viral infections, and yet many people expect their doctors to prescribe antibiotics for colds and flu, which of course are viral.
Disinfectants, such as bleach, are powerful chemicals used to kill micro-organisms in the environment. Antiseptics are weaker chemicals applied to wounds and sores to prevent micro-organisms from multiplying. Specific fungicidal chemicals are effective against the few fungal micro-organisms that live on our skin such as ringworm and athlete’s foot.
Decomposers Fungi and most bacteria are saprotrophic and have an important role in an ecosystem as decomposers, breaking down dead or waste organic matter and releasing inorganic molecules. These nutrients are taken up by green plants which are in turn consumed by animals, and the products of these plants and animals are eventually again broken down by decomposers.
Sewage treatment employs bacteria which break down harmful substances in sewage into less harmful ones. Aerobic bacteria decompose organic matter in sewage in the presence of oxygen. Once the oxygen is used up the aerobic bacteria can no longer function, and anaerobic bacteria continue the decomposition of organic matter into methane gas and carbon dioxide, along with water and other minerals. The digested sludge is rich in nitrates and phosphates and can be spread on the land as fertiliser. Some sewage treatment plants have used the methane as a cheap form of fuel (biogas). Anaerobic micro-organisms are also being used to convert carbohydrate-rich crops, such as cane sugar and maize, into ethanol which is used as a substitute for petrol in cars. This biofuel (or gasohol) is used widely in Brazil, which has meagre oil resources.
The carbon cycle Fats, carbohydrates and proteins all contain carbon atoms, so dead and waste organic matter contains a lot of carbon. In breaking this down, saprophytic bacteria and fungi take up some carbon to build their own bodies, and release some as carbon dioxide during respiration. However, the carbon cycle need not involve decomposers because autotrophs can access carbon from the abundant carbon dioxide in the air.
The nitrogen cycle All living things need nitrogen, it is an essential component of all proteins. It makes up 79% of the air we breathe, but the N2 molecules are very stable and unreactive, and are not readily accessible to plants and animals in this form. Nitrogen-fixing bacteria are able to convert (or fix) nitrogen gas from the air into nitrogen compounds. Plants take up these nitrogen compounds through their roots, combine them with products of photosynthesis, and make proteins. Animals obtain the protein they need by eating the plants or other animals. Some nitrogen-fixing bacteria are free- living in the soil, others live in small swellings, or nodules, on the roots of some plants, particularly members of the legume family (such as clover, peas and beans). This is a symbiotic arrangement, the plant gets nitrogen compounds and the bacteria receive carbohydrates from the plant. Dead and waste organic matter contains ammonium compounds which are converted by nitrifying bacteria into nitrates, and these are assimilated by plants. Denitrifying bacteria remove nitrates and ammonium compounds from the soil by converting them into nitrogen gas.
Digestion Despite the vast quantities of cellulose eaten by herbivores, mammals themselves cannot digest cellulose and rely entirely on the action of carbohydrate-digesting bacteria in their guts. These secrete the enzyme cellulase which splits the cellulose into monosaccharides which can be absorbed by the gut. Ruminants (cud-chewing) mammals such as cows have a large chamber in the stomach called a rumen which contains huge numbers of these bacteria. Non-ruminant herbivores such as rabbits and horses have cellulose-digesting micro-organisms in their appendix and caecum which act as ‘fermentation-chambers’. Huge numbers of bacteria, particularly Escherichia coli, also inhabit the human colon. There are an estimated four hundred species and it has been suggested that the action of some of these on carbohydrate can contribute up to 10% of our energy requirements. Other bacteria synthesise vitamins and amino acids, and others may contribute to our resistance to disease by competing for space in the gut with harmful bacteria. It is important therefore to maintain a healthy gut flora.
Biotechnology The manipulation of cells, particularly micro-organisms, to produce useful substances is referred to as biotechnology. Micro-organisms are exploited extensively in the fields of medicine, agriculture, food production, waste disposal and many other industries. We make use of some saprophytic bacteria which do not produce waste products harmful to humans. The bacterium Lactobacillus feeds on milk, turning it into yoghurt. Other bacteria and fungi help in cheese-making and are responsible for distinctive flavours. Most industrial enzymes (protein catalysts) come from micro-organisms. Special strains of fungi and bacteria are developed by genetic engineering. They are grown in large fermenters where they secrete enzymes into their nutrient solution. The enzyme is isolated and concentrated for use. Examples of such enzymes include amylases for producing chocolates, fruit juices and syrups; cellulases for softening vegetables; proteases for tenderising meat and for removing biological stains when put in biological washing powders.
Yeast is a single-celled fungus that lives naturally on the surface of fruit. It is economically important in brewing and bread-making. Yeast respires anaerobically (i.e. without the use of oxygen) and breaks down glucose with the production of carbon dioxide, ethanol (alcohol) and energy. In wine-making the yeast feeds on fruit sugars in the grapes, and in beer-making it feeds on the maltose sugar in germinating barley. The term fermentation, is usually applied to this process of anaerobic respiration in which alcohol is produced. Controlled oxidation of alcohol can be carried out to produce vinegar (ethanoic acid). Bread-making uses the carbon dioxide produced by anaerobic respiration, not the ethanol. Starch in the dough breaks down to sugar, which feeds the yeast. The carbon dioxide bubbles make the dough rise before it is baked into bread.
Yeast, including that left over from brewing, and other micro-organisms are also cultivated as an important food source for farm animals, and for humans. When fed on simple sugars and inorganic salts in controlled conditions, these micro-organisms can double their mass within hours (plants and animals may take weeks). They are rich in protein and contain most of the essential vitamins and amino acids required by animals. The mould Fusarium is grown in this way to form a mycoprotein which is as nutritious as meat, but lower in cholesterol and higher in fibre. It is marketed as the meat substitute ‘Quorn’. This kind of high protein food produced from micro-organisms is called single-cell protein, and it is increasingly grown on the nutrients present in industrial waste (e.g. from food, paper-making and agricultural industries).
micro-organisms Micro-organisms can be grown in a sterile Petri dish on agar jelly which contains appropriate nutrients. After introducing a small sample of water, soil, leaf, etc., the lid should be permanently sealed. After several days the micro-organisms will have grown and multiplied. The colonies become visible due to the multiplication of the cells, not due to cells getting larger! Fungi usually appear as furry clumps and bacterial colonies are often smooth and shiny-looking. After inspection, the sealed dishes should be sterilised in an autoclave or strong disinfectant, in case any pathogens have been incubated.
ContentsDiversity of organisms