Microorganisms PDF

## 2 Microorganisms **Learning Objectives** - list the conditions required for the growth of microorganisms - explain the role of microorganisms in increasing soil fertility through the nitrogen cycle - explain the role of microorganisms in cleaning the environment and their use for commercial pur...

## 2 Microorganisms **Learning Objectives** - list the conditions required for the growth of microorganisms - explain the role of microorganisms in increasing soil fertility through the nitrogen cycle - explain the role of microorganisms in cleaning the environment and their use for commercial purposes - discuss the harmful effects of microorganisms - list the methods of preserving food **What are Microorganisms?** A wide variety of living organisms live among us and also inside us. Some are large, while some are small enough to be seen only under a microscope. The tiny ones include millions of single-celled organisms called microorganisms or microbes. Some of them are useful, while others are harmful and may cause diseases. **Types of Microorganisms** There are five major groups of microorganisms. They are: - Bacteria (singular: bacterium) - Viruses - Protozoa (singular: protozoan) - Fungi (singular: fungus) - Algae (singular: alga) **Bacteria** Bacteria were among the first living things to appear on the Earth. They can be found in almost all environments and inside organisms. Bacteria are of several shapes. Some are spherical (cocci), some are spiral (spirilla) and some are rod-shaped (bacilli). Examples of bacteria include Streptococcus and Lactobacillus. **Viruses** Viruses infect all life forms. They come in various shapes and sizes, and can be seen only with the help of powerful microscopes. Some examples of viruses are the papillomavirus, which can cause cancer, and the varicella zoster virus (chickenpox virus). **Protozoa** Protozoa are single-celled organisms that thrive in soil and aquatic environments. Amoeba and Euglena are some examples of protozoa. **Fungi** Fungi are plant-like organisms that do not contain chlorophyll. Some examples of fungi are mushrooms and yeast. **Algae** Algae include single-celled and multicellular organisms. They are simple plant-like organisms found in water bodies, rocks and snow. Spirogyra and Chlorella are some examples of algae. **Milestones in the discovery of microorganisms** - **Antonie van Leeuwenhoek (1632-1723)**, a Dutch man, first observed microorganisms with the help of his newly invented microscope. - **Louis Pasteur (1822-1895)**, a French man, discovered that microorganisms in the air spoil food. He concluded that they are the cause of fermentation and decay, and also the main cause of diseases. - **Robert Koch (1843-1910)**, a German, designed techniques for handling bacteria that are used to this day. He studied the disease anthrax in cattle and proved that it was caused by specific microorganisms that spread the disease from infected animals to other healthy animals. - **Joseph Lister (1827-1912)**, an Englishman, was a famous surgeon. He was the first to eliminate bacteria from the operating theatre by using carbolic acid, a disinfectant. This greatly improved the chances of a patient surviving an operation. **Where are Microorganisms Found?** Microorganisms are found in almost every kind of environment, including inside other organisms! A single gram of soil can contain anywhere from 100 million to 1000 million bacteria, while the collections of dead cells on your scalp contain 500 million or more bacteria. The air you breathe, the clothes you wear, your skin, fingernails, and even your intestines, harbour microorganisms. **Conditions for the Growth of Microorganisms** Different microorganisms have different needs; they grow under a wide range of conditions. - **Oxygen** Many microorganisms require oxygen for respiration (are aerobic). However, many others, like yeast and the tetanus-causing bacterium, do not require oxygen for respiration (are anaerobic). - **Water** Moisture is essential for most microorganisms to thrive. - **Suitable temperature** Some microorganisms can tolerate and grow at extreme temperatures of -10 °C or above 100 °C. But in general, microorganisms thrive between 25 °C and 38 °C. - **Dark places** Many microorganisms thrive in dark places. Direct sunlight often kills them. However, photosynthetic microorganisms need light to grow. - **Suitable food supply** Microorganisms live and feed on a wide range of materials. Some are saprophytes (most fungi and some bacteria) and are responsible for the decay and decomposition of dead organisms. Others, like some bacteria, fungi and protozoa, are parasites and obtain food from the tissues of their host organisms. Some bacteria, protozoa and algae can photosynthesise. **Useful Microorganisms** Microorganisms are useful to us in many ways. **Increasing Soil Fertility** - Bacteria and fungi decompose dead organisms and organic matter like dung, straw and grass. They thus enable the nutrients in dead organisms and organic waste to go back to the soil. This not only prevents the accumulation of dead organic matter in the soil but also increases soil fertility. - Microorganisms have a very important role to play in the fixing of nitrogen. Plants such as beans, peas and soya bean, belonging to the legume family, have swollen parts in their roots called **nodules**. Different species of Rhizobium bacterium live in these swellings and convert nitrogen to ammonia. This process is called **nitrogen fixation**. This helps to make the soil rich in nitrogen compounds, which are essential for the growth of plants. Let us look at nitrogen fixation in detail. **Nitrogen Fixation** Nitrogen is the most abundant element (makes up nearly 78% of the volume of air) in the Earth's atmosphere. It is a vital element for living organisms since it is a component of important molecules such as proteins. However, atmospheric nitrogen cannot be used directly by most organisms. Plants can use nitrogen only in the form of nitrate (NO3-) or ammonium (NH4+) compounds. But these compounds are available in limited quantity in the soil. As crop plants generally require relatively large amounts of nitrogen for their growth, these compounds in the soil get used up. Some bacteria convert atmospheric nitrogen to ammonia (NH3). Some nitrogen-fixing bacteria convert ammonia into nitrites, and others convert nitrites into nitrates. This process is called **nitrification**. The latter bacteria are called **nitrifying bacteria** or **nitrifiers**. They make the soil rich in nitrates. Many bacteria and fungi decompose organic matter and release fixed nitrogen for reuse by other organisms. All these processes contribute to the **nitrogen cycle**. **Nitrogen Cycle** As you have read, nitrifying bacteria fix ammonia in the soil in the form of nitrates. In addition, small amounts of nitrogen and oxygen combine in the atmosphere during lightning to form nitrogen oxides. The same happens during combustion in factories or in engines of motor vehicles. These oxides react with rain water to form dilute nitric acid. The nitric acid reacts with minerals in the soil to form nitrates. Plants take in nitrates through their roots and assimilate them into proteins. Animals obtain the proteins they need by eating plants or the flesh of other animals. When plants and animals eventually die, the nitrogen compounds-proteins and so on-are broken down to give ammonia. This process is known as **ammonification**. The ammonia produced can enter different processes. Some bacteria directly break down ammonia to release atmospheric nitrogen. Nitrifying bacteria convert (fix) ammonia into nitrates. Nitrates may be stored in humus and taken up again by plants or leached from the soil and carried to lakes and streams. They may also be converted to free nitrogen by **denitrifying bacteria** through a process called **denitrification**. The nitrogen is returned to the atmosphere. This process of circulation of nitrogen between the atmosphere, soil, plants and animals is called the **nitrogen cycle**. **Cleaning the Environment** - Raw sewage containing faeces and urine is a health hazard. Bacteria break down raw sewage into harmless and even useful substances, which can be used in landfills. They can also be used to replenish the soil with nitrates and phosphates, and also improve the soil's water retention capacity. - In the villages of some countries like India and China, **biogas fermenters** are used to produce methane. Human and animal faeces and leafy waste from crops are broken down by bacteria in the absence of oxygen (anaerobic conditions) to produce methane (biogas), a valuable fuel for cooking and lighting. **Commercial Uses of Microorganisms** **Fermentation** Fermentation is the breakdown of substances by the activity of fungi and bacteria, usually leading to the production of carbon dioxide and alcohol/acid. Alcohol is made by the fermentation of sugar using yeast. Yeast is used to ferment sugars in grapes to make wine. Sugars in rice and barley are fermented to produce beer. - **Yeast** is used in bakeries to make bread. When it is added to dough, it produces carbon dioxide, which makes the dough rise. Bread made from the dough is soft and fluffy due to the gas. Yeast is also used to make dhokla light and fluffy. - **Idli and dosa** are made by grinding rice and pulses to a fine paste. The mixture is allowed to stand for a few hours. Fermentation occurs naturally due to bacteria in the mixture. **Retting of fibres** Linen is made from fibres obtained from the flax plant. Similarly, jute fibres are obtained from jute plants. The plants are tied in bundles and placed in water. Bacteria enter the stems and gradually destroy the stem tissues, loosening the supporting fibres. This is known as **retting**. These fibres are used to make threads. **Production of leather** The action of microbes was traditionally used in the leather-making industry. During the processing of hide (animal skin) to produce leather, the action of bacteria on the skins makes them soft and pliable. **Food industry** - **Making of curd and cheese** Some bacteria break down lactose (milk sugar) to form lactic acid, which turns milk sour. Lactic acid acts on **casein**, a protein present in milk. This helps in the formation of curd. The making of cheese is similar in some ways. Bacteria convert the lactose found in milk to lactic acid. An enzyme is added to set the curd. The semi-solid curdled material is separated from the fluid and is made into cheese. Fungi and bacteria act on fresh cheese and give it its characteristic smell and flavour. - **Making of vinegar** Yeast cells break down the sugar found in fruits and convert it into alcohol. Bacteria act on this alcohol to form vinegar. **As a stabiliser and a thickener** Algae are used in the preparation of agar (vegetarian gelatine). Sodium compounds obtained from algae keep ice cream smooth and free of crystals. Products from algae are also used as food stabilisers and as a thickener in salad dressings.. **As a source of food** Algae are potentially an unlimited source of wholesome and nutritious food. In China and Japan, many types of seaweed are consumed as food. Chlorella is a single-celled spherical alga that has been the subject of research in the study of photosynthesis and as a source of human food (especially on space flights). **Making of antibiotics** An antibiotic is a substance that kills microorganisms or stops their growth. Fungi secrete antibiotics such as **penicillin** and **cephalosporin**, while bacteria secrete the antibiotics **streptomycin**, **chloramphenicol** and **tetracycline**. Antibiotics can kill disease-causing microorganisms without severe side effects such as injury to body tissues or disturbance of body functions. Today, over a hundred different antibiotics are available to doctors to cure anything from minor illnesses to life-threatening infections. Each antibiotic works differently and acts on different types of bacteria. Antibiotics are also used to cure plant and animal diseases. They are mixed with food to cure microbial infections in livestock (poultry and cattle). **Immunity and making of vaccines** A foreign substance or a disease-causing microbe that enters our body is called an antigen. Our body produces proteins, called antibodies, to attack and destroy the antigen. Antibodies produced against a particular antigen are specific and will attack only that antigen. For example, antibodies produced against the measles virus will not attack typhoid bacteria. The ability of our body to resist infection is known as immunity. - Immunity against some diseases may be **natural**, that is, inherited from parents. - Immunity against some diseases is **acquired** after recovering from an attack of the disease, as in the case of measles and chickenpox. Once you have recovered from these diseases, the antibodies remain in the blood for a short time. However, the ability to produce the antibodies again is greatly increased. Therefore, any further infection by the bacterium or virus is likely to be stopped, and you are **immune** to the disease. - Immunity against a particular disease can also be induced in the body by introducing dead or weakened microbes of the disease. This substance introduced into the body is called a **vaccine**. The body recognises the antigen and produces antibodies against it. This technique is called **vaccination** or **immunisation**. Through this process, it is now possible to be protected against a number of infectious diseases like tetanus, typhoid, polio, measles and hepatitis B. **Vaccines can be a dose of:** - dead microbes, as in the case of cholera and typhoid vaccines - live microbes, which have been altered so that they are no longer dangerous and cannot cause the disease, as in the case of polio and measles vaccines - **toxoids**, harmless forms of the **toxins** (poisonous substances) produced by disease-causing germs, as in the case of the tetanus vaccine All vaccines are made from microorganisms **Harmful Microorganisms** Many kinds of microorganisms cause diseases in humans. Such microorganisms are called **pathogens**. Diseases caused by microorganisms and transmitted from an infected person or animal to another person or animal are called **communicable diseases** or **infectious diseases**. Communicable diseases are carried from person to person in a number of ways. The main sources of infection are: - Through **direct contact** with a sick person, as in the case of ringworm and chickenpox. - Through **air**, also known as **droplet infection**, as in the common cold and flu. When a sick person sneezes or coughs, microbes are released into the air. These can infect a healthy person breathing the air. - Through **contaminated food and water**, as in cholera and typhoid. For example, the parasites that cause amoebic dysentery enter the water when an infected person passes faeces near a water body. Other people get infected when they use the water from such a source. - Through **insects** such as flies, fleas and mosquitoes, which are **carriers¹** of microbes. When a mosquito bites an infected human, it carries microbes with it and transfers them to any healthy person it bites afterwards. Diseases such as malaria, dengue fever and yellow fever spread in this way. Fleas transfer the bacterium that causes bubonic plague from rats to humans. Flies carry microbes from contaminated food to clean food. | Disease | Cause | Method of spread | Symptoms | Prevention | |----------------|---------------------------|---------------------|-----------------------------------------------------------------------------------------|-----------------------------| | Cholera | Bacterium: Vibrio cholerae | Contaminated water or food | Severe diarrhoea, dehydration, vomiting, abdominal pains | Vaccination | | Tuberculosis | Bacterium: Mycobacterium | Droplet infection | Persistent cough, fever, fatigue, weight loss, blood in sputum | BCG vaccination | | (TB) | tuberculosis | | | | | Typhoid | Bacterium: Salmonella | Contaminated water or food | High fever, acute headache, constipation, dry cough | Anti-typhoid inoculation | | | typhi | | | | | Gastroenteritis | Many viruses and bacteria | Food-borne mainly | Diarrhoea and vomiting within hours of eating an infected meal | No vaccine; antibiotics can be taken for bacterial infections | | Common cold | Many viruses | Droplet infection | Fever, headache, runny nose, sneezing and coughing | No vaccine; avoid crowded places; cover nose and mouth when coughing and sneezing | | Poliomyelitis | Polio virus | Through human faeces | Fever, headache, destruction of nerve cells causing stiffness, paralysis and muscle wastage | Polio vaccine by mouth and injection | | (polio) | | | | | | Malaria | Protozoa: Plasmodium | Through infected mosquitoes | Recurring chills and fever | Check breeding of mosquitoes | | | species | | | | | Ringworm | Fungus | Transmitted to humans from pets or livestock, or from infected bedding | Itchy ring-shaped patches | Maintain hygiene; infection can be treated with antifungal medication | - Through **animal bites**, as in the case of rabies. The bite of an infected dog can spread the disease to a human. - Through **cuts and wounds**, as in the case of tetanus. Different pathogens act in different ways. Some produce **toxins**, while others invade cells or tissues, and then produce toxins. The symptoms of the disease appear only after a brief lapse of time, during which the microbes multiply rapidly. The period between the infection and the appearance of symptoms is called the **incubation period**. The symptoms may be fever, headache, nausea, vomiting, rashes and so on. - **Viruses** cause diseases such as the common cold, measles and influenza. These diseases spread through the air. **Fungi and bacteria** grow on food, releasing toxic substances and spoiling the food. This leads to **food poisoning**. - **Bacteria** cause pneumonia, tetanus, tuberculosis and cholera. - **Protozoa** cause malaria and amoebic dysentery. Microorganisms also cause diseases in plants and animals. - **Leaf mosaic viruses** enter and damage the outer cells of leaves. - **Potato blight** in potato plants is caused by a fungus. - **Mildew** in grapes, roses, gooseberries and other flowering plants is caused by fungi. - **Rust** in wheat and other cereals is caused by fungi. - **Foot-and-mouth disease** in cattle is caused by a virus. **Food Preservation ** Have you ever felt ill soon after eating a meal? If so, you may have eaten spoilt food. Food has to be kept free of the growth of moulds and bacteria, which spoil food and cause food poisoning and other diseases. Many fungi and bacteria are **decomposers**. They are responsible for the decomposition of dead organic matter. When fungi and bacteria come in contact with food, they begin to decompose the food and release **chemical toxins**. These contaminate or spoil food, making it unfit to be eaten. When food is stored for use at a later date, it is important to prevent it from getting damaged by the activities of: - **microbes** - bacteria and fungi - **insects** - flies, weevils and so on - **rodents** - mice, rats and so on - **natural chemical changes** which take place within food To prevent spoilage of food by insects and rodents, food should be kept covered or in closed containers. However, it is not always possible to keep food away from microbes. It is, therefore, much better to kill the microbes or make sure that the conditions are not suitable for their growth. This will prevent food-borne diseases and keep food fit to be eaten for a longer period. **Processing of food to prevent its spoilage and to retain its nutritive value for long periods is called **food preservation**. **Food preservation helps us in many ways:** - It reduces wastage of food. - It increases the storage period.. - It enables food to be transported to distant places. - It helps to store food for use in seasons when it is not available. **Methods of Food Preservation** Food is preserved using many methods. - **Canning** Food and drink can be preserved by putting them in sealed, airtight containers. - **Pasteurisation** Most bacteria in milk and other liquids can be killed by heating them to high temperatures and quickly cooling them down. This method ensures that the flavour of the food is not affected. - **Sun drying** Microbes cannot grow in dry conditions. So, water is removed from some vegetables like cauliflowers and fruits like mangoes by drying them in sunlight. - **Salting** Fruits and vegetables can be preserved by adding salt and then drying them. Salt prevents the growth of microbes. - **Pickling** Raw mangoes, gooseberries, lemons, and vegetables like cauliflowers, onions and cucumbers are preserved in vinegar or brine (salty water) to prevent the growth of microbes and give them a sharp or spicy flavour. They are then eaten with other foods. - **Adding sugar** Fruits are preserved in the form of jams and jellies by adding sugar. Sugar removes water from cells, preventing the growth of microbes. - **Refrigeration** Freezing stops the growth and multiplication of microbes but does not kill them. Frozen food should not be left in the open for long as the microbes will start multiplying again. **Mark ✓ if you have understood the concept.** - Microorganisms or microbes are tiny organisms that can only be seen under a microscope. ✓ - The five major groups of microorganisms are bacteria, viruses, protozoa, fungi and algae. ✓ - The ideal conditions for most types of microorganisms to survive well are enough oxygen, moisture, moderate temperatures of about 25-38 °C, darkness and suitable food supply. ✓ - The main uses of bacteria are: increasing soil fertility, cleaning the environment, in the food, leather and fibre industry, and in making medicines (antibiotics and vaccines). ✓ - Nitrogen gas can be fixed by bacteria in the root nodules of legumes. ✓ - Denitrifying bacteria free nitrogen from the nitrates. ✓ - The process of circulation of nitrogen between the atmosphere, soil, plants and animals is called the nitrogen cycle. ✓ - Harmful microorganisms are called pathogens. They cause communicable diseases such as cholera, polio, flu, dysentery and hepatitis. They also cause diseases in plants and animals. ✓ - Communicable diseases spread through direct contact, air, contaminated food and water, insects, animal bites, and through cuts and wounds. ✓ - Pathogens cause diseases by damaging tissues or by producing toxins. ✓ - Food has to be preserved for later use and should be kept fresh. If microbes grow on it, the food gets spoilt and eating it may cause food poisoning. ✓ - Microbial activity in food is controlled by treating the food in a process called food preservation. Some methods of food preservation are canning, sun drying, salting, pickling, adding sugar and refrigeration. ✓