In the five-kingdom classification of Whittaker, there is no mention of some acellular organisms like viruses and viroids and lichens. Viruses did not find a place in classification since they are not truly ‘living’ if we understand living as those organisms that have a cell structure. They are non-cellular organisms that are characterized by having an inert crystalline structure outside the living cell.

Once they infect a cell they take over the machinery of the host cell to replicate themselves, killing the host. The name virus that means venom or poisonous fluid was given by Pasteur.

Evidence that Show Viruses are Living:

• They have genetic material (RNA or DNA)
• They can grow
• They can be transmitted from one host to another.
• They are capable of multiplication in the host.
• They can mutate.
• They show irritability because they react to heat, radiation, and chemicals.

Evidence that Show Viruses are Non-Living:

• They can be crystallized and stored for a very long time.
• They don’t have a cell wall or cytoplasm.
• They are inert outside the host.
• They don’t have cell organelles or don’t have a metabolism.
• They can not function outside the host.
• They do not show cell division.

Structure of Viruses:

They were found to be smaller than bacteria because they passed through bacteria-proof filters. W.M. Stanley (1935) showed that viruses could be crystallized and crystals consist largely of proteins. In addition to proteins, viruses also contain genetic material, that could be either RNA or DNA. No virus contains both RNA and DNA. A virus is a nucleoprotein and the genetic material is infectious.

In general, viruses that infect plants have single-stranded RNA and viruses that infect animals have either single or double-stranded RNA or double-stranded DNA. Bacterial viruses or bacteriophages (viruses that infect the bacteria) are usually double-stranded DNA viruses.

The protein coat called capsid made of small subunits called capsomeres protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms.

Types of Viruses:

Plant Viruses:

• The viruses which infect and attack plants are called plant viruses.
• They have single-stranded RNA either ss-RNA or ds-RNA.
• They show helical symmetry
• They are mostly rod-shaped or cylindrical.

Animal Viruses:

• The viruses which infect and attack animals are called animal viruses.
• They have either single or double-stranded RNA or double-stranded DNA.
• They show radial symmetry.
• They are mostly rod polyhedral in shape.

Bacterial Viruses:

• They are also called Bacteriophages.
• The viruses which infect and attack bacteria are called bacterial viruses.
• They have a DNA strand.
• They show radial symmetry
• They are mostly tadpole in shape.

Economic Importance of Viruses:

Plant Diseases:

Little leaf of brinjal:

The characteristic symptom of the disease is the marked reduction in the size of the leaves. The newly formed leaves become progressively smaller. The petioles are very much shortened. The leaves appear appressed to the stem and become narrow, soft, glabrous, and yellow. The internodes are shortened. The axillary buds are stimulated to sprout, and they grow into short branches with very small leaves. In severe cases, affected plants do not bear any fruit, or, if formed, it becomes hard and tough.

Yellow vein mosaic of lady’s finger(Bhendi or okra):

Yellow vein mosaic was first reported in okra plants in 1924 in India and Sri Lanka. The symptoms include alternate green and yellow patches, vein clearing, and vein chlorosis of leaves. The yellow network of veins is very conspicuous, and vein and veinlets are thickened. In severe cases, the chlorosis may extend to the interveinal area and may result in complete yellowing of leaves. Fruits are dwarfed, malformed, and yellow-green.

Potato Leaf Roll:

The potato leaf roll virus causes a very well visible leaf rolling up. the leaflets are curled up to form a small boat. Infected plants are dwarf, the leaves are very fragile, and the whole plant is light.

Papaya leaf curl:

It was first reported in Tamil Nadu in 1939. The most prominent symptoms are the rolling of the leaves downward and inward in the form of an inverted cup and the thickening of veins. Sometimes all the leaves at the top of the plant are affected by these symptoms. In advanced stages of the disease, defoliation takes place and the growth of the plant is arrested.

Bunchy top of banana:

The leaves remain bunched up, with yellow fringes and stand erect. The bunchy top symptom is usually most visible on young plants. Bunchy top symptom can be more subtle on older banana plants. The production of banana fruit becomes uneconomical and unprofitable.

Grassy shoot of sugarcane:

This disease is characterized by the production of numerous small and thin tillers having narrow leaves. Diseased plants exhibit varying degrees of loss of chlorophyll, ranging from total green to white. Premature & excessive tillering gives a crowded appearance like ‘grass’ to the clump. The root system of the affected plant reduced and plants are usually reduced in height (stunted growth). Affected clumps hardly produce one or two weak canes.

Tobacco Mosaic Disease:

Symptoms induced by Tobacco mosaic virus (TMV) are somewhat dependent on the host plant and can include mosaic, mottling, necrosis, stunting, leaf curling, and yellowing of plant tissues. The symptoms are very dependent on the age of the infected plant, the environmental conditions, the virus strain, and the genetic background of the host plant.

Animal Diseases:

Viroids:

In 1971, T. O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease. It was found to be a free RNA; it lacked the protein coat that is found in viruses, hence the name viroid. The RNA of the viroid was of low molecular weight. These are mainly plant pathogens. The viroids do not show dormant state. The diseases caused by viroids are  Citrus exocortis Chysanthemum stunt Cucumber bale fruit Potato spindle tuber.

Science > Biology > Classification of Microbes, Protists, and Fungi > Viruses

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Microbes are omnipresent i.e. they exist everywhere: in soil, air, water and also in the human body and the bodies of plants and other animals! They also exist in places where no other life-form exists. Bacteria, fungi, protozoa, viroids are examples of microorganisms. The general presumption is that the microbes are harmful to us because some of them cause diseases to both plants and animals including humans. However, there are many microorganisms, who are useful to us in many ways. In this article, we shall study the use of microbes in household activities.

Edible mushrooms are cultivated and consumed as food worldwide. Algae are used as thickening agents and in ice creams. They are nutritionally important in vegetarian diets. Yeasts are used as a common flavoring agent of food and for food production. Bacteria are used to produce dairy products like cheese and yogurt.

Fermentation:

The process of anaerobic respiration in which the complex molecules incompletely brakes into simple ones by the microbial action is called fermentation. There are many types of fermentation that are distinguished by the end products formed from pyruvate or its derivatives. The two fermentations most commonly used by humans to produce commercial foods are ethanol fermentation (used in beer and bread) and lactic acid fermentation (used to flavor and preserve dairy and vegetables). While there are a number of products from fermentation, the most common are ethanol, lactic acid, carbon dioxide, and hydrogen gas (H₂). These products are used commercially in foods, vitamins, pharmaceuticals, or as industrial chemicals. 

Fermentation of dough is done for making dosa, idli. CO₂ produced in the process gets trapped in gluten and makes idli puffy. Microorganisms involved in the process are Bacillus Candida, Saccharomyces cerevisiae. For making bread the microbe used in Baker‘s yeast (Saccharomyces cerevisiae). Toddy is made from the sap of palm using the process of fermentation.

The products obtained in the dairy industry by microbe activities are cheese, yogurt, buttermilk, paneer. Microbes used are species of Streptomyces, Penicillium, Lactobacillus

Bread Making:

The flour, salt, and yeast are mixed. Then the oil and water are mixed with it. The dough is kneaded until the dough becomes smooth. Then the dough is allowed to ferment overnight. The dough is moulded into balls and put on a baking tray. The oven is preheated to 220˚C and the balls are baked for 25-30 minutes until they turn golden brown, Then they are allowed to cool on wire mesh.

Cheese Making:

The process is used in cheese making (eg. Swiss cheese by Propionibacterium sharmanii, Roquefort cheese by fungi). Different varieties of cheese are known by their characteristic texture, flavour, and taste, the specificity coming from the microbes used. The bacterium Propionibacterium sharmanii is used in ‘Swiss cheese’ to give it its characteristic holes by producing a large amount of carbon dioxide. ‘Roquefort cheese’ is ripened by growing certain fungi on them to give them their specific flavour.

Curd Making:

The increased acidity causes the milk proteins (casein) to tangle into solid masses or curds. Milk that has been left to sour (raw milk alone or pasteurized milk with added lactic acid bacteria) will also naturally produce curds.

A small amount of curd added to the fresh milk as inoculum or starter contains millions of LAB, which at suitable temperatures multiply, thus converting milk to curd. During growth, the LAB produces acids that coagulate and partially digest the milk proteins. The microorganism involved is Lactobacillus Lactic acid Bacteria (LAB). The process increases nutritional value. It checks disease-causing microbes in our stomach. It increases Vitamin B₁₂

Yogurt Making:

Milk contains the sugar lactose. and some bacteria will ferment lactose to produce lactic acid. Yogurt is produced by batch culture, where pasteurized milk has the bacteria Lactobacillus bulgaricus and Streptococcus thermophilus added to it. The mixture is maintained at a temperature of around 40°C. The bacteria will produce lactic acid by respiration, and this lowers the pH. When it reaches a target the product is harvested.

Microbes as the Source of Food:

With the continuous rise in the global population, the demand for food has also increased. The conventional methods of agriculture and animal husbandry are not sufficient to fulfill the nutritional requirements of the growing population. This resulted in malnutrition due to deficiency of protein in food.

Some microbes or their fruiting bodies are directly used as a source of food. The microorganisms utilize the carbon and nitrogen present in these materials and convert them into high-quality proteins which can be used as a supplement in both human and animal feed.  They are rich in protein. Single-cell protein (SCP) is the protein extracted, or the cells themselves, from cultivated microbial biomass. The microbes involved are bacteria, fungi, and algae. SCP has high protein content. They can be grown on waste products such as whey, which makes it more of an economically- feasible protein source.

SCP can be obtained from bacteria (Bacillus subtilis), fungi (Candida and Saccharomyces cerevisiae), algae (Chlorella).  Bacterial sources of SCP are Pseudomonas fluorescens, Lactobacillus, Bacillus megaterium. The fungal sources of SCP are Aspergillus fumigatus, Aspergillus niger, Rhizopus cyclopean. Yeasts used as source of SCP are Saccharomyces cerevisiae, Candida tropicalis, Candida utilis. The algal sources of SCP are Spirulina sps, Chlorella pyrenoidosa, Chondrus crispus.

Mushrooms:

Fungus like mushrooms and truffles (bacidomycetes) are directly used as food. They are sugar-free, fat-free. They have low-calorie value. They are rich in proteins, vitamins, minerals, and amino acids. Some common edible mushrooms are white button mushroom (Agaricus bisporus), Paddy straw mushroom (Volvariella Volvacea), Oyster mushroom (Pleurotus florida). Toadstools are poisonous mushrooms.

Algae, which include seaweeds and other aquatic plants, contain chlorophyll and require carbon dioxide and continuous sunlight. Those without chlorophyll, particularly yeasts and fungi, show more promise for commercial production.

Next Topic: Industrial Uses of Microbes

Biology > Microbes in Human Welfare > Household Uses of Microbes

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