Microorganisms

Living organisms are all around us, but many are too small to be seen without magnification. These tiny organisms are called microorganisms or microbes. For instance, the greyish-white patches seen on moist bread in the rainy season are formed by microbes, which appear as tiny, rounded structures under a magnifying glass.

Observing water from a pond or moist soil under a microscope reveals a multitude of tiny moving organisms. While soil and water are teeming with life, not all are considered microbes. True microorganisms are invisible to the naked eye, though some, like bread mould, can be seen with a magnifying glass. Most require a microscope for observation, hence the name microorganisms.

Microorganisms are broadly categorised into four main groups:

• Bacteria
• Fungi
• Protozoa
• Some Algae

Viruses are also microscopic but differ significantly from other microbes. They can only reproduce by entering and taking over the cells of host organisms, which can be bacteria, plants, or animals. Many common illnesses like colds, influenza, and coughs are caused by viruses. More serious diseases such as polio and chicken pox are also viral infections.

Some diseases like dysentery and malaria are caused by protozoa, while diseases such as typhoid and tuberculosis (TB) are caused by bacteria.

Where Do Microorganisms Live?

Microorganisms can be either single-celled (unicellular), like some bacteria, algae, and protozoa, or composed of multiple cells (multicellular), like many algae and fungi. They are incredibly adaptable and can survive in virtually all types of environments on Earth. This includes extreme conditions ranging from very cold icy regions to hot springs, and from dry deserts to wet marshy lands. They are also found living inside the bodies of other organisms, including animals and humans. Some microbes live on the surface of other organisms, while many others exist independently.

Microorganisms And Us

Microorganisms significantly impact our lives, acting as both friends and foes. Many are beneficial, helping us in various ways, while others are harmful, causing diseases or spoiling things.

---

Friendly Microorganisms

Microorganisms have many useful applications, including:

• Producing food items like curd, bread, and cakes.
• Cleaning the environment.
• Producing medicines.
• Increasing soil fertility in agriculture.
• Used for commercial production of alcohol and vinegar.

---

Making Of Curd And Bread

Curd formation from milk is facilitated by specific bacteria, particularly Lactobacillus. This bacterium multiplies in milk and converts lactose sugar into lactic acid, causing the milk to curdle and form curd. Bacteria are also used in making cheese, pickles, and fermenting batters for idli and dosa.

Yeast, a type of fungus, is essential for making bread, pastries, and cakes. When mixed with flour and sugar, yeast respires, producing carbon dioxide gas. These gas bubbles cause the dough to rise and increase in volume, resulting in soft and fluffy baked goods.

---

Commercial Use Of Microorganisms

Microorganisms are used on a large scale in industries for producing alcohol, wine, and acetic acid (vinegar). Yeast is specifically used for the industrial production of alcohol and wine. It ferments natural sugars found in grains like barley, wheat, rice, or fruit juices, converting them into alcohol.

The process by which sugar is converted into alcohol by microorganisms like yeast is called fermentation. Louis Pasteur discovered fermentation in 1857.

---

Medicinal Use Of Microorganisms

Microorganisms are a source of important medicines called antibiotics. Antibiotics are chemicals that kill or inhibit the growth of disease-causing microorganisms. Many antibiotics, such as streptomycin, tetracycline, and erythromycin, are derived from specific bacteria and fungi. These are used to treat various diseases in humans and animals. Antibiotics are also added to livestock feed to prevent microbial infections.

It is crucial to take antibiotics only as prescribed by a qualified doctor and to complete the full course. Taking antibiotics unnecessarily or in incorrect doses can reduce their effectiveness in the future and may kill beneficial bacteria in the body. Antibiotics are not effective against viral infections like colds and flu.

---

Vaccine

A vaccine is a preparation containing weakened or dead disease-causing microbes, or parts of them. When introduced into a healthy body, the immune system responds by producing antibodies to fight the invaders. The body also develops a memory, so if the actual pathogen enters in the future, the body can quickly produce the necessary antibodies to fight it off, thus preventing the disease. This process provides long-lasting immunity.

Vaccination is a preventive measure against many diseases, including cholera, tuberculosis, smallpox, and hepatitis. Edward Jenner discovered the smallpox vaccine in 1798. Vaccination is particularly important for children and infants to protect them from various infectious diseases. Programs like Pulse Polio aim to vaccinate children to eradicate diseases like polio.

---

Increasing Soil Fertility

Some microorganisms play a vital role in improving soil fertility. Certain bacteria, such as Rhizobium (found in the root nodules of leguminous plants like beans and peas), and some blue-green algae (cyanobacteria), can directly convert atmospheric nitrogen gas into nitrogen compounds that plants can absorb from the soil. These microbes are known as biological nitrogen fixers. This process enriches the soil with nitrogen, an essential nutrient for plant growth.

---

Cleaning The Environment

Microorganisms act as natural decomposers, playing a crucial role in cleaning up the environment. They break down dead organic matter, including dead plants, animals, and waste materials like vegetable peels and animal excreta, into simpler, harmless, and usable substances (like manure). Without microbial decomposition, dead organisms and waste would accumulate. The nutrients released during decomposition are returned to the soil, becoming available for plants again. Thus, microbes help degrade waste and clean the surroundings, reducing foul odours.

Harmful Microorganisms

Besides their beneficial roles, microorganisms can also be harmful in numerous ways. Some are responsible for causing diseases in humans, plants, and animals. Disease-causing microbes are called pathogens. Other microbes can spoil food, clothing, and leather.

---

Disease Causing Microorganisms In Humans

Pathogens can enter the human body through contaminated air, water, food, or direct contact with an infected individual or an animal carrier. Diseases that spread from an infected person to a healthy one through these means are called communicable diseases. Examples include cholera, common cold, chicken pox, and tuberculosis.

When someone with a common cold sneezes, tiny moisture droplets containing viruses are released into the air, which can then be inhaled by a healthy person, causing infection. Covering the mouth and nose while sneezing and maintaining distance from infected individuals are simple preventive measures.

Some insects and animals act as carriers, transmitting pathogens without being affected themselves. For example, houseflies can carry pathogens on their bodies after sitting on garbage or excreta. When they land on uncovered food, they transfer these pathogens, making the food contaminated and potentially causing illness in anyone who eats it. Keeping food covered is essential.

Mosquitoes are also important carriers. The female Anopheles mosquito transmits the malaria parasite (Plasmodium), while the female Aedes mosquito carries the dengue virus. Preventing the spread of malaria and dengue involves controlling mosquito populations by preventing water from collecting anywhere (coolers, tires, pots), as mosquitoes breed in standing water. Keeping surroundings clean and dry helps reduce mosquito breeding sites.

Here is a summary of some common human diseases caused by microbes:

Disease	Causative Microorganism	Mode of Transmission	Preventive Measures (General)
Tuberculosis	Bacteria	Air	Isolation of patient, vaccination, avoiding contact with patient's belongings
Measles	Virus	Air	Vaccination at suitable age
Chicken Pox	Virus	Air/Contact	Vaccination at suitable age
Polio	Virus	Air/Water	Vaccination (Oral drops are a vaccine)
Cholera	Bacteria	Water/Food	Personal hygiene, good sanitation, consuming properly cooked food and boiled water
Typhoid	Bacteria	Water	Personal hygiene, good sanitation, consuming properly cooked food and boiled water, vaccination
Hepatitis A	Virus	Water	Drinking boiled water, vaccination
Malaria	Protozoa	Mosquito (Female Anopheles)	Using mosquito nets and repellents, spraying insecticides, controlling mosquito breeding (preventing water collection)

Robert Köch discovered the bacterium (Bacillus anthracis) causing anthrax disease in 1876.

---

Disease Causing Microorganisms In Animals

Microorganisms also cause diseases in animals. For instance, anthrax is a severe bacterial disease affecting both humans and cattle. Foot and mouth disease in cattle is caused by a virus.

---

Disease Causing Microorganisms In Plants

Many microorganisms cause diseases in commercially important plants like wheat, rice, potato, sugarcane, orange, and apple, leading to reduced crop yields. Examples include Citrus canker (bacterial, spread by air), Rust of wheat (fungal, spread by air and seeds), and Yellow vein mosaic of bhindi (viral, spread by insects). These plant diseases can be managed by applying specific chemicals that kill the responsible microbes.

Plant Disease	Causative Microorganism	Mode of Transmission
Citrus canker	Bacteria	Air
Rust of wheat	Fungi	Air, seeds
Yellow vein mosaic of bhindi (Okra)	Virus	Insect

---

Food Poisoning

Food can become poisonous due to the growth of certain microorganisms that produce toxic substances. Consuming this spoiled food can cause serious illness, including vomiting, and in severe cases, can even be fatal. Therefore, preventing food spoilage is extremely important.

Food Preservation

Preventing food spoilage by microorganisms is crucial. Various methods are used at home and commercially to preserve food.

---

Chemical Method

Common chemicals like salts and edible oils are used as preservatives because they inhibit microbial growth. Salt or acid preservatives are added to pickles. Chemicals like sodium benzoate and sodium metabisulphite are common industrial preservatives used in jams and squashes to prevent spoilage.

---

Preservation By Common Salt

Salting has been used traditionally to preserve meat and fish by covering them with dry salt, which prevents bacterial growth. Salting is also used for preserving fruits like amla, raw mangoes, and tamarind.

---

Preservation By Sugar

Sugar is used to preserve jams, jellies, and squashes. Sugar reduces the moisture content in the food, creating an environment unsuitable for bacterial growth.

---

Preservation By Oil And Vinegar

Using oil and vinegar prevents spoilage of pickles because bacteria cannot survive in such acidic and oily conditions. This method is also used to preserve vegetables, fruits, fish, and meat.

---

Heat And Cold Treatments

Applying heat or cold helps preserve food. Boiling milk kills many microorganisms. Storing food in a refrigerator at low temperatures slows down or inhibits the growth of microbes.

Pasteurisation is a process developed by Louis Pasteur. It involves heating milk to about $70^\circ\text{C}$ for 15 to 30 seconds, then rapidly cooling and storing it. This kills harmful microbes without significantly affecting the milk's nutritional value, making it safe to consume without boiling. Pasteurised milk stored in packets is treated this way.

---

Storage And Packing

Modern methods include storing food items like dry fruits and even vegetables in sealed, airtight packets. This prevents moisture entry and protects the food from microbial attack.

Nitrogen Fixation

Although nitrogen is the most abundant gas ($78\%$) in the atmosphere, most organisms cannot use it directly. Nitrogen is an essential element for life, being a part of proteins, chlorophyll, nucleic acids, and vitamins. Certain microorganisms, like the bacterium Rhizobium (found in the root nodules of legumes) and some blue-green algae, are capable of converting atmospheric nitrogen gas into usable nitrogen compounds (like nitrates) in the soil. This process is called nitrogen fixation.

Nitrogen can also be fixed in smaller amounts by lightning during thunderstorms, where the high energy converts atmospheric nitrogen and oxygen into oxides of nitrogen that dissolve in rain and fall to the earth.

Nitrogen Cycle

The nitrogen cycle describes the continuous movement of nitrogen between the atmosphere, soil, plants, and animals. Here are the key steps:

1. Atmospheric Nitrogen: The atmosphere contains nitrogen gas ($\text{N}_2$).
2. Nitrogen Fixation: Atmospheric nitrogen is converted into usable nitrogen compounds by:
   • Biological fixation by bacteria (like Rhizobium) and blue-green algae in the soil.
   • Atmospheric fixation by lightning.
3. Assimilation: Plants absorb nitrogen compounds from the soil through their roots and use them to synthesize proteins and other essential molecules.
4. Consumption: Animals obtain nitrogen by eating plants or other animals.
5. Ammonification: When plants and animals die, or animals excrete waste, decomposer bacteria and fungi break down the nitrogenous organic matter into ammonia ($\text{NH}_3$) or ammonium compounds ($\text{NH}_4^+$).
6. Nitrification: Specific bacteria in the soil convert ammonium compounds first into nitrites ($\text{NO}_2^-$) and then into nitrates ($\text{NO}_3^-$). Nitrates are the form most readily absorbed by plants.
7. Denitrification: Other bacteria in the soil convert some of the nitrates back into nitrogen gas ($\text{N}_2$), which is then released back into the atmosphere.

This cyclical process ensures that the total amount of nitrogen in the atmosphere remains relatively constant, while nitrogen is made available for living organisms.

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer: