Microorganisms

Beyond the plants and animals we commonly see, there exist other living organisms that are usually invisible to our naked eye. These tiny life forms are known as microorganisms or microbes.

For example, moist bread left exposed during the rainy season might develop greyish-white patches. Looking at these patches through a magnifying glass reveals small, dark, rounded structures, which are types of microorganisms.

Activities observing drops of water from soil or a pond under a microscope show a world filled with numerous tiny organisms, though not all of them are strictly classified as microbes.

Microorganisms are exceptionally small, making them undetectable without magnification. Some, like the fungus on bread, can be observed with a magnifying glass, but most require a microscope to be seen.

Microorganisms are categorised into four main groups:

• Bacteria
• Fungi
• Protozoa
• Certain Algae

Viruses are also microscopic but are distinct from other microorganisms. They can only reproduce inside the cells of a host organism, which could be a bacterium, plant, or animal cell.

Many common illnesses are caused by microorganisms:

• Viruses: Common cold, influenza (flu), most coughs, serious diseases like polio and chicken pox.
• Protozoa: Diseases like dysentery and malaria.
• Bacteria: Diseases like typhoid and tuberculosis (TB).

Where Do Microorganisms Live?

Microorganisms are found in a vast array of environments. They can be single-celled (like bacteria, some algae, and protozoa) or multicellular (like many algae and fungi).

They are capable of surviving in extreme conditions, ranging from freezing cold climates and hot springs to dry deserts and marshy wetlands. They are also found living inside the bodies of animals, including humans.

Some microbes live and grow on other organisms, while others exist independently and freely in nature.

Microorganisms And Us

Microorganisms interact with us in significant ways. While some are beneficial and useful, others are harmful and can cause diseases.

Friendly Microorganisms

Microbes are used for various beneficial purposes, including in food preparation, commercial production, medicine, increasing soil fertility, and environmental cleaning.

Making Of Curd And Bread

The transformation of milk into curd is a process driven by microorganisms. Specifically, the bacterium Lactobacillus is crucial for curd formation. It multiplies in milk, converting it into curd.

Bacteria are also involved in making other food products like cheese, pickles, and are important ingredients in dishes like idlis and bhaturas where fermentation by bacteria and yeast helps the batter rise.

For bread and cakes, yeast is commonly used. Yeast is a fungus that reproduces very quickly. During respiration, yeast produces carbon dioxide gas. These gas bubbles cause dough to expand and rise, which is fundamental to the baking process.

The process by which yeast converts sugar into alcohol is called fermentation. This discovery is credited to Louis Pasteur in 1857.

Commercial Use Of Microorganisms

Microorganisms are employed on a large scale for the commercial production of substances like alcohol, wine, and acetic acid (vinegar).

Yeast is particularly important for the commercial production of alcohol and wine. It is grown on natural sugars found in grains such as barley, wheat, rice, or in crushed fruit juices to carry out fermentation.

Medicinal Use Of Microorganisms

When you are ill, a doctor might prescribe antibiotics. These medicines, derived from microorganisms like bacteria and fungi, work by killing or inhibiting the growth of disease-causing microbes.

Common antibiotics include Streptomycin, tetracycline, and erythromycin. They are produced by cultivating specific microorganisms and are used to treat a range of diseases.

Antibiotics are also mixed into animal feed (livestock and poultry) to prevent microbial infections and are used to control diseases in plants.

Important precautions when taking antibiotics:

• Only take antibiotics on the advice of a qualified doctor.
• Always complete the entire course prescribed by the doctor.
• Taking antibiotics when they are not needed or in incorrect doses can make them less effective in the future.
• Unnecessary antibiotic use can also kill beneficial bacteria naturally present in the body.
• Antibiotics are not effective against viral diseases like the common cold and flu.

The discovery of penicillin, the first antibiotic, by Alexander Fleming in 1929, occurred when he observed that mould growing on a culture plate prevented bacterial growth.

Vaccine

Vaccination helps protect children and adults from various diseases.

When disease-causing microbes enter the body, the body's immune system produces substances called antibodies to fight them. The body retains the memory of how to fight that specific microbe.

A vaccine works by introducing weakened or dead disease-causing microbes into a healthy body. The body responds by producing appropriate antibodies to fight these introduced microbes. These antibodies remain in the body, providing protection against future invasion by the live disease-causing microbes.

Vaccination is effective in preventing diseases such as cholera, tuberculosis, smallpox, and hepatitis.

Edward Jenner discovered the vaccine for smallpox in 1798. A worldwide effort using vaccination has led to the near eradication of smallpox globally.

Modern vaccines are produced on a large scale using microorganisms to protect humans and animals from numerous infectious diseases. Programmes like the Pulse Polio Programme involve administering oral polio vaccine drops to children.

Increasing Soil Fertility

Certain microorganisms in the soil contribute to increased soil fertility by fixing atmospheric nitrogen. Bacteria like Rhizobium live in the root nodules of leguminous plants (like beans and peas) and convert nitrogen gas from the air into usable nitrogen compounds in the soil. Some blue-green algae (cyanobacteria) also perform nitrogen fixation.

These microbes are often referred to as biological nitrogen fixers.

Cleaning The Environment

Microorganisms play a crucial role in decomposing dead organic matter. When plant waste, vegetable peels, animal remains, or faeces are left in a suitable environment, microorganisms break them down into simpler, harmless substances.

This decomposition process converts organic waste into valuable manure, releasing nutrients back into the soil for plants to use again. This is why piles of dead organic matter eventually disappear from the ground.

Materials like plastic bags, glass bottles, and certain plastic toys do not decompose through microbial action.

By breaking down dead organic waste and converting harmful, smelly substances into simpler ones, microorganisms effectively help in cleaning up the environment and recycling nutrients.

Harmful Microorganisms

While many microbes are beneficial, some are detrimental in several ways. They can cause diseases in humans, plants, and animals, and also spoil food, clothing, and leather.

Microorganisms that cause diseases are known as pathogens.

Disease Causing Microorganisms In Humans

Pathogens can enter the human body through various routes: the air we breathe, the water we drink, or the food we eat. They can also spread through direct contact with an infected person or be transmitted by animals or insects acting as carriers.

Diseases that spread from an infected person to a healthy person through air, water, food, or direct contact are called communicable diseases. Examples include cholera, common cold, chicken pox, and tuberculosis.

When a person with a common cold sneezes, they release tiny droplets containing thousands of viruses into the air. A healthy person inhaling this contaminated air can get infected.

Certain insects and animals serve as carriers, transmitting disease-causing microbes. A housefly can pick up pathogens from garbage or animal waste and transfer them to uncovered food, making the person who eats that food ill. Therefore, keeping food covered is essential.

Another well-known carrier is the female Anopheles mosquito, which carries the protozoan parasite responsible for malaria (Plasmodium). The female Aedes mosquito is a carrier for the dengue virus.

Preventing mosquitoes from breeding is key to controlling the spread of malaria and dengue. Mosquitoes lay their eggs in water. Thus, avoiding water collection in coolers, tyres, flower pots, and other areas helps prevent mosquito breeding. Keeping surroundings clean and dry reduces mosquito populations.

Here is a table outlining some common human diseases caused by microorganisms:

Disease	Causative Microorganism	Mode of Transmission	Preventive Measures (General)
Tuberculosis (TB)	Bacteria	Air	Keep patient isolated; keep personal belongings separate; vaccination (BCG) at suitable age.
Measles	Virus	Air	Keep patient isolated; vaccination at suitable age.
Chicken Pox	Virus	Air/Contact	Keep patient isolated; vaccination at suitable age.
Polio	Virus	Air/Water	Keep patient isolated; Vaccination (Polio drops/injection).
Cholera	Bacteria	Water/Food	Maintain personal hygiene and sanitation; consume properly cooked food and boiled water; vaccination.
Typhoid	Bacteria	Water	Maintain personal hygiene and sanitation; consume properly cooked food and boiled water; vaccination.
Hepatitis A	Virus	Water	Drink boiled drinking water; vaccination.
Malaria	Protozoa	Mosquito (Female Anopheles)	Use mosquito nets and repellents; spray insecticides; control mosquito breeding by preventing water collection.

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

Disease Causing Microorganisms In Animals

Microorganisms cause diseases not just in humans and plants, but also in various animals. For instance, Anthrax is a severe disease affecting both humans and cattle, caused by a bacterium (Bacillus anthracis). Foot and mouth disease in cattle is caused by a virus.

Disease Causing Microorganisms In Plants

Several microorganisms inflict diseases on important crop plants such as wheat, rice, potato, sugarcane, orange, and apple. These plant diseases lead to a reduction in crop yield and quality.

Here is a table showing some common plant diseases caused by microorganisms:

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

These plant diseases can often be controlled by applying specific chemicals that kill the responsible microbes.

Food Poisoning

Consuming food that has been spoiled by certain microorganisms can lead to food poisoning. Microbes growing on food can produce poisonous substances (toxins). These toxins, when ingested, can cause severe illness and, in some cases, even be fatal.

Preventing food from being spoiled by microorganisms is therefore crucial for health.

Food Preservation

We need methods to prevent food spoilage, which is caused by microorganisms. Spoiled food can be identified by unpleasant smell, bad taste, and changes in colour and texture. Food spoilage is essentially a chemical reaction caused by microbial growth.

Various methods are used to preserve food at home and on a larger scale to protect it from microbial attack.

Chemical Method

Certain chemicals commonly used to inhibit microbial growth are called preservatives. Common examples include salts and edible oils.

Adding salt or acid preservatives to pickles prevents microbial spoilage. Chemical preservatives like sodium benzoate and sodium metabisulphite are used in jams and squashes to prevent their spoilage.

Preservation By Common Salt

Using common salt for preservation has been a practice for centuries, especially for meat and fish. Covering meat and fish with dry salt creates an environment that hinders bacterial growth. Salting is also used for preserving fruits like amla, raw mangoes, and tamarind.

Preservation By Sugar

Jams, jellies, and squashes are preserved using sugar. Sugar acts by reducing the water content (moisture) in the food, making it difficult for spoilage-causing bacteria to grow.

Preservation By Oil And Vinegar

Using oil and vinegar in food items, particularly pickles, prevents spoilage. Bacteria cannot survive in the acidic environment created by vinegar or the low moisture environment created with oil.

Heat And Cold Treatments

Applying heat or cold significantly reduces microbial activity.

• Heating: Boiling milk kills many microorganisms present in it before storage or consumption.
• Cooling: Storing food in a refrigerator at low temperatures slows down or stops the growth of microbes.

Pasteurisation is a specific heat treatment for milk (and other liquids). Milk is heated to about $70^\circ\text{C}$ for $15$ to $30$ seconds, and then rapidly cooled (chilled) and stored. This process prevents the growth of microbes and allows pasteurised milk to be consumed without boiling. This method was developed by Louis Pasteur.

Storage And Packing

Storing food in sealed, airtight packets also helps prevent microbial attack. This method is commonly used for preserving dry fruits and even some vegetables these days, as it prevents contact with air, moisture, and microbes.

Nitrogen Fixation

Nitrogen is an essential element for plant growth. The atmosphere contains a large amount of nitrogen gas ($78\%$). However, plants cannot directly use atmospheric nitrogen in its gaseous form.

Certain microorganisms, particularly bacteria like Rhizobium, play a vital role in converting atmospheric nitrogen into forms usable by plants. This process is called nitrogen fixation.

Rhizobium bacteria live in a symbiotic relationship within the root nodules of leguminous plants (plants that bear pulses, like beans and peas). In this relationship, the bacteria receive shelter and nutrients from the plant, and in return, they provide the plant with usable nitrogen compounds they fix from the air.

Nitrogen fixation can also occur naturally, albeit to a lesser extent, through atmospheric processes like lightning, which converts nitrogen and oxygen into oxides of nitrogen.

Despite these processes, the total amount of nitrogen in the atmosphere remains relatively constant due to the nitrogen cycle.

Nitrogen Cycle

Nitrogen is a fundamental component of important biological molecules in all living organisms, including proteins, chlorophyll, nucleic acids (DNA, RNA), and vitamins. The atmospheric nitrogen ($78\%$) is not directly usable by most life forms.

The Nitrogen Cycle describes the continuous movement of nitrogen between the atmosphere, soil, water, and living organisms. This cycle ensures that nitrogen is available in different forms and maintains its overall balance in the environment.

Key steps in the Nitrogen Cycle include:

1. Nitrogen Fixation: Atmospheric nitrogen gas ($N_2$) is converted into usable nitrogen compounds (like ammonia) by certain bacteria (e.g., Rhizobium, Azotobacter) and blue-green algae (cyanobacteria) present in soil or root nodules. Lightning also contributes to this process.
2. Nitrification: Ammonia is converted into nitrates by other soil bacteria (nitrifying bacteria) in two steps: ammonia to nitrites, and then nitrites to nitrates. Nitrates are the primary form of nitrogen absorbed by plants.
3. Assimilation: Plants absorb nitrogen compounds (mainly nitrates) from the soil through their roots and use them to synthesize proteins and other organic molecules. Animals obtain nitrogen by eating plants or other animals.
4. Ammonification: When plants and animals die, and when animals excrete waste, decomposer microorganisms (bacteria and fungi) break down the organic nitrogen compounds in dead matter and waste products, converting them back into ammonia in the soil.
5. Denitrification: Certain soil bacteria (denitrifying bacteria) convert some of the nitrates back into nitrogen gas ($N_2$), which is then released back into the atmosphere. This step completes the cycle.

This continuous cycling process ensures that the percentage of nitrogen in the atmosphere remains relatively stable.

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