Microbes in Human WelfareClass 12 Biology Notes

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Microbes in Household Products

While we often think of microbes as germs, many are incredibly useful and a part of our daily lives, especially in the kitchen!

Curd Production

A common example of a microbe at work is the conversion of milk into curd.

• Microbe Used: Lactic Acid Bacteria (LAB), such as Lactobacillus.
• Process:
  1. A small amount of curd, which acts as a starter or inoculum, is added to fresh, warm milk. This starter contains millions of LAB.
  2. At a suitable temperature, the LAB multiply.
  3. During their growth, they produce lactic acid. This acid causes the milk proteins to coagulate (clump together) and partially digest, turning the milk into curd.
• Benefits:
  • The process increases the nutritional value by raising the amount of vitamin $B_{12}$.
  • In our stomachs, LAB are beneficial as they help check the growth of disease-causing microbes.

Fermented Dough and Foods

The puffed-up texture of many of our favorite foods is due to gas produced by microbes.

• Dosa and Idli: The dough used for these foods is fermented by bacteria. The fermentation process produces carbon dioxide ($CO_2$) gas, which causes the dough to rise and gives it a "puffed-up" appearance.
• Bread: The dough for bread is fermented using baker's yeast (Saccharomyces cerevisiae). This yeast also produces $CO_2$, which makes the bread light and spongy.
• 'Toddy': This is a traditional drink from southern India, made by fermenting the sap collected from palm trees.
• Other Foods: Microbes are also used to ferment fish, soyabean, and bamboo shoots to create various food products.

Cheese Making

Cheese is one of the oldest food items made using microbes. The specific microbe used determines the final texture, flavor, and taste of the cheese.

• Swiss Cheese: The characteristic large holes in Swiss cheese are created by large amounts of $CO_2$ gas produced by a bacterium called Propionibacterium sharmanii.
• Roquefort Cheese: This type of cheese is ripened by growing a specific fungus on it, which imparts a distinct flavor.

Microbes in Industrial Products

Microbes are like tiny factories, used on a massive scale to produce valuable products for humanity. This large-scale production is often done in very large vessels called fermentors.

Fermented Beverages

Yeast has been used for thousands of years to produce alcoholic beverages.

• Microbe Used: Saccharomyces cerevisiae, also known as brewer's yeast.
• Process: This yeast ferments malted cereals and fruit juices to produce ethanol (alcohol).
• Types of Drinks: The final product depends on the raw material and the processing method.
  • Without Distillation: Wine and beer are produced without distilling the fermented liquid.
  • With Distillation: Whisky, brandy, and rum are produced by distilling the fermented broth, which increases the alcohol concentration.

Antibiotics

The discovery of antibiotics was a major breakthrough in 20th-century medicine.

• Antibiotics are chemical substances produced by some microbes that can kill or stop the growth of other, harmful (disease-causing) microbes. The name means 'against life', referring to the life of pathogens.
• Penicillin: The First Antibiotic:
  • It was a chance discovery by Alexander Fleming. He noticed that a mould, Penicillium notatum, growing on one of his culture plates was preventing the growth of Staphylococci bacteria around it.
  • He named the chemical produced by the mould Penicillin.
  • Its full potential as an effective antibiotic was later established by Ernest Chain and Howard Florey.
  • This discovery was so important that Fleming, Chain, and Florey were awarded the Nobel Prize in 1945.
• Impact: Antibiotics have drastically improved our ability to treat deadly diseases like plague, whooping cough (kali khansi), diphtheria (gal ghotu), and leprosy (kusht rog).

Chemicals, Enzymes, and other Bioactive Molecules

Microbes are used for the commercial production of various organic acids, enzymes, and other special molecules.

Organic Acid Producers:

• Citric Acid: Aspergillus niger (a fungus)
• Acetic Acid: Acetobacter aceti (a bacterium)
• Butyric Acid: Clostridium butylicum (a bacterium)
• Lactic Acid: Lactobacillus (a bacterium)

Enzymes and their Uses:

• Lipases: Used in detergent formulations to help remove oily stains from laundry.
• Pectinases and Proteases: Used to clarify bottled fruit juices, making them clearer than homemade juices.
• Streptokinase: Produced by the bacterium Streptococcus. It is used as a 'clot buster' to remove blood clots from the vessels of patients who have suffered a heart attack.

Other Bioactive Molecules:

• Cyclosporin A: Produced by the fungus Trichoderma polysporum. It is used as an immunosuppressive agent in organ transplant patients to prevent their body from rejecting the new organ.
• Statins: Produced by the yeast Monascus purpureus. They are used as blood-cholesterol lowering agents. They work by competitively inhibiting the enzyme that is responsible for making cholesterol in the body.

Microbes in Sewage Treatment

Cities and towns generate huge quantities of wastewater, or sewage, which contains human excreta, organic matter, and many disease-causing microbes. This water cannot be released directly into rivers and streams. It must first be treated in Sewage Treatment Plants (STPs) to make it less polluting. This treatment relies heavily on heterotrophic microbes.

The treatment is carried out in two main stages:

Primary Treatment

This is a physical process designed to remove large and small solid particles.

1. Filtration: Floating debris is removed by passing the sewage through screens.
2. Sedimentation: The sewage is then held in a tank where grit (soil and small pebbles) settles down.
3. The solids that settle form the primary sludge, and the liquid part, called the effluent, is taken for secondary treatment.

Secondary Treatment or Biological Treatment

This stage uses microbes to clean the water.

1. Aeration: The primary effluent is pumped into large aeration tanks, where it is constantly mixed and air is pumped in.
2. Floc Formation: This allows useful aerobic microbes to grow vigorously, forming masses of bacteria and fungal filaments called flocs.
3. BOD Reduction: As these microbes grow, they consume the organic matter present in the effluent. This significantly reduces the Biochemical Oxygen Demand (BOD) of the water.
   • BOD is a measure of the amount of oxygen that would be needed by bacteria to oxidise all the organic matter in one liter of water.
   • A higher BOD means the water is more polluted. The goal of sewage treatment is to lower the BOD.
4. Sedimentation: Once the BOD is low, the effluent is moved to a settling tank. Here, the bacterial 'flocs' are allowed to settle down. This sediment is called activated sludge.
5. Inoculum and Digestion:
   • A small part of the activated sludge is pumped back into the aeration tank to serve as an inoculum (starter) for the next batch.
   • The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, anaerobic bacteria digest the bacteria and fungi in the sludge, producing a mixture of gases like methane ($CH_4$), hydrogen sulfide ($H_2S$), and carbon dioxide ($CO_2$). This gas mixture is called biogas and can be used as fuel.
6. Release: The treated effluent from the secondary treatment plant is now clean enough to be released into natural water bodies like rivers.

Microbes in Production of Biogas

Biogas is a mixture of flammable gases, predominantly methane, produced by microbial activity. It is a valuable source of fuel.

• The Microbes: Certain anaerobic bacteria, collectively called methanogens (e.g., Methanobacterium), produce large amounts of methane along with $CO_2$ and $H_2$ when they break down cellulosic material.
• Where are they found?
  • In the anaerobic sludge during sewage treatment.
  • In the rumen (a part of the stomach) of cattle, where they help digest the cellulose in the cattle's food.
• Gobar Gas: Because methanogens are present in the rumen, cattle dung (gobar) is rich in these bacteria. This dung can be used to generate biogas, commonly called gobar gas.

A Typical Biogas Plant

1. A concrete tank (10-15 feet deep) is used to collect bio-wastes and a slurry (a mix of dung and water) is fed into it.
2. A floating cover is placed over the slurry. As the methanogens digest the waste and produce gas, the cover rises.
3. An outlet pipe connected to the cover supplies the biogas to nearby houses for cooking and lighting.
4. The spent slurry is removed through another outlet and can be used as an excellent fertiliser.

Microbes as Biocontrol Agents

Biocontrol is the use of biological methods to control plant pests and diseases, offering an alternative to toxic chemical insecticides and pesticides that pollute our environment. The idea is not to eradicate pests, but to keep them at manageable levels using natural predators and pathogens.

Examples of Biocontrol

• Insects as Predators: The common Ladybird beetle (with red and black markings) is very useful for controlling aphids, and Dragonflies are used to get rid of mosquitoes.
• Bacteria as Biocontrol Agents:
  • The bacterium Bacillus thuringiensis (Bt) is used to control butterfly caterpillars.
  • Dried spores of Bt are sold in sachets, mixed with water, and sprayed onto vulnerable plants like brassicas and fruit trees.
  • When the insect larvae eat the plant, the toxin is released in their gut, and they are killed. This method is specific and does not harm other insects.
  • Using genetic engineering, scientists have even introduced the Bt toxin gene directly into plants, creating pest-resistant crops like Bt-cotton.
• Fungi as Biocontrol Agents:
  • Fungi of the genus Trichoderma are free-living in root ecosystems. They are effective biocontrol agents against several plant pathogens.
• Viruses as Biocontrol Agents:
  • Baculoviruses, particularly those from the genus Nucleopolyhedrovirus, are pathogens that attack insects and other arthropods.
  • They are excellent candidates for species-specific, narrow-spectrum insecticidal applications. This means they only target specific pests and have no negative effects on plants, mammals, birds, fish, or even beneficial insects. This makes them perfect for Integrated Pest Management (IPM) programs.

Microbes as Biofertilisers

The overuse of chemical fertilisers pollutes the environment. A sustainable alternative is organic farming, which uses biofertilisers.

• Biofertilisers are organisms that enrich the nutrient quality of the soil. The main sources are bacteria, fungi, and cyanobacteria.

Bacteria as Biofertilisers

These bacteria enrich the nitrogen content of the soil.

• Symbiotic: Rhizobium bacteria form a symbiotic association with the roots of leguminous plants, creating nodules. They fix atmospheric nitrogen into organic forms that the plant can use as a nutrient.
• Free-living: Bacteria like Azospirillum and Azotobacter live freely in the soil and can also fix atmospheric nitrogen.

Fungi as Biofertilisers

Fungi can form symbiotic associations with plant roots, known as mycorrhiza.

• Example: Many members of the genus Glomus form mycorrhiza.
• Function: The fungal partner absorbs phosphorus from the soil and passes it to the plant. In return, the fungus gets nutrients from the plant.
• Other Benefits: Plants with mycorrhizal associations also show resistance to root-borne pathogens, tolerance to salinity and drought, and overall better growth.

Cyanobacteria as Biofertilisers

Cyanobacteria (also called blue-green algae) are autotrophic microbes that can fix atmospheric nitrogen.

• Examples: Anabaena, Nostoc, Oscillatoria.
• Use: In paddy (rice) fields, cyanobacteria serve as an important biofertiliser. They also add organic matter to the soil, increasing its fertility.