Unit 3: Biology and Human Welfare - Chapter 2: Microbes in Human Welfare

3.2 Microbes in Human Welfare

Microbes (microorganisms) are microscopic organisms that are ubiquitous and play crucial roles in various aspects of human welfare, both beneficial and harmful. This chapter focuses on their beneficial applications.

1. Microbes in Household Products

• Curd:

  • Microbe: Lactobacillus (Lactic Acid Bacteria - LAB).
  • Process: LAB grow in milk and convert lactose sugar into lactic acid. Lactic acid coagulates and partially digests the milk proteins, forming curd.
  • Benefits: Increases nutritional value by increasing Vitamin B12. Checks disease-causing microbes in the stomach.

• Bread:

  • Microbe: Saccharomyces cerevisiae (Baker's yeast).
  • Process: Yeast ferments sugars in dough, producing CO2 gas, which causes the dough to rise (leavening).

• Cheese:

  • Process: Involves partial degradation of milk by microbes.
  • Swiss Cheese: Large holes are due to the production of a large amount of CO2 by a bacterium called Propionibacterium shermanii.
  • Roquefort Cheese: Ripened by growing specific fungi on them, which gives them a characteristic flavor.

• Dosa and Idli: Dough is fermented by bacteria, producing CO2.

2. Microbes in Industrial Uses

Microbes are used to produce a variety of industrial products on a large scale in bioreactors.

• Fermented Beverages:

  • Microbe: Saccharomyces cerevisiae (Brewer's yeast).
  • Products: Ethanol (alcohol), wine, beer, whisky, brandy, rum.
  • Process: Fermentation of malted cereals and fruit juices.
  • Wine and beer are produced without distillation, while whisky, brandy, and rum are produced by distillation of the fermented broth.

• Antibiotics:

  • Definition: Chemical substances produced by some microbes that can kill or retard the growth of other (disease-causing) microbes.
  • Penicillin:
    • Discovery: Alexander Fleming (1928) observed that Penicillium notatum inhibited the growth of Staphylococcus bacteria.
    • Full Potential: Ernst Chain and Howard Florey (1940s) established its full potential as an effective antibiotic. Awarded Nobel Prize (1945) jointly with Fleming.
    • Uses: Used to treat various bacterial infections (e.g., pneumonia, bronchitis, diphtheria).

• Organic Acids:

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

• Enzymes:

  • Lipases: Used in detergent formulations to remove oily stains from laundry.
  • Pectinases and Proteases: Used in bottled fruit juices to make them clear.
  • Streptokinase: Produced by the bacterium Streptococcus. Used as a 'clot buster' for removing clots from the blood vessels of patients who have undergone myocardial infarction (heart attack).

• Bioactive Molecules:

  • Cyclosporin A: Produced by the fungus Trichoderma polysporum. Used as an immunosuppressive agent in organ transplant patients.
  • Statins: Produced by the yeast Monascus purpureus. Used as blood-cholesterol lowering agents (act by competitively inhibiting the enzyme responsible for cholesterol synthesis).

3. Microbes in Sewage Treatment

Sewage (municipal wastewater) contains large amounts of organic matter and pathogenic microbes. It cannot be discharged directly into natural water bodies. Sewage treatment plants (STPs) use microbes to treat sewage.

• Primary Treatment (Physical Treatment):

  • Involves physical removal of large and small particles from sewage through sequential filtration and sedimentation.
  • Steps:
    1. Filtration: Floating debris is removed by sequential filtration.
    2. Grit Removal: Grit (soil and small pebbles) is removed by sedimentation in settling tanks.
  • The solid waste (primary sludge) settles down, and the supernatant (effluent) is taken for secondary treatment.

• Secondary Treatment (Biological Treatment):

  • The primary effluent is passed into large aeration tanks, where it is constantly agitated mechanically and air is pumped into it.
  • Steps:
    1. Aeration: This allows vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh-like structures).
    2. BOD Reduction: These microbes consume the major part of the organic matter in the effluent, significantly reducing the Biochemical Oxygen Demand (BOD) of the sewage. BOD is a measure of the organic matter present in the water.
    3. Settling: Once the BOD is reduced, the effluent is passed into a settling tank where the bacterial flocs are allowed to sediment (activated sludge).
    4. Inoculation: A small part of the activated sludge is pumped back into the aeration tank to serve as an inoculum.
    5. Anaerobic Sludge Digesters: The remaining major part of the activated sludge is pumped into anaerobic sludge digesters, where other kinds of bacteria (anaerobic bacteria) grow and digest the bacteria and fungi in the sludge. During this digestion, a mixture of gases (biogas) like methane, H2S, and CO2 is produced.
  • The effluent from the secondary treatment plant is generally released into natural water bodies.

4. Microbes in Biogas Production

Biogas is a mixture of gases (primarily methane, with CO2 and H2S) produced by the anaerobic breakdown of organic matter by microbes. It is used as fuel.

• Methanogens:

  • A group of anaerobic bacteria (e.g., Methanobacterium) that produce methane.
  • They are found in the anaerobic sludge during sewage treatment and in the rumen of cattle (where they help in the digestion of cellulose and produce methane).

• Biogas Plant Structure:

  • Consists of a concrete tank (10-15 feet deep) in which bio-wastes (dung, agricultural waste) are collected and a slurry of dung and water is fed.
  • A floating cover is placed over the slurry, which rises as biogas is produced.
  • The biogas is drawn out through an outlet pipe and used for cooking and lighting.
  • The spent slurry is removed through another outlet and used as fertilizer.

5. Microbes as Biocontrol Agents

Biocontrol refers to the use of biological methods for controlling plant diseases and pests, reducing reliance on chemical pesticides and insecticides.

• Examples:
  • Bt (Bacillus thuringiensis):
    • Used to control insect pests (e.g., butterfly caterpillars).
    • Spores of Bt are mixed with water and sprayed onto vulnerable plants. The insect larvae ingest the spores, and the toxin released in their gut kills them.
    • Genetically engineered plants (e.g., Bt cotton) express the Bt toxin gene, making them resistant to insect pests.
  • Trichoderma:
    • A free-living fungus commonly found in root ecosystems.
    • Effective biocontrol agent against several plant pathogens.
  • Nucleopolyhedrovirus (NPV):
    • Viruses belonging to the genus Nucleopolyhedrovirus are excellent candidates for species-specific, narrow-spectrum insecticidal applications.
    • They do not harm plants, mammals, birds, fish, or even non-target insects.
    • Beneficial in Integrated Pest Management (IPM) programs.

6. Microbes as Biofertilizers

Biofertilizers are organisms that enrich the nutrient quality of the soil.

• Bacteria:

  • Rhizobium: Symbiotic bacterium that lives in the root nodules of leguminous plants. Fixes atmospheric nitrogen into organic forms, which can be used by the plant.
  • Azotobacter and Azospirillum: Free-living nitrogen-fixing bacteria that enrich the nitrogen content of the soil.

• Cyanobacteria (Blue-green algae):

  • Autotrophic microbes that fix atmospheric nitrogen (e.g., Anabaena, Nostoc).
  • Important biofertilizers in paddy fields.

• Mycorrhiza:

  • Symbiotic association between fungi and the roots of higher plants.
  • The fungal symbiont absorbs phosphorus from the soil and passes it to the plant.
  • Also provides resistance to root-borne pathogens, increases tolerance to salinity and drought, and enhances overall plant growth.

Integrated Pest Management (IPM) and Chemical Pesticide Alternatives

Integrated Pest Management (IPM) is an ecological approach to pest control that combines various methods (biological, cultural, physical, chemical) to manage pest populations in a way that minimizes economic damage and reduces risks to human health and the environment.

• Alternatives to Chemical Pesticides:
  • Biocontrol agents: As discussed above (Bt, Trichoderma, NPV).
  • Crop Rotation: Breaks pest life cycles.
  • Resistant Varieties: Using crop varieties that are naturally resistant to pests.
  • Biological Pesticides: Derived from natural materials like animals, plants, bacteria, and certain minerals.

This approach promotes a holistic understanding of the agro-ecosystem and aims for sustainable agriculture.