Microbes in Human Welfare

Microbes are fundamental components of Earth’s biological systems, found in diverse environments including soil, water, air, inside living organisms, and even extreme locations like thermal vents (up to 100°C), deep soil, thick snow layers, and highly acidic environments. They are incredibly diverse, encompassing protozoa, bacteria, fungi, microscopic animal and plant viruses, viroids, and prions.

While some microbes cause diseases in humans, animals, and plants, many are incredibly beneficial. This chapter highlights their significant contributions to human welfare.

Microbes in Household Products

Microbes and their products are used daily in household applications.

Curd Production

• Microbes: Lactobacillus and other Lactic Acid Bacteria (LAB).
• Process: LAB grow in milk, producing acids that coagulate and partially digest milk proteins, converting it to curd.
• Inoculum: A small amount of curd (starter) contains millions of LAB, which multiply at suitable temperatures.
• Benefits: Improves nutritional quality by increasing Vitamin B12; LAB also play a beneficial role in the stomach by checking disease-causing microbes.

Dough Fermentation

• Examples: Dough for dosa, idli, and bread.
• Effect: Puffed-up appearance is due to the production of CO2 gas.
• Microbe for bread: Baker’s yeast (Saccharomyces cerevisiae).

Traditional Foods and Drinks

• Toddy: A traditional South Indian drink made by fermenting sap from palms.
• Fermented foods: Microbes are used to ferment fish, soyabean, and bamboo-shoots.

Cheese Production

• One of the oldest food items involving microbes.
• Specificity: Different varieties are known by their characteristic texture, flavour, and taste, which comes from the specific microbes used.
• Examples:
  • Swiss cheese: Large holes are due to the production of a large amount of CO2 by the bacterium Propionibacterium sharmanii.
  • Roquefort cheese: Ripened by growing specific fungi on them, which imparts a particular flavour.

Microbes in Industrial Products

Microbes are used on an industrial scale to synthesize valuable products such as beverages, antibiotics, chemicals, and enzymes. This requires growing them in very large vessels called fermentors.

Fermented Beverages

• Microbes: Primarily yeasts, especially Saccharomyces cerevisiae (also known as brewer’s yeast).
• Process: Used for fermenting malted cereals and fruit juices to produce ethanol.
• Types of Beverages:
  • Without distillation: Wine and beer.
  • With distillation: Whisky, brandy, and rum (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. The term “antibiotic” means ‘against life’ in the context of disease-causing organisms, but ‘pro life’ for human beings.
• Penicillin:
  • First antibiotic discovered: A chance discovery by Alexander Fleming.
  • Discovery Process: Fleming observed a mould (Penicillium notatum) growing in an unwashed culture plate, around which Staphylococci bacteria could not grow. He named the chemical produced by the mould “Penicillin”.
  • Full Potential: Established much later by Ernest Chain and Howard Florey.
  • Impact: Extensively used to treat American soldiers wounded in World War II.
  • Nobel Prize: Awarded to Fleming, Chain, and Florey in 1945 for this discovery.
• Impact on Human Health: Greatly improved capacity to treat deadly diseases like plague, whooping cough, diphtheria, and leprosy.

Chemicals, Enzymes and other Bioactive Molecules

Microbes are commercially and industrially used to produce various chemicals and bioactive 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).
• Ethanol: Commercial production using yeast (Saccharomyces cerevisiae).
• Enzymes:
  • Lipases: Used in detergent formulations to remove oily stains from laundry.
  • Pectinases and Proteases: Used to clarify bottled fruit juices, making them clearer than homemade ones.
  • Streptokinase: Produced by the bacterium Streptococcus (and modified by genetic engineering), used as a ‘clot buster’ to remove clots from blood vessels of heart attack patients (myocardial infarction).
• Bioactive Molecules:
  • Cyclosporin A: An immunosuppressive agent used in organ-transplant patients, produced by the fungus Trichoderma polysporum.
  • Statins: Blood-cholesterol lowering agents, produced by the yeast Monascus purpureus. They act by competitively inhibiting the enzyme responsible for cholesterol synthesis.

Microbes in Sewage Treatment

Large quantities of waste water (municipal waste-water or sewage) are generated daily in cities and towns, containing significant amounts of organic matter and pathogenic microbes. This sewage cannot be discharged directly into natural water bodies.

Sewage is treated in Sewage Treatment Plants (STPs) to reduce its polluting potential, primarily using heterotrophic microbes naturally present in the sewage. Treatment occurs in two main stages:

Primary Treatment (Physical)

• Process: Involves physical removal of large and small particles from sewage through filtration and sedimentation.
  • Filtration: Removes floating debris.
  • Sedimentation: Removes grit (soil and small pebbles).
• Products: Solids that settle form the primary sludge, and the supernatant liquid is the effluent.
• Next Step: The effluent from the primary settling tank proceeds to secondary treatment.

Secondary Treatment (Biological)

• Aeration Tanks: Primary effluent is passed into large aeration tanks where it is constantly agitated mechanically, and air is pumped in.
• Flocs Formation: This promotes the vigorous growth of useful aerobic microbes, forming flocs (masses of bacteria associated with fungal filaments, forming mesh-like structures).
• Organic Matter Consumption: These microbes consume the major part of the organic matter in the effluent, significantly reducing the Biochemical Oxygen Demand (BOD).
• BOD Significance:
  • BOD refers to the amount of oxygen that would be consumed if all organic matter in one litre of water were oxidized by bacteria.
  • It measures the rate of oxygen uptake by microorganisms and indirectly quantifies the organic matter in water.
  • Higher BOD indicates greater polluting potential of the waste water.
• Settling Tank: Once the BOD is significantly reduced, the effluent is transferred to a settling tank where the bacterial flocs are allowed to sediment.
• Activated Sludge: This sediment is called activated sludge.
  • A small portion is pumped back into the aeration tank to serve as an inoculum.
  • The remaining major part is pumped into large anaerobic sludge digesters.
• Anaerobic Digestion: In these digesters, other types of bacteria that grow anaerobically digest the bacteria and fungi in the sludge.
  • During this digestion, bacteria produce a mixture of gases, including methane, hydrogen sulphide, and carbon dioxide. These gases form biogas, which is inflammable and can be used as an energy source.
• Discharge: The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.

Microbial treatment is highly effective and has been practiced for over a century; no man-made technology has been able to rival it. To combat increasing pollution due to urbanization and insufficient sewage treatment plants, the Ministry of Environment and Forests has initiated plans like the Ganga Action Plan and Yamuna Action Plan to build more STPs and ensure only treated sewage is discharged into rivers.

Microbes in Production of Biogas

Biogas is a mixture of gases, predominantly methane, produced by microbial activity, and can be used as fuel. The type of gas produced depends on the microbes and the organic substrates they utilize.

• Methanogens: Certain bacteria that grow anaerobically on cellulosic material produce large amounts of methane along with CO2 and H2.
  • Example: Methanobacterium is a common methanogen.
  • Occurrence: These bacteria are found in anaerobic sludge during sewage treatment and in the rumen (part of the stomach) of cattle.
  • Role in Cattle: In the rumen, they help break down cellulose, playing an important role in cattle nutrition.
• Gobar Gas: Cattle excreta (dung or gobar) is rich in these bacteria and can be used for biogas generation, commonly called gobar gas.
• Biogas Plant Structure and Function:
  • Consists of a concrete tank (10-15 feet deep) where bio-wastes and dung slurry are collected.
  • A floating cover is placed over the slurry, which rises as gas is produced by microbial activity.
  • An outlet pipe supplies biogas to nearby houses for cooking and lighting.
  • The spent slurry is removed through another outlet and can be used as fertilizer.
• Prevalence: Biogas plants are more common in rural areas due to the large availability of cattle dung.
• Development in India: The technology for biogas production was mainly developed in India due to efforts by the Indian Agricultural Research Institute (IARI) and Khadi and Village Industries Commission (KVIC).

Microbes as Biocontrol Agents

Biocontrol refers to using biological methods to control plant diseases and pests. This approach offers an alternative to chemical insecticides and pesticides, which are toxic and harmful to humans, animals, and the environment.

Principles of Biological Control in Agriculture

• Organic Farming Philosophy: Emphasizes that biodiversity promotes health and sustainability.
• Pest Management: Instead of eradication, pests are kept at manageable levels through a complex system of checks and balances within a vibrant ecosystem.
• Holistic Approach: Focuses on understanding the intricate interactions between organisms in the field’s fauna and flora.
• Reduced Chemical Dependence: Biocontrol measures significantly reduce the reliance on toxic chemicals and pesticides.
• Understanding Ecosystem: Organic farmers familiarize themselves with predators, pests, their life cycles, feeding patterns, and preferred habitats to develop appropriate biocontrol means.

Examples of Biocontrol Agents

• Insects as Predators:
  • Ladybird beetles (red and black markings): Useful for controlling aphids.
  • Dragonflies: Help get rid of mosquitoes.
• Microbial Biocontrol Agents:
  • Bacillus thuringiensis (Bt): Bacteria available as dried spores.
    • Application: Mixed with water and sprayed onto vulnerable plants (e.g., brassicas, fruit trees).
    • Mechanism: When eaten by insect larvae, a toxin is released in their gut, killing the caterpillars.
    • Specificity: Kills caterpillars but leaves other insects unharmed.
    • Genetic Engineering: Scientists have introduced Bt toxin genes into plants (e.g., Bt-cotton), making them resistant to insect pests.
  • Trichoderma (fungus): Free-living fungi common in root ecosystems.
    • Role: Effective biocontrol agents against several plant pathogens.
  • Baculoviruses: Pathogens that attack insects and other arthropods.
    • Genus: Most used as biological control agents belong to the genus Nucleopolyhedrovirus.
    • Characteristics: Excellent candidates for species-specific, narrow-spectrum insecticidal applications.
    • Safety: Have no negative impacts on plants, mammals, birds, fish, or non-target insects. This makes them desirable for Integrated Pest Management (IPM) programmes or ecologically sensitive areas.

Microbes as Biofertilisers

Biofertilisers are organisms that enrich the nutrient quality of the soil. Their use is crucial in moving towards organic farming and reducing reliance on chemical fertilisers, which contribute significantly to environmental pollution.

The main sources of biofertilisers are bacteria, fungi, and cyanobacteria.

Bacteria

• Rhizobium: Form symbiotic associations (nodules) on the roots of leguminous plants. They fix atmospheric nitrogen into organic forms that plants can use as nutrients.
• Free-living Nitrogen Fixers: Other bacteria can fix atmospheric nitrogen while living freely in the soil.
  • Examples: Azospirillum and Azotobacter.
  • Effect: Enrich the nitrogen content of the soil.

Fungi

• Mycorrhiza: Fungi form symbiotic associations with plants.
  • Example: Many members of the genus Glomus form mycorrhiza.
  • Role: The fungal symbiont absorbs phosphorus from the soil and passes it to the plant.
  • Benefits to Plants: Plants with such associations show increased resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in plant growth and development.

Cyanobacteria (Blue-Green Algae)

• Nature: Autotrophic microbes widely distributed in aquatic and terrestrial environments.
• Nitrogen Fixation: Many can fix atmospheric nitrogen.
  • Examples: Anabaena, Nostoc, Oscillatoria.
• Role in Agriculture: Serve as important biofertilisers, particularly in paddy fields. They also add organic matter to the soil, increasing its fertility.

A number of biofertilisers are commercially available and regularly used by farmers in India to replenish soil nutrients and reduce dependence on chemical fertilisers.