Unit 8: Microbes in Human Welfare
Microbes in Household Products
Microbes and their products are used daily in households. A familiar example is the production of curd from milk using microorganisms.
Lactic Acid Bacteria (LAB)
Bacteria such as Lactobacillus and others commonly called lactic acid bacteria (LAB) grow in milk and convert it to curd.
Definition: Lactic Acid Bacteria (LAB) are bacteria that multiply in milk and produce acids that coagulate and partially digest the milk proteins.
Step-by-step Explanation:
- A small amount of curd (termed inoculum or starter) is added to fresh milk.
- The inoculum contains millions of LAB.
- At suitable temperatures, LAB grow in milk and produce acids.
- The acid coagulates milk proteins, turning milk into curd.
Exam-Oriented Notes:
- LAB improve the nutritional quality of curd by increasing Vitamin B12.
- In the stomach, LAB play a very beneficial role in checking disease-causing microbes.
Other Household Fermentations
Dosa and Idli: The puffed-up appearance of dough is due to the production of carbon dioxide gas during the fermentation process by bacteria.
Toddy: A traditional drink of some parts of southern India is made by fermenting sap from palms.
Bread: Bakers yeast (Saccharomyces cerevisiae) is used for fermenting dough used in making bread.
Microbes in Industrial Products
Even in industry, microbes are used for synthesis of a number of products valuable to human beings. Beverages and antibiotics are some examples.
Fermented Beverages
Microbes, especially yeasts, have been used for centuries for the production of beverages like wine, beer, whisky, brandy or rum.
Definition: Brewer's yeast (Saccharomyces cerevisiae) is used for fermenting malted cereals and fruit juices to produce ethanol.
Depending on the type of raw material used for fermentation and the type of processing (with or without distillation), different alcoholic drinks are obtained.
- Wine and beer are produced without distillation.
- Whisky, brandy, and rum are produced by distillation of the fermented broth.
Antibiotics
Definition: Antibiotics are chemical substances, which are produced by some microbes and can kill or retard the growth of other (disease-causing) microbes.
Anti means against, and biotic means life; together they mean 'against life' (in the context of disease-causing organisms); whereas with reference to human beings they are pro-life.
Key Discovery: Alexander Fleming discovered penicillin from the mould Penicillium notatum while working on Staphylococci bacteria. Its full potential as an effective antibiotic was established later by Ernest Chain and Howard Florey. Penicillin was extensively used to treat American soldiers wounded in World War II.
Chemicals, Enzymes and Other Bioactive Molecules
Microbes are used for commercial and industrial production of certain chemicals like organic acids, alcohols and enzymes.
| Chemical / Molecule | Microbe Name | Commercial Application |
|---|---|---|
| Citric acid | Aspergillus niger | Organic acid production |
| Acetic acid | Acetobacter aceti | Organic acid production |
| Butyric acid | Clostridium butylicum | Organic acid production |
| Lactic acid | Lactobacillus | Organic acid production |
| Ethanol | Saccharomyces cerevisiae | Commercial alcohol production |
| Lipases | -- | Used in detergent formulations and helpful in removing oily stains from laundry |
| Pectinases and Proteases | -- | Used for clarifying bottled juices |
| Streptokinase | Streptococcus | Used as a 'clot buster' for removing clots from the blood vessels of patients who have undergone myocardial infarction (heart attack) |
| Cyclosporin A | Trichoderma polysporum | Used as an immunosuppressive agent in organ transplant patients |
| Statins | Monascus purpureus | Used as blood-cholesterol lowering agents by competitively inhibiting the enzyme responsible for synthesis of cholesterol |
Microbes in Sewage Treatment
Municipal waste-water is also called sewage. It contains large amounts of organic matter and microbes. Sewage cannot be discharged directly into natural water bodies like rivers and streams because it is polluted. Therefore, sewage is treated in Sewage Treatment Plants (STPs) before disposal.
Step-by-step Sewage Treatment Process:
- Primary Treatment:
- This step physically removes large and small particles from the sewage through filtration and sedimentation.
- Floating debris is removed by sequential filtration.
- The grit (soil and small pebbles) is removed by sedimentation.
- All solids that settle form the primary sludge, and the effluent forms the supernatant (primary effluent).
- 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.
- This allows rapid growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh-like structures).
- While growing, these microbes consume the major part of the organic matter in the effluent. This significantly reduces the BOD (Biochemical Oxygen Demand) of the effluent.
Definition: Biochemical Oxygen Demand (BOD) refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidized by bacteria.
- Once the BOD of sewage is reduced significantly, the effluent is then passed into a settling tank where the bacterial 'flocs' are allowed to sediment. This sediment is called activated sludge.
- A small part of the activated sludge is pumped back into the aeration tank to serve as the inoculum.
- The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters.
- In the digesters, other kinds of bacteria, which grow anaerobically, digest the bacteria and fungi in the sludge. During this digestion, bacteria produce a mixture of gases such as methane, hydrogen sulphide and carbon dioxide. These gases form biogas.
- The effluent from secondary treatment plant is generally released into natural water bodies like rivers and streams.
- The biogas plant consists of a concrete tank (10-15 feet deep) in which bio-wastes are collected and a slurry of dung is fed.
- A floating cover is placed over the slurry, which keeps rising as the biogas is produced in the tank due to the microbial activity.
- The biogas plant has an outlet, which is connected to a pipe to supply biogas for cooking and lighting.
- The spent slurry is removed through another outlet and may be used as fertiliser.
- Chemical pesticides and insecticides are toxic and extremely harmful to human beings and animals, and pollute our environment.
- Biological control of pests and diseases is a method that relies on natural predation rather than introduced chemicals.
- Ladybird and Aphids: The ladybird (beetle with red and black markings) is useful to get rid of aphids.
- Dragonflies: Used to get rid of mosquitoes.
- Bacillus thuringiensis (Bt): Used to control butterfly caterpillars. These are available in sachets as dried spores which are sprayed on vulnerable plants such as brassicas and fruit trees, where they are eaten by the insect larvae. In the gut of the larvae, the toxin is released and kills the larvae.
- Trichoderma species: Free-living fungi that are very common in the root ecosystems. They are effective biocontrol agents of several plant pathogens.
- Baculoviruses: Pathogens that attack insects and other arthropods. The majority of baculoviruses used as biological control agents are in the genus Nucleopolyhedrovirus. These viruses are excellent candidates for species-specific, narrow-spectrum insecticidal applications. They have no negative impacts on plants, mammals, birds, fish or even on non-target insects.
Microbes in Production of Biogas
Definition: Biogas is a mixture of gases (containing predominantly methane) produced by the microbial activity and which can be used for cooking and lighting.
Certain bacteria, which grow anaerobically on cellulosic material, produce large amounts of methane along with CO2 and H2. These bacteria are collectively called methanogens, and one common bacterium is Methanobacterium.
Rural Biogas Plant Structure:
Microbes as Biocontrol Agents
Biocontrol refers to the use of biological methods for controlling plant diseases and pests.
Key Advantages Over Chemical Pesticides
Examples of Biocontrol Agents
Microbes as Biofertilisers
Definition: Biofertilisers are organisms that enrich the nutrient quality of the soil.
The main sources of biofertilisers are bacteria, fungi, and cyanobacteria.
1. Rhizobium
Rhizobium bacteria form symbiotic associations with the root nodules of legumes. They fix atmospheric nitrogen into organic forms, which is used by the plant as nutrient.
2. Free-Living Nitrogen Fixers
Other bacteria can fix atmospheric nitrogen while free-living in the soil (e.g., Azospirillum and Azotobacter), thus enriching the nitrogen content of the soil.
3. Mycorrhiza
Fungi form symbiotic associations with plants (Mycorrhiza). Many members of the genus Glomus form mycorrhiza.
Benefits of Mycorrhiza:
4. Cyanobacteria
Cyanobacteria are autotrophic microbes widely distributed in aquatic and terrestrial environments many of which can fix atmospheric nitrogen, e.g., Anabaena, Nostoc, Oscillatoria, etc.
In paddy fields, cyanobacteria serve as an important biofertiliser. Blue-green algae also add organic matter to the soil and increase its fertility.