Microbes are omnipresent (soil, water, air, inside bodies, thermal vents at 100°C, deep soil, under snow). They include protozoa, bacteria, fungi, microscopic plant viruses, viroids, and prions (proteinaceous infectious agents).

Microbes in Household Products:

• Curd: Lactobacillus and other Lactic Acid Bacteria (LAB) grow in milk and convert it to curd. During growth, the LAB produce acids that coagulate and partially digest the milk proteins. A small amount of curd added to fresh milk as inoculum (starter) contains millions of LAB, which at suitable temperatures multiply, converting milk to curd, which also improves its nutritional quality by increasing Vitamin B12. LAB also checks disease-causing microbes in the stomach.

• Dough: The dough used for making dosa and idli is fermented by bacteria (puffed-up appearance due to CO2 production). The dough for making bread is fermented using Baker’s yeast (Saccharomyces cerevisiae).

• Toddy: A traditional drink of some parts of southern India made by fermenting sap from palms.

• Cheese: Different varieties are known by their texture, flavour, and taste. The large holes in ‘Swiss cheese’ are due to production of a large amount of CO2 by a bacterium named Propionibacterium shermanii. The ‘Roquefort cheese’ is ripened by growing a specific fungus on them.

Microbes in Industrial Products: Production on an industrial scale requires growing microbes in very large vessels called Fermenters.

• Fermented Beverages: Saccharomyces cerevisiae (Brewer’s yeast) is used for fermenting malted cereals and fruit juices to produce ethanol. Wine and beer are produced without distillation (low alcohol); whisky, brandy, and rum are produced by distillation of the fermented broth (high alcohol).

• Antibiotics: Chemical substances produced by some microbes and can kill or retard the growth of other (disease-causing) microbes. Penicillin was the first antibiotic to be discovered by Alexander Fleming (working on Staphylococci bacteria, he observed a mould Penicillium notatum prevented bacterial growth). Its full potential as an effective antibiotic was established much later by Ernest Chain and Howard Florey. They were awarded the Nobel Prize in 1945. Antibiotics have greatly improved our capacity to treat deadly diseases like plague, whooping cough (kali khasi), diphtheria (gal ghotu), and leprosy (kusht rog).

• Chemicals, Enzymes, and other Bioactive Molecules:

  • Acids: Aspergillus niger (fungus) $\to$ Citric acid; Acetobacter aceti (bacterium) $\to$ Acetic acid; Clostridium butylicum (bacterium) $\to$ Butyric acid; Lactobacillus (bacterium) $\to$ Lactic acid.

  • Enzymes: Lipases are used in detergent formulations (remove oily stains). Pectinases and Proteases are used to clarify bottled juices. Streptokinase produced by Streptococcus and modified by genetic engineering is used as a ‘clot buster’ for removing clots from blood vessels of patients who have undergone myocardial infarction.

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

Microbes in Sewage Treatment: Municipal waste-water (sewage) contains large amounts of organic matter and microbes. Treatment is done in Sewage Treatment Plants (STPs).

• • Primary Treatment: Physical removal of particles (large and small) through filtration and sedimentation. Floating debris is removed by sequential filtration. Grit (soil and small pebbles) are removed by sedimentation. The solid that settles forms the primary sludge, and the supernatant forms the 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 vigorous growth of useful aerobic microbes into flocs (masses of bacteria associated with fungal filaments to form mesh-like structures). These microbes consume the major part of the organic matter in the effluent, significantly reducing the BOD (Biochemical Oxygen Demand). BOD refers to the amount of oxygen that would be consumed if all the organic matter in one liter of water were oxidized by bacteria. Greater BOD implies higher polluting potential. Once BOD is reduced, the effluent is 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 is pumped into large tanks called anaerobic sludge digesters. Here, other kinds of bacteria (anaerobic) grow and 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 (Biogas). The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.

Microbes in Production of Biogas: Biogas is a mixture of gases (predominantly methane) produced by microbial activity. Bacteria that grow anaerobically on cellulosic material produce a large amount of methane along with CO2 and H2. These bacteria are collectively called methanogens (e.g., Methanobacterium). These are found in the anaerobic sludge during sewage treatment and in the rumen (part of stomach) of cattle (help in cellulose breakdown). The excreta (dung) of cattle, commonly called gobar, is rich in these bacteria. 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 on rising as gas is produced. The biogas plant has an outlet connected to a pipe to supply biogas. The spent slurry is removed through another outlet and may be used as fertilizer. The technology was developed in India mainly due to the efforts of IARI (Indian Agricultural Research Institute) and KVIC (Khadi and Village Industries Commission).

Microbes as Biocontrol Agents: Use of biological methods for controlling plant diseases and pests.

• Ladybird (beetle with red and black markings) controls aphids.

• Dragonflies control mosquitoes.

• Bacillus thuringiensis (Bt): Available in sachets as dried spores which are mixed with water and sprayed onto vulnerable plants (like brassicas and fruit trees). These are eaten by the insect larvae. In the gut of the larvae, the toxin is released and the larvae get killed. This kills caterpillars but leaves other insects unharmed.

• Trichoderma species: Free-living fungi found in root ecosystems. Effective biocontrol agents of several plant pathogens.

• Baculoviruses (genus Nucleopolyhedrovirus): Pathogens that attack insects and other arthropods. They are species-specific, narrow spectrum insecticidal applications. Desirable when beneficial insects are being conserved (IPM – Integrated Pest Management).

Microbes as Biofertilizers: Organisms that enrich the nutrient quality of the soil.

• Bacteria: Rhizobium forms symbiotic associations with root nodules of leguminous plants and fixes atmospheric nitrogen. Azospirillum and Azotobacter are free-living bacteria in the soil that fix atmospheric nitrogen.

• Fungi (Mycorrhiza): Fungi form symbiotic associations with plants (e.g., genus Glomus). The fungal symbiont absorbs phosphorus from soil and passes it to the plant. Plants also show resistance to root-borne pathogens, tolerance to salinity and drought, and an overall increase in plant growth.

• Cyanobacteria: Autotrophic microbes widely distributed in aquatic and terrestrial environments (e.g., Anabaena, Nostoc, Oscillatoria). In paddy fields, cyanobacteria serve as an important biofertilizer. Blue-green algae also add organic matter to the soil and increase its fertility.