Microbes in Human Welfare — Notes

• Curd: Lactobacillus and other lactic acid bacteria (LAB) grow in milk, produce acids that coagulate and partially digest milk proteins, increase vitamin B₁₂, and check disease-causing microbes in the stomach. A small amount of curd added to fresh milk acts as the inoculum.
• Dosa and idli: dough fermented by bacteria; CO₂ makes it puff up.
• Bread: dough fermented by baker's yeast (Saccharomyces cerevisiae).
• Toddy: traditional drink made by fermenting palm sap.
• Cheese: large holes in Swiss cheese from CO₂ produced by Propionibacterium sharmanii; Roquefort cheese ripened by specific fungi for flavour.

Beverages: brewer's yeast (Saccharomyces cerevisiae) ferments malted cereals and fruit juices to ethanol. Wine and beer are produced without distillation; whisky, brandy and rum by distilling the fermented broth.

Antibiotics: chemicals produced by some microbes that kill or retard the growth of disease-causing microbes. Penicillin — discovered by Alexander Fleming while working on Staphylococci: a mould, Penicillium notatum, prevented their growth; its full potential was established by Ernst Chain and Howard Florey; used for wounded soldiers in World War II; Nobel Prize 1945. Antibiotics have greatly improved treatment of plague, whooping cough, diphtheria and leprosy.

Chemicals and enzymes:

Lipases in detergents remove oily stains; pectinases and proteases clarify bottled juices.

Bioactive molecules: streptokinase (from Streptococcus, modified by genetic engineering) — "clot buster" for patients who have had a myocardial infarction; cyclosporin A (from Trichoderma polysporum) — immunosuppressant for organ-transplant patients; statins (from the yeast Monascus purpureus) — lower blood cholesterol by competitively inhibiting an enzyme of cholesterol synthesis.

Sewage (municipal waste water) contains large amounts of organic matter and microbes, many pathogenic; it is treated in sewage treatment plants (STPs) before release into natural water bodies.

Primary treatment (physical): floating debris removed by sequential filtration; grit (soil, small pebbles) by sedimentation. Settled solids = primary sludge; supernatant = effluent.

Secondary (biological) treatment: effluent passed into large aeration tanks, agitated and pumped with air; useful aerobic microbes grow into flocs (bacteria associated with fungal filaments in mesh-like structures), which consume most of the organic matter, significantly reducing the BOD. Then the effluent goes to a settling tank, where flocs sediment as activated sludge; a small part is pumped back into the aeration tank as inoculum; the major part goes to anaerobic sludge digesters, where other bacteria digest the bacteria and fungi, producing biogas (methane, hydrogen sulphide, CO₂). The treated effluent is released into rivers.

BOD (biochemical oxygen demand): the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria; higher BOD = more polluting potential.

The Ministry of Environment and Forests initiated the Ganga Action Plan and Yamuna Action Plan to save these rivers from pollution.

Biogas is a mixture of gases, predominantly methane, produced by microbial activity; it is used as fuel. Methanogens such as Methanobacterium grow anaerobically on cellulosic material and produce large amounts of methane, with CO₂ and H₂. They occur in anaerobic sludge and in the rumen of cattle (where they help digest cellulose), so cattle dung (gobar) is rich in them.

Biogas plant: a concrete tank (10–15 feet deep) into which bio-wastes and a slurry of dung are fed; a floating cover over the slurry rises as gas is produced; an outlet pipe supplies gas for cooking and lighting; spent slurry is removed through another outlet and used as fertiliser. The technology was developed in India mainly by IARI (Indian Agricultural Research Institute) and KVIC (Khadi and Village Industries Commission).

Biocontrol uses biological methods to control plant diseases and pests, reducing dependence on chemicals that are toxic to humans and animals and pollute the environment. Organic farmers aim to keep pests at manageable levels through a balanced ecosystem.

• Ladybird beetles (aphids) and dragonflies (mosquitoes).
• Bacillus thuringiensis (Bt): spores sprayed on plants; caterpillars that eat them are killed by a toxin released in their gut; other insects are unharmed.
• Trichoderma: free-living fungi in the root ecosystem; biocontrol of several plant pathogens.
• Baculoviruses (genus Nucleopolyhedrovirus): species-specific, narrow-spectrum insecticides with no negative impact on plants, mammals, birds, fish or non-target insects; useful in integrated pest management (IPM).

Biofertilisers enrich the nutrient quality of soil:

• Rhizobium — symbiotic in root nodules of leguminous plants; fixes atmospheric nitrogen.
• Azospirillum, Azotobacter — free-living nitrogen fixers in soil.
• Mycorrhiza (fungi of genus Glomus) — absorb phosphorus from soil for the plant; also give resistance to root pathogens and tolerance to salinity and drought.
• Cyanobacteria (Anabaena, Nostoc, Oscillatoria) — fix nitrogen, especially in paddy fields; add organic matter.

Judicious use of antibiotics: use only on prescription, at the correct dose, for the full course; never for viral infections; misuse selects resistant bacteria.