Microbes in Human Welfare

In this chapter: microbes in household products; industrial products (beverages, antibiotics, acids, enzymes, bioactive molecules); sewage treatment and BOD; biogas; biocontrol agents; biofertilisers.

Microbes in household products

• Curd: Lactobacillus and other lactic acid bacteria (LAB) produce acids that coagulate and partially digest milk proteins. A little curd added to milk acts as the starter (inoculum). LAB also increase vitamin B₁₂ and, in our stomach, check disease-causing microbes.
• Dosa and idli batter: fermented by bacteria; the puffed-up look comes from CO₂.
• Bread dough: fermented by baker's yeast, Saccharomyces cerevisiae.
• Toddy: a traditional drink in parts of southern India, made by fermenting palm sap.
• Microbes are also used to ferment fish, soybean and bamboo shoots.
• Cheese: the large holes in Swiss cheese come from CO₂ made by Propionibacterium sharmanii; Roquefort cheese is ripened by growing specific fungi on it.

Microbes in industry

Fermented beverages

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

Antibiotics

Penicillin was the first antibiotic, discovered by chance. Alexander Fleming, working with Staphylococci, noticed that a mould growing on an unwashed plate stopped the bacteria growing around it. The chemical was named penicillin after the mould Penicillium notatum. Its full potential was established later by Ernest Chain and Howard Florey; it was used to treat wounded American soldiers in World War II. All three received the Nobel Prize in 1945. Antibiotics treat plague, whooping cough (kali khansi), diphtheria (gal ghotu) and leprosy (kusht rog).

Chemicals, enzymes and bioactive molecules

Sewage treatment

Municipal waste water (sewage) contains a lot of organic matter and microbes, many of them pathogenic. It is treated in sewage treatment plants (STPs).

Primary treatment (physical)

1. Floating debris is removed by sequential filtration.
2. Grit (soil and small pebbles) is removed by sedimentation.
3. Settled solids form the primary sludge; the liquid above is the primary effluent, which goes to secondary treatment.

Secondary treatment (biological)

1. The primary effluent goes into large aeration tanks, constantly agitated with air pumped in.
2. Useful aerobic microbes grow vigorously as flocs (masses of bacteria with fungal filaments forming a mesh).
3. They consume most of the organic matter, greatly reducing the BOD.
4. The effluent passes to a settling tank, where flocs settle as activated sludge. A small part is pumped back into the aeration tank as inoculum.
5. The rest goes to anaerobic sludge digesters, where anaerobic bacteria digest the bacteria and fungi in the sludge and produce biogas (methane, hydrogen sulphide, CO₂), which is inflammable and can be used for energy.
6. The treated effluent is released into rivers and streams.

BOD (biochemical oxygen demand) is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. It indirectly measures organic matter: the higher the BOD, the greater the polluting potential.

The Ministry of Environment and Forests started the Ganga Action Plan and Yamuna Action Plan to build STPs so that only treated sewage enters these rivers.

Biogas

• A mixture of gases, predominantly methane, produced by microbes and used as fuel.
• Produced by methanogens (e.g. Methanobacterium), which grow anaerobically on cellulosic material and produce methane, CO₂ and H₂. They occur in anaerobic sludge and in the rumen of cattle, where they help digest cellulose.
• Cattle dung (gobar) is rich in these bacteria, so it is used to make gobar gas.
• Biogas plant: a concrete tank (10 to 15 feet deep) fed with a slurry of dung; a floating cover rises as gas is produced; an outlet pipe supplies gas to nearby houses; spent slurry is removed and used as fertiliser. Used for cooking and lighting, mostly in rural areas.
• The technology was developed in India mainly by IARI (Indian Agricultural Research Institute) and KVIC (Khadi and Village Industries Commission).

Biocontrol agents

Biofertilisers

• Organisms that enrich soil nutrients: bacteria, fungi and cyanobacteria.
• Rhizobium forms root nodules on legumes (symbiotic) and fixes atmospheric nitrogen.
• Azospirillum and Azotobacter are free-living soil bacteria that fix nitrogen.
• Mycorrhiza: fungi (many of genus Glomus) in symbiosis with plant roots absorb phosphorus and pass it to the plant. Such plants also resist root-borne pathogens, tolerate salinity and drought, and grow better.
• Cyanobacteria such as Anabaena, Nostoc and Oscillatoria fix nitrogen; important biofertilisers in paddy fields, also adding organic matter to soil.

Common traps: (1) Swiss cheese holes: Propionibacterium sharmanii; Roquefort: fungi. (2) Statins from Monascus purpureus (yeast); cyclosporin A from Trichoderma polysporum. (3) Citric acid from a fungus (Aspergillus niger), not a bacterium. (4) Activated sludge is from the settling tank after aeration, not from primary treatment. (5) Mycorrhiza helps with phosphorus, not nitrogen.

NEET focus

• Household microbes; vitamin B₁₂ from LAB; distilled vs undistilled drinks.
• Penicillin story and Nobel Prize year.
• The product-microbe table (acids, enzymes, bioactive molecules).
• Sewage treatment sequence; BOD meaning; Ganga and Yamuna Action Plans.
• Biogas plant and methanogens; biocontrol agents; biofertilisers.

Practice questions