How Tiny Microbes Set Your Curd, Clean Sewage and Feed Crops

Microbes are often blamed for disease, but many are indispensable. They set curd, raise idli batter, make medicines, clean waste water, produce cooking gas and supply crops with nitrogen.

This lesson covers microbes in food and industry, in sewage treatment and biogas, and as biocontrol agents and biofertilisers.

Microbes ferment foods at home, and in industry they are grown in large fermenters to make beverages, antibiotics, organic acids, enzymes and other useful molecules.

Household products:

- Curd — Lactobacillus and other lactic acid bacteria turn milk into curd
- Idli and dosa batter — fermented by bacteria that release carbon dioxide, making it rise
- Bread — dough is leavened by baker's yeast, Saccharomyces cerevisiae
- Cheese — the large holes in Swiss cheese come from carbon dioxide made by Propionibacterium sharmanii, and some cheeses are ripened by fungi

Industrial products:

- Beverages — wine and beer are made by fermenting fruit juices and malted cereals with brewer's yeast, while spirits such as whisky, brandy and rum are also distilled
- Antibiotics — penicillin from the fungus Penicillium notatum kills disease-causing bacteria
- Organic acids — citric acid from Aspergillus niger, acetic acid from Acetobacter aceti, butyric acid from Clostridium butylicum and lactic acid from Lactobacillus
- Enzymes — lipases in detergents; pectinases and proteases to clarify fruit juices; streptokinase from Streptococcus to dissolve blood clots in heart attack patients
- Bioactive molecules — cyclosporin A from Trichoderma polysporum, used to suppress immunity in transplant patients, and statins from Monascus purpureus, which lower blood cholesterol

An everyday example. A spoonful of old curd stirred into warm milk overnight supplies the lactic acid bacteria that set the next batch.

The substance. Curd is more nutritious than the milk it came from — lactic acid bacteria raise its vitamin B12 content and help check disease-causing microbes in the stomach.

Sewage is treated first by physically removing solids and then by letting aerobic microbes digest organic matter in aeration tanks, while biogas is produced when methanogenic bacteria break down organic waste such as cattle dung without oxygen.

Primary treatment:

- Floating debris is removed by sequential filtration, and grit and soil by sedimentation
- The settled solids form primary sludge, and the liquid above is the effluent

Secondary (biological) treatment:

- The effluent is pumped into aeration tanks and stirred with air
- Aerobic bacteria and fungal filaments form mesh-like masses called flocs, which consume organic matter
- This sharply lowers the BOD (biochemical oxygen demand) — the oxygen microbes need to break down the organic matter in water
- In settling tanks the flocs sink as activated sludge; part is returned to the aeration tank as an inoculum
- The rest is digested in anaerobic sludge digesters, releasing gases such as methane, hydrogen sulphide and carbon dioxide
- The treated effluent is released into rivers and streams

Biogas production:

- Methanogens such as Methanobacterium grow without oxygen on cellulose-rich material
- They live in the rumen of cattle, so cattle dung (gobar) is rich in them
- In a biogas plant, a slurry of dung and water is fed into a sealed tank, where methanogens produce biogas, mainly methane with carbon dioxide, which is piped for cooking and lighting
- The spent slurry is used as manure

An everyday example. Gobar gas plants in rural India give families clean cooking fuel and manure from cattle dung.

The substance. A high BOD means heavily polluted water — more organic matter needs more oxygen to break it down.

Biocontrol agents are living organisms used to control pests and diseases in place of chemicals, biofertilisers are microbes that enrich soil nutrients, and integrated pest management combines biological, cultural and limited chemical methods to keep pests below damaging levels.

Biocontrol agents:

- Ladybird beetles eat aphids, and dragonflies eat mosquitoes
- Bt bacteria — spores of Bacillus thuringiensis sprayed on crops kill caterpillars without harming most other insects
- Trichoderma fungi — free-living in root systems, they control several plant pathogens
- Baculoviruses of the genus Nucleopolyhedrovirus — attack specific insects while sparing beneficial ones

Biofertilisers:

- Bacteria — Rhizobium in the root nodules of legumes fixes nitrogen, and free-living Azospirillum and Azotobacter also add nitrogen to soil
- Fungi — mycorrhiza formed by fungi such as Glomus absorb phosphorus from the soil for the plant
- Cyanobacteria — Anabaena, Nostoc and Oscillatoria fix nitrogen in paddy fields and add organic matter

Integrated pest management (IPM):

- Pests are monitored, and action begins only when they reach damaging levels
- Natural predators and biocontrol agents are encouraged
- Crop rotation, resistant varieties and traps are used
- Chemical pesticides are a last resort, protecting the environment and beneficial insects

An everyday example. Organic farmers across India use Trichoderma, neem-based sprays and pheromone traps as part of integrated pest management.

The substance. Killing every insect harms farming — beneficial predators and pollinators die too, so pests often return in larger numbers.