Microbes in Human Welfare

Microbes are not only agents of disease — they are enormously useful, and many work quietly in our own kitchens. A classic example is curd. It is made from milk by lactic acid bacteria (LAB), chiefly Lactobacillus: a small amount of curd (the 'starter' or inoculum) added to warm milk multiplies and produces lactic acid, which coagulates and partially digests the milk proteins. LAB also increase vitamin B12 in the curd and, in the stomach, help check disease-causing microbes.

The dough used to make bread, dosa and idli is also fermented by microbes. For idli and dosa the batter is fermented by LAB, while the puffiness of bread comes from baker's yeast (Saccharomyces cerevisiae), which releases carbon dioxide that makes the dough rise.

Cheese is one of the oldest microbial products, and different microbes give different cheeses their character — a popular NEET fact. The large holes in 'Swiss cheese' are produced by a large amount of CO₂ released by a bacterium, Propionibacterium sharmanii. The 'Roquefort cheese' is ripened by growing a specific fungus, Penicillium roqueforti, on it, which gives a particular flavour.

Many traditional foods of the world are likewise fermented, such as the South Indian foods and the Bharuch (Gujarat) drink, and 'toddy' is a traditional fermented drink made from sap of palms. The common thread is that helpful microbes — mainly bacteria and yeasts — transform simple ingredients into nutritious, flavourful products. For NEET, fix the microbe for each product: curd = LAB/Lactobacillus; bread dough rising = yeast (Saccharomyces cerevisiae); Swiss cheese holes = Propionibacterium (CO₂); Roquefort cheese = Penicillium.

On an industrial scale, microbes are grown in large vessels called fermentors (bioreactors) to make valuable products. One of the oldest uses is making fermented (alcoholic) beverages. The yeast Saccharomyces cerevisiae (called brewer's yeast here) ferments sugars to ethanol. Wine and beer are produced without distillation, whereas whisky, brandy and rum are produced by distillation of the fermented broth.

The most important medical products are antibiotics — chemicals made by microbes that kill or stop other microbes. The story begins with Alexander Fleming, who discovered penicillin from the mould Penicillium notatum (its full potential as a drug was later established by Chain and Florey). Penicillin was used widely to treat wounded soldiers in World War II, and antibiotics have since transformed medicine.

Microbes also yield useful organic acids, enzymes and other bioactive molecules. Organic acids include citric acid (Aspergillus niger), acetic acid (Acetobacter aceti), butyric acid (Clostridium butylicum) and lactic acid (Lactobacillus). Industrially important enzymes include lipases (used in detergents to remove oily stains), pectinases (to clarify bottled juices) and streptokinase (from Streptococcus, used as a 'clot buster' to dissolve blood clots in heart-attack patients).

Two other named bioactive molecules are frequently asked. Cyclosporin A, from the fungus Trichoderma polysporum, is used as an immunosuppressant in organ transplants. Statins, produced by the yeast Monascus purpureus, are used to lower blood cholesterol (they block an enzyme in cholesterol synthesis). For NEET, fix the alcohol products (with/without distillation), penicillin (Fleming, Penicillium notatum), citric acid (Aspergillus niger), streptokinase (clot buster), cyclosporin A (immunosuppressant) and statins (cholesterol-lowering).

Cities produce huge amounts of sewage (municipal waste water), rich in organic matter and microbes. It cannot be released into rivers directly, so it is treated in sewage treatment plants (STPs) using heterotrophic microbes, in two main stages.

Primary treatment is essentially physical: the sewage is passed through filters and grit chambers to remove floating debris and grit, and then allowed to settle so that the solids form the primary sludge while the supernatant forms the effluent. Secondary (biological) treatment then acts on this effluent. It is passed into large aeration tanks where it is constantly agitated and air is pumped in; this lets useful aerobic microbes grow into masses called 'flocs' (mesh-like bacterial colonies). These microbes consume the organic matter, drastically reducing the BOD (biochemical oxygen demand) of the water — a lower BOD means cleaner water. This is the activated-sludge process.

Once the BOD is low, the sewage is passed to settling tanks where the bacterial flocs sediment as the activated sludge; a part of this is used as the inoculum for the next batch, and the rest is pumped into large tanks called anaerobic sludge digesters. Here other, anaerobic bacteria digest the sludge, producing a mixture of gases including methane — biogas.

This links directly to biogas production. Biogas is a mixture of gases (predominantly methane, with CO₂ and H₂S) produced by the anaerobic breakdown of organic matter. The microbes responsible are the methanogens — methane-producing bacteria such as Methanobacterium — which also occur in the rumen of cattle (so cattle dung, 'gobar', is rich in them). In a biogas (gobar gas) plant, dung slurry is digested anaerobically to release biogas used as fuel for cooking and lighting, and the spent slurry is used as manure. For NEET, fix the two stages of sewage treatment (primary = physical; secondary = activated-sludge, lowers BOD) and biogas (methane, methanogens like Methanobacterium, cattle rumen/dung).

Modern agriculture is moving away from harmful chemicals toward biological methods, and microbes are central to this. Biological control (biocontrol) is the use of living organisms to control plant pests and diseases instead of chemical pesticides — this protects beneficial insects and reduces pollution.

Several biocontrol agents are heavily examined. The bacterium Bacillus thuringiensis (Bt) produces a protein toxin that kills the larvae of certain insects; it is sprayed as a powder, and its toxin gene has also been introduced into crops to make Bt cotton. The fungus Trichoderma is a free-living fungus used to control several plant pathogens (it protects the roots). Baculoviruses (genus Nucleopolyhedrovirus, NPV) are used as narrow-spectrum (species-specific) insecticides that do not harm other organisms. Among animals, the ladybird beetle controls aphids and dragonflies control mosquitoes.

Biofertilisers are organisms that enrich the nutrient quality of the soil, mainly by adding nitrogen and helping with phosphorus — a clean alternative to chemical fertilisers. The most important nitrogen-fixers are the bacteria. Rhizobium lives symbiotically in the root nodules of legumes and fixes atmospheric nitrogen; Azospirillum and Azotobacter are free-living nitrogen-fixing bacteria in the soil.

Other groups also act as biofertilisers. Cyanobacteria (blue-green algae) such as Anabaena, Nostoc and Oscillatoria fix nitrogen and are especially important in paddy fields (the water fern Azolla with Anabaena is a famous biofertiliser for rice). Finally, the fungal symbiosis called mycorrhiza — especially the genus Glomus — associates with plant roots and helps absorb phosphorus (and gives tolerance to drought and disease). For NEET, fix the biocontrol agents (Bt against larvae, Trichoderma against pathogens, baculovirus/NPV species-specific) and the biofertilisers (Rhizobium symbiotic, Azospirillum/Azotobacter free-living, cyanobacteria in paddy, mycorrhiza/Glomus for phosphorus).