Summary

Chapter Summary

Microorganisms, though most familiar to the public as agents of disease, are overwhelmingly beneficial, and human societies -- long before understanding the underlying biology -- have exploited their metabolic activity for practical benefit across an extraordinary range of applications, spanning food, industry, sanitation, energy, agriculture and medicine.

In food processing, lactic-acid bacteria such as Lactobacillus ferment milk's lactose into lactic acid, coagulating milk proteins to form curd, while the yeast Saccharomyces cerevisiae ferments sugars in dough to release carbon dioxide, leavening bread. Naturally occurring bacteria similarly ferment idli/dosa batter, and specific bacteria such as Propionibacterium shermanii give Swiss cheese its characteristic holes and flavour.

Fermented beverages -- wine, beer and toddy -- are produced by yeast fermentation of grape juice, malted grain, and palm sap respectively, consumed as-is; whisky, rum and brandy begin the same way but are additionally distilled, concentrating their alcohol content well beyond what fermentation alone can achieve.

In industrial production, microbes manufacture organic acids (citric acid from Aspergillus niger, lactic acid from Lactobacillus, butyric acid from Clostridium butylicum), industrial ethanol (from Saccharomyces), enzymes used across manufacturing (lipases, pectinases, the clot-dissolving drug streptokinase from Streptococcus), and complex bioactive pharmaceuticals such as the immunosuppressant cyclosporin A (from Trichoderma polysporum) and cholesterol-lowering statins (from Monascus purpureus).

In sewage treatment, primary treatment physically removes solids, after which secondary/biological treatment uses aerated tanks in which bacterial and fungal flocs consume dissolved organic matter, sharply lowering the sewage's Biochemical Oxygen Demand (BOD) -- the standard measure of organic-pollution load. Leftover activated sludge is further digested anaerobically, a process that also yields biogas as a useful by-product.

Biogas, generated the same way at dedicated rural biogas plants using cattle-dung slurry, is produced chiefly by methanogenic archaea (such as Methanobacterium) fermenting organic matter anaerobically; a simple biogas plant design channels the gas through a rising gas-holder while also yielding a nutrient-rich spent slurry usable as fertiliser.

As biocontrol agents, Bacillus thuringiensis (Bt) produces an insect-gut-toxic protein exploited both as a sprayed biopesticide and, via genetic engineering, in Bt crop varieties; the fungus Trichoderma suppresses root-pathogenic fungi like Pythium and Fusarium; and species-specific Nucleopolyhedrovirus (NPV) biopesticides offer narrow-target insect control safe for non-target organisms.

As biofertilisers, the bacterium Rhizobium fixes nitrogen symbiotically within legume root nodules; free-living bacteria such as Azotobacter and Azospirillum fix nitrogen independently in soil; mycorrhizal fungi (e.g. Glomus) extend a plant's effective root surface to boost phosphorus and water uptake; and nitrogen-fixing cyanobacteria such as Anabaena and Nostoc enrich paddy soils with both fixed nitrogen and organic matter. …