Microbes in Human Welfare

The One Thing

Microorganisms function as biological agents that convert organic materials into useful products, remove pollutants from sewage and improve plant nutrition. Their controlled use supports industrial production, wastewater treatment and more sustainable agriculture.

Reactions, Processes and Experiments

Key Terms

• Fermentation: A microbial process in which microorganisms convert organic substances into useful products under controlled conditions.
• Industrial microbiology: The use of microorganisms or their enzymes to manufacture products on a large scale.
• Saccharomyces cerevisiae: Baker’s yeast used to produce bread, wine and beer by converting sugars into alcohol and carbon dioxide.
• Ethanol fermentation: The anaerobic conversion of sugars into ethanol and carbon dioxide by yeast: C6H12O6 → 2C2H5OH + 2CO2 + energy.
• Lactic acid: An organic acid produced by Lactobacillus and used in food processing and other industries.
• Acetic acid: The main acid in vinegar, produced commercially by Acetobacter aceti through oxidation of ethanol.
• Citric acid: An organic acid produced commercially using Aspergillus niger and used in food, beverages and medicines.
• Antibiotics: Chemical substances produced by microorganisms that kill or inhibit the growth of disease-causing microbes.
• Penicillin: The first widely used antibiotic, obtained from Penicillium notatum; it acts against many bacterial infections.
• Streptokinase: An enzyme produced by Streptococcus that helps dissolve blood clots and is used as a clot-busting medicine.
• Cyclosporin A: An immunosuppressive substance produced by Trichoderma polysporum and used during organ transplantation to reduce rejection.
• Statins: Cholesterol-lowering substances produced by Monascus purpureus that inhibit cholesterol synthesis.
• Sewage: Wastewater containing human excreta, domestic waste, microbes and other organic and inorganic materials.
• Primary treatment: The physical removal of large and suspended particles from sewage by filtration, screening and sedimentation.
• Secondary treatment: The biological treatment of sewage in which aerobic microbes degrade organic matter and form microbial flocs.
• Activated sludge: The microbial-rich sediment formed after secondary treatment; a portion is recycled as inoculum and the rest enters an anaerobic sludge digester.
• Biochemical oxygen demand (BOD): The amount of dissolved oxygen required by microorganisms to decompose organic matter in water. A high BOD indicates greater organic pollution.
• Biogas: A combustible gas mixture, mainly methane, produced when anaerobic microbes decompose organic matter.
• Methanogens: Anaerobic archaea that produce methane; Methanobacterium is commonly found in cattle rumen and sludge digesters.
• Biofertilizers: Preparations containing living microorganisms that increase the availability of nutrients to plants.
• Rhizobium: A symbiotic nitrogen-fixing bacterium found in the root nodules of leguminous plants.
• Mycorrhiza: A mutually beneficial association between fungal hyphae and plant roots; Glomus improves phosphorus absorption and resistance to stress.
• Cyanobacteria: Photosynthetic microorganisms such as Anabaena and Nostoc that fix atmospheric nitrogen and enrich soil, especially in paddy fields.
• Biological nitrogen fixation: The conversion of atmospheric nitrogen into usable nitrogen compounds by nitrogen-fixing microorganisms.
• Biocontrol: The use of beneficial organisms or their products to control pests and diseases instead of relying mainly on chemical pesticides.

Easily Confused

• Primary treatment vs secondary treatment: Primary treatment physically removes solids by screening, filtration and sedimentation, whereas secondary treatment uses aerobic microbes to degrade biodegradable organic matter.
• Activated sludge vs biogas: Activated sludge is the microbial-rich sediment formed after secondary treatment, whereas biogas is the methane-rich gas produced when sludge undergoes anaerobic digestion.
• Fermentation vs anaerobic digestion: Fermentation produces industrial products such as ethanol, while anaerobic digestion decomposes sewage sludge and produces biogas.
• BOD vs oxygen content: BOD is the amount of dissolved oxygen microorganisms require to decompose organic matter; it is not the amount of oxygen already present in water.
• Rhizobium vs cyanobacteria: Rhizobium fixes nitrogen symbiotically in the root nodules of legumes, whereas cyanobacteria such as Anabaena and Nostoc fix atmospheric nitrogen and enrich soil, particularly in paddy fields.
• Mycorrhiza vs biological nitrogen fixation: Mycorrhiza mainly improves phosphorus and water absorption and stress resistance, whereas biological nitrogen fixation converts atmospheric nitrogen into usable nitrogen compounds.
• Antibiotics vs statins: Antibiotics kill or inhibit disease-causing microbes, whereas statins inhibit cholesterol synthesis.
• Penicillin vs streptokinase: Penicillin is an antibiotic obtained from Penicillium notatum, whereas streptokinase is a clot-dissolving enzyme produced by Streptococcus.

What Gets Asked

• Identify the microorganism and product: Questions may pair Saccharomyces cerevisiae with ethanol and carbon dioxide, Aspergillus niger with citric acid, Acetobacter aceti with acetic acid, and Clostridium butylicum with butyric acid. A common error is assigning the wrong microorganism to an organic acid.
• Write or interpret ethanol fermentation: The required equation is C6H12O6 → 2C2H5OH + 2CO2 + energy. Marks are lost by omitting carbon dioxide, energy or the anaerobic nature of the process.
• Explain penicillin discovery and action: The named example is Alexander Fleming’s 1928 observation that Penicillium mould inhibited bacterial growth. The key observation is the clear region where bacteria fail to grow.
• Describe sewage treatment in sequence: The expected sequence is screening and sedimentation, aeration in an aeration tank, settling of activated sludge and anaerobic digestion of excess sludge. A frequent error is treating primary treatment as a microbial process rather than mainly physical solids removal.
• Explain BOD and its significance: Secondary treatment reduces BOD because aerobic microbes consume organic matter. A lower BOD indicates less biodegradable organic matter remains; reversing this relationship costs marks.
• Compare biofertilizers and their functions: Rhizobium fixes nitrogen in legume root nodules, mycorrhiza improves phosphorus and water absorption, and cyanobacteria such as Anabaena, Nostoc and Azolla-Anabaena add nitrogen to soil. The main error is attributing phosphorus absorption to Rhizobium or nitrogen fixation to mycorrhiza.