Microbes — bacteria, fungi, cyanobacteria, and viruses — are not merely agents of disease. They are indispensable partners in human civilization, driving food production, industrial manufacturing, environmental management, agricultural fertility, and biocontrol. NEET consistently dedicates 2–3 questions per year to this chapter, focusing on precise microbe-product associations, the mechanics of sewage treatment, the BOD concept, and the distinction between different biofertilizer organisms.

Food Processing

The conversion of milk to curd is performed by Lactobacillus through lactic acid fermentation. A frequently overlooked fact is that this process simultaneously increases the vitamin B12 content of curd compared to raw milk. Saccharomyces cerevisiae (baker's yeast) is responsible for both bread leavening (via CO2 production) and ethanol generation in beer and wine production. In cheese making, the microbe matters enormously for NEET purposes: Propionibacterium shermanii produces CO2 during the ripening of Swiss cheese, creating its large characteristic holes. Penicillium roqueforti imparts the blue-green veining and sharp flavor to Roquefort cheese through mold ripening. Aspergillus niger is exploited industrially to produce citric acid, while Acetobacter aceti oxidizes ethanol to acetic acid (vinegar).

Industrial Microbiology

Alexander Fleming discovered penicillin in 1928 from Penicillium notatum — a landmark event in the history of medicine. Streptomyces species are prolific antibiotic producers, yielding streptomycin (against TB and gram-negative bacteria), erythromycin, and tetracycline. Key enzymes of industrial significance include streptokinase from Streptococcus — a thrombolytic agent that dissolves blood clots, used clinically in treating myocardial infarctions. Students must remember that streptokinase is NOT an antibiotic; it is an enzyme. Lipases and proteases from various bacteria and fungi are incorporated into laundry detergents to remove fat and protein stains at low temperatures. Pectinases, produced by Aspergillus and other fungi, clarify fruit juices by breaking down pectin in cell walls. Among bioactive molecules, lovastatin from Monascus purpureus competitively inhibits HMG-CoA reductase — the rate-limiting enzyme in cholesterol biosynthesis — thereby lowering blood cholesterol levels. Cyclosporin A, produced by Trichoderma polysporum, is a powerful immunosuppressant critical for preventing organ rejection after transplantation.

Sewage Treatment

Municipal sewage undergoes sequential physical and biological treatment before effluent is released. Primary treatment is entirely physical: raw sewage passes through screens to remove large solids, then through grit chambers, and finally into settling tanks where suspended particles sediment as primary sludge. The liquid overflow is primary effluent. Secondary (biological) treatment is the BOD-reducing stage: primary effluent enters aeration tanks where aerobic microbes grow vigorously, consuming dissolved organic matter. The resulting activated sludge settles in a secondary settling tank; part is recycled as inoculum to seed new aeration tanks, and the remainder is pumped into anaerobic sludge digesters. BOD (Biochemical Oxygen Demand) quantifies the dissolved oxygen consumed by microbes to break down organic matter in a water sample. High BOD = high organic pollution. In anaerobic digesters, methanogens — principally Methanobacterium — decompose organic sludge, producing biogas: a mixture of methane (CH4), carbon dioxide (CO2), and hydrogen sulphide (H2S). Biogas serves as a renewable fuel in many rural communities.

Biocontrol Agents

Bacillus thuringiensis (Bt) produces crystalline proteins — notably Cry1Ac and Cry2Ab — that are toxic specifically to lepidopteran (moth and butterfly) larvae. These proteins form the molecular basis of Bt crop technology in agricultural genetic engineering. Baculoviruses, particularly Nucleopolyhedrovirus (NPV), are highly species-specific biocontrol agents that infect and kill insects without affecting plants, mammals, or non-target insects. Trichoderma species function as biocontrol fungi against soil-borne pathogens. Ladybird beetles (coccinellids) are classical biological control agents that feed on aphids.

Biofertilizers

Biofertilizers enrich soil fertility through natural biological mechanisms. Rhizobium is a symbiotic bacterium that forms root nodules on legumes (pea, soybean, groundnut) and fixes atmospheric nitrogen (N2) into bioavailable ammonia. Free-living nitrogen fixers — Azotobacter in aerobic soils and Azospirillum in the rhizosphere — operate independently of a host plant. Cyanobacteria such as Anabaena and Nostoc are especially important in paddy (rice) fields: they fix atmospheric nitrogen and simultaneously add organic matter as they die and decompose. Mycorrhiza (e.g., Glomus) is a symbiotic fungus-root association whose primary contribution is enhancing phosphorus uptake from soil, along with improved water absorption and resistance to soil-borne diseases. Mycorrhiza does NOT fix nitrogen — this is a classic NEET trap.

The master skill for this chapter is accurate microbe-product-function mapping with no conflation between superficially similar options.