Microbes in Human Welfare — Fermentation, Sewage Treatment and Biocontrol
This is a short, purely factual chapter built almost entirely of which microbe does which job, so the whole battle is learning the name-to-use pairings cleanly. Group them by application — food and drink, industrial products, sewage, biogas, biocontrol, biofertilisers — and each group is a small list. Two names recur and are worth fixing first because they anchor half the chapter: *Lactobacillus* (curd, and the setting of many fermented foods) and *Saccharomyces cerevisiae* (brewer's/baker's yeast, behind bread, alcohol and more). The classic traps are giving a product the wrong microbe, or confusing the two stages of sewage treatment. Drill the pairings and the marks are guaranteed.
Sewage Treatment and Biogas
- Sewage treatment has two stages. Primary treatment is physical — filtration and sedimentation remove solids (the settled solids are "primary sludge"). Secondary (biological) treatment uses microbes in aeration tanks: they form flocs and consume the organic matter, greatly lowering the BOD.
- BOD (biochemical oxygen demand) measures the organic pollution: the more organic matter, the more oxygen microbes consume, so a high BOD means highly polluted water. Secondary treatment reduces the BOD before the water is released.
- Biogas (mainly methane) is produced by methanogens — anaerobic archaebacteria such as Methanobacterium — which also live in the rumen of cattle, which is why cattle dung is the usual feed for a gobar-gas plant.
🎯 Why sewage is cleaned with the very microbes that live in it
It seems backwards to clean dirty water by adding more microbes, but that is exactly how secondary treatment works, and understanding why makes the two-stage design obvious. The problem with sewage is not the microbes themselves but the enormous load of organic matter, which would strip a river of oxygen and kill fish if released directly — that is what a high BOD measures. Primary treatment only removes the physical solids; the dissolved and suspended organic matter is still there. So in secondary treatment the sewage is pumped into aeration tanks and vigorously oxygenated, and the naturally present aerobic microbes multiply, clump into flocs, and consume the organic matter as food — in effect eating the pollution. As they do, the BOD drops sharply, and once it is low enough the water is safe to release. The microbes are then allowed to settle as activated sludge, part of which is recycled as inoculum and the rest sent to anaerobic digesters that produce biogas. So the logic is: the pollution is food, and letting microbes eat it under controlled aeration converts a disposal problem into both clean water and fuel.Biocontrol and Biofertilisers
- Biocontrol uses living organisms against pests instead of chemicals: the ladybird beetle against aphids, dragonflies against mosquitoes, and the bacterium Bacillus thuringiensis (Bt) whose toxin kills insect larvae — sprayed, or engineered into Bt-crops.
- *Trichoderma* (a fungus) controls plant pathogens, and Baculoviruses (genus Nucleopolyhedrovirus) are narrow-spectrum insect pathogens — a plus, because they spare beneficial insects and are truly species-specific.
- Biofertilisers enrich soil nutrients: Rhizobium fixes nitrogen in legume root nodules; free-living Azotobacter and the cyanobacterium Anabaena fix nitrogen; and mycorrhiza (a fungus–root partnership, e.g. Glomus) helps the plant absorb phosphorus.
ℹ️ Why species-specific biocontrol beats a broad chemical spray
A chemical pesticide is a blunt instrument: it kills the target pest but also the pollinators, the pest's natural predators and countless harmless insects, and it lingers in the soil and food chain. Biocontrol is prized because good biocontrol agents are narrow-spectrum — they attack only the target and leave everything else alone. The Baculoviruses are the textbook example: a given virus infects essentially one insect species, so releasing it wipes out a crop pest without touching the bees that pollinate the crop or the ladybirds that eat other pests. Bacillus thuringiensis is similar — its toxin is activated only in the alkaline gut of specific insect larvae and is harmless to humans and most other animals. This species-specificity is the whole selling point, and it is also why biocontrol fits an ecological, sustainable approach to farming: instead of poisoning the whole community to remove one member, you deploy that member's specific natural enemy. When a question asks "why is a Baculovirus preferred in integrated pest management", the answer it wants is exactly this narrow host range that spares non-target and beneficial species.Key takeaways
- Curd = Lactobacillus (raises vitamin B₁₂); bread and alcohol = Saccharomyces cerevisiae (yeast).
- Penicillin from Penicillium (Fleming); citric acid from Aspergillus niger; statins from Monascus.
- Sewage: primary = physical (solids removed); secondary = biological (microbes in flocs lower the BOD).
- High BOD = highly polluted water; secondary treatment cuts the BOD before release.
- Biogas (methane) is made by methanogens (e.g. Methanobacterium), also found in cattle rumen.
- Biocontrol: ladybird vs aphids, Bacillus thuringiensis (Bt) toxin kills larvae; Baculoviruses are species-specific.
- Biofertilisers: Rhizobium (nodules) and Azotobacter/Anabaena fix nitrogen; mycorrhiza helps phosphorus uptake.
- Narrow-spectrum biocontrol spares beneficial insects — the key advantage over chemical pesticides.
Practice questions (8)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
The microorganism used in the production of curd from milk is:
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Answer: A — Lactobacillus
Lactobacillus (lactic acid bacteria) curdles milk into curd and even raises its vitamin B₁₂ content. Saccharomyces is the yeast used for bread and alcohol, so naming it confuses the curd anchor with the fermentation anchor.A high biochemical oxygen demand (BOD) of a water sample indicates that the water is:
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Answer: A — Highly polluted with organic matter
The more organic matter present, the more oxygen microbes use to break it down, so a high BOD means heavy organic pollution. Clean water has a low BOD, which is why the "very clean" option inverts the meaning of the measure.Biogas (mainly methane) is produced by the activity of:
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Answer: A — Methanogens (anaerobic archaebacteria)
Methanogens such as Methanobacterium generate methane anaerobically, and they also live in the cattle rumen, which is why dung feeds a biogas plant. Lactobacillus and Penicillium are used for curd and antibiotics respectively, not for methane production.In sewage treatment, the removal of suspended and dissolved organic matter by microbes forming flocs occurs during:
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Answer: A — Secondary (biological) treatment
Secondary treatment aerates the sewage so microbes multiply, form flocs and consume the organic load, sharply lowering the BOD. Primary treatment only removes physical solids by sedimentation and filtration, so it cannot handle the dissolved organic matter.The antibiotic penicillin is obtained from the fungus:
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Answer: A — Penicillium notatum
Penicillin comes from Penicillium notatum, discovered by Alexander Fleming. Aspergillus niger makes citric acid and Monascus makes statins, so those are industrial products of different fungi rather than the source of penicillin.Rhizobium acts as a biofertiliser because it:
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Answer: A — Fixes atmospheric nitrogen in the root nodules of legumes
Rhizobium lives symbiotically in legume root nodules and converts atmospheric nitrogen into a form the plant can use, enriching the soil. Absorbing phosphorus is the job of mycorrhiza, so that option assigns Rhizobium another biofertiliser's function.A key advantage of Baculoviruses as biocontrol agents in integrated pest management is that they:
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Answer: A — Are species-specific, sparing beneficial and non-target insects
Baculoviruses have a very narrow host range, attacking essentially one insect species and leaving pollinators and predators unharmed — the whole point of biocontrol. Killing every insect would be the failing of broad chemical pesticides, which is the opposite of what makes these viruses valuable.The rising of bread dough and the fermentation of sugar to ethanol are both brought about by:
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Answer: A — Saccharomyces cerevisiae
Yeast (Saccharomyces cerevisiae) releases CO₂ that leavens dough and ferments sugars into ethanol, so both effects trace to the same organism. Lactobacillus makes curd and Rhizobium fixes nitrogen, neither of which leavens bread or produces ethanol.