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Microbes in Human Welfare is one of the shortest chapters in Class 12 biology, and one of the easiest to lose marks on. It is mostly a long list of names: which bacterium makes which acid, which fungus gives which drug. This post turns the chapter into a map you can recall, works through the one calculation it hides, and lists the mix-ups that cost students marks.
What does the syllabus ask for?
The NEET (UG) 2026 syllabus notified by the National Medical Commission lists the topic as microbes "in household food processing, industrial production, sewage treatment, energy generation and as biocontrol agents and biofertilizers." Those six headings are the six sections of Chapter 8 in the NCERT Class 12 Biology textbook. Check the syllabus for your own exam year, since NMC can revise it.
How do you remember which microbe makes what?
Build one table per section, with three columns: organism, product, and what the product does. From the NCERT chapter:
Household: *Lactobacillus* and other lactic acid bacteria turn milk to curd and increase its vitamin B12. Baker's yeast, *Saccharomyces cerevisiae*, raises bread dough. *Propionibacterium sharmanii* makes the large holes in Swiss cheese by releasing CO2.
Acids: *Aspergillus niger* (a fungus) for citric acid, *Acetobacter aceti* for acetic acid, *Clostridium butylicum* for butyric acid, *Lactobacillus* for lactic acid.
Drugs and enzymes: streptokinase from *Streptococcus* as a clot buster, cyclosporin A from the fungus *Trichoderma polysporum* as an immunosuppressant in transplant patients, statins from *Monascus purpureus* to lower blood cholesterol, and lipases in detergents.
Biocontrol: *Bacillus thuringiensis* (Bt) against caterpillars, the fungus *Trichoderma* against plant pathogens, and baculoviruses of the genus *Nucleopolyhedrovirus* against insects.
Biofertilisers: *Rhizobium* in root nodules, free-living *Azospirillum* and *Azotobacter*, *Glomus* forming mycorrhiza that pass phosphorus to plants, and cyanobacteria such as *Anabaena* and *Nostoc* in paddy fields.
Write the table once from the book, then twice more from memory a few days apart. The gaps you leave are your revision list.
What does the antibiotics section actually say?
NCERT tells the penicillin story briefly: Alexander Fleming noticed a mould on an unwashed plate of *Staphylococcus* bacteria, and the bacteria could not grow around it. He published the work in 1929 in the British Journal of Experimental Pathology. Ernst Chain and Howard Florey later showed its full value as a medicine, and the three shared the 1945 Nobel Prize in Physiology or Medicine "for the discovery of penicillin and its curative effect in various infectious diseases".
The story has a modern sequel worth knowing for essays and interviews. The WHO fact sheet on antimicrobial resistance says bacterial resistance was estimated to be associated with more than 4.7 million deaths worldwide in 2021, and names misuse and overuse of antimicrobials as a driver.
How do sewage treatment and biogas fit together?
Learn these as one flow, because the chapter links them.
Primary treatment is physical: filtering out floating debris and letting grit settle. The settled solids are primary sludge.
Secondary treatment is biological. The effluent goes into aeration tanks, where aerobic microbes grow into flocs and consume most of the organic matter. That lowers the BOD, the oxygen that bacteria would use to oxidise the organic matter in one litre of water.
The flocs settle as activated sludge. A small part goes back into the aeration tank as inoculum. The rest goes to anaerobic sludge digesters, where other bacteria produce methane, hydrogen sulphide and CO2.
That gas mixture is biogas. The same methanogens, such as *Methanobacterium*, live in the rumen of cattle, which is why cattle dung (gobar) is the feed for village biogas plants. NCERT credits the technology in India mainly to IARI and KVIC.
A high BOD means more polluting water, not cleaner water. That one sign error appears in many wrong answers.
Worked example: how much ethanol can yeast make?
The chapter asks you to recall the reactions behind brewing and baking. Yeast ferments glucose anaerobically:
C6H12O6 → 2 C2H5OH + 2 CO2
Question. Yeast fully ferments 360 g of glucose. What mass of ethanol and of CO2 forms?
Molar masses: glucose = 6(12) + 12(1) + 6(16) = 180 g/mol. Ethanol = 2(12) + 6(1) + 16 = 46 g/mol. CO2 = 12 + 2(16) = 44 g/mol.
Moles of glucose = 360 ÷ 180 = 2 mol.
Each mole gives 2 mol ethanol and 2 mol CO2, so 4 mol of each.
Ethanol = 4 × 46 = 184 g. CO2 = 4 × 44 = 176 g.
Check: 184 + 176 = 360 g, so mass is conserved.
The same CO2 is what puffs up idli batter and bread dough. In real fermentations yeast also uses some sugar to grow, so actual yields are lower than this ideal figure.
Which mistakes cost the most marks?
Mixing up acid producers. *Aspergillus* (citric) is a fungus; *Acetobacter* (acetic) and *Clostridium* (butyric) are bacteria.
**Swapping the two *Trichoderma* roles.** *T. polysporum* gives cyclosporin A; *Trichoderma* species as a group are biocontrol agents.
Saying biogas is pure methane. NCERT calls it a mixture containing predominantly methane, with CO2 and H2 from methanogens on cellulose.
Mixing up distilled and undistilled drinks. Wine and beer are made without distillation; whisky, brandy and rum are distilled.
For the organisms behind these names, our posts on the principles of biotechnology for NEET and human health and immunity cover the chapters on either side.
A practice routine for this chapter
Day 1: Read the chapter once and build the five tables above.
Day 2: Cover the organism column and fill it in from the product. Then cover the product column.
Day 4: Draw the sewage-to-biogas flow from memory, with BOD marked.
Day 7: Do ten mixed questions from previous papers and mark each error as a wrong name, a wrong process or a misread question.
If one process will not stick, such as why the rumen and a sludge digester hold the same bacteria, read a fuller explanation of that one idea. The Fermi Concept Library covers every concept in biology with its own lesson, so you can fill one gap and come back to the chapter.
Tonight, write the five organism and product tables from memory and mark every blank.
