Microbes in Human WelfareNEET Zoology · Class 12 · NCERT Chapter 8

You can expect 1 question from Microbes in Human Welfare in NEET 2027. The most commonly tested facts are: Lactobacillus in curd, penicillin from Penicillium notatum (or P. chrysogenum), BOD and sewage treatment, Bacillus thuringiensis as a biocontrol agent, and Rhizobium in root nodules. Know the specific microbe-product pairs well.

Lactobacillus (also called lactic acid bacteria or LAB) is used to make curd. It ferments the lactose in milk, producing lactic acid. The lactic acid lowers the pH of the milk, causing the milk proteins to coagulate and form curd. As a bonus, LAB also produce small amounts of vitamin B12. The same bacteria keep harmful microbes in check in your gut.

Saccharomyces cerevisiae (baker's yeast) ferments the sugars in the dough, producing carbon dioxide (CO2) and ethanol. The CO2 gas gets trapped in the dough and causes it to rise, giving bread its soft, spongy texture. The ethanol evaporates during baking. The same process (but at different scales) is used to make idli and dosa batter, where the fermentation also slightly increases the nutritional value by producing B vitamins.

Alexander Fleming discovered penicillin in 1928 when he noticed that the mould Penicillium notatum was killing bacteria on his culture plates. He realised the mould was secreting something that killed bacteria, and he named it penicillin. Later, Ernst Chain and Howard Florey developed penicillin into a usable medicine (1940s). Fleming, Chain and Florey shared the Nobel Prize in Physiology or Medicine in 1945. Penicillin is the first antibiotic to be used in medicine.

BOD stands for Biological Oxygen Demand. It is the amount of oxygen that bacteria need to break down the organic waste in a given volume of water. High BOD means a lot of organic waste is present. In sewage treatment, the goal of secondary (biological) treatment is to reduce the BOD of the effluent. In the aeration tank, bacteria form flocs and use oxygen to break down organic matter, reducing the BOD significantly. The treated water (with low BOD) can then be released without causing oxygen depletion in receiving water bodies.

Bacillus thuringiensis (Bt) is a soil bacterium that naturally produces proteins toxic to certain insects, especially caterpillars (lepidopteran larvae). Farmers spray dried spore preparations of Bt on crops. When caterpillars eat the leaves, they also ingest the Bt spores. Inside the insect gut, the spores release protein crystals (Bt toxin) that kill the larvae. The toxin is harmless to plants, mammals and birds. This is a safe, eco-friendly alternative to chemical pesticides.

A chemical fertiliser is a manufactured inorganic compound (like urea or ammonium nitrate) that adds nutrients directly to the soil. It acts fast but can cause soil degradation and water pollution over time. A biofertiliser uses living microorganisms that improve plant nutrition naturally. Examples: Rhizobium bacteria fix atmospheric nitrogen in root nodules of legumes; mycorrhizal fungi (Glomus) increase phosphate absorption; cyanobacteria (Anabaena, Nostoc, Oscillatoria) fix nitrogen in wet rice fields. Biofertilisers are sustainable and improve soil health.