Microbes in Human Welfare: common doubts, answered
The questions students ask most often about Microbes in Human Welfare, each with a short answer. For the full chapter, read the Microbes in Human Welfare notes.
About the chapter
How are microbes helpful in household food preparation?
Microbes are used in everyday foods through fermentation. Lactobacillus converts milk to curd, bacteria raise the dough for idli and dosa, baker's yeast raises bread, and a bacterium and a fungus ripen cheeses. Toddy is made by fermenting the sap of palms. In every case the microbe changes the food's texture, flavour or nutrition.
Microbes all around
Read this section in the notes →What are prions and how do they differ from bacteria?
Prions are infectious agents made of protein alone, without any nucleic acid, whereas bacteria are living cells with DNA. NCERT mentions them among the microbes found everywhere, and unlike bacteria or viruses they have no genetic material of their own. Bacteria are cellular, so they can grow and divide on their own.
Curd, dough and cheese
Read this section in the notes →How does milk turn into curd?
Lactic acid bacteria (LAB), mainly Lactobacillus, grow in warm milk and convert some of the sugar into lactic acid. The acid coagulates and partially digests the milk proteins and so milk sets as curd. A small amount of curd added as inoculum starts the process, and the vitamin B₁₂ content also increases.
Why does the dough of idli and dosa rise?
The dough rises because bacteria in the fermenting batter release carbon dioxide gas, which gets trapped and puffs it up. It is a bacterial fermentation. Bread dough is different, since there the puffing is done by baker's yeast, Saccharomyces cerevisiae.
What makes the holes in Swiss cheese?
The holes are made by carbon dioxide produced in large amounts by the bacterium Propionibacterium sharmanii, which is used to ripen it. The gas forms bubbles inside the cheese. Roquefort cheese is ripened by a fungus instead, which gives it a particular flavour.
Fermented beverages
Read this section in the notes →What is the difference between wine and whisky?
Wine and beer are made by fermentation of juices or malted cereals with yeast, without distillation. Whisky, brandy and rum are produced by fermentation followed by distillation, which gives a higher alcohol content. The yeast used for both is Saccharomyces cerevisiae, called brewer's yeast.
Antibiotics
Read this section in the notes →Who discovered penicillin and which organism produces it?
Alexander Fleming discovered penicillin, which is made by the mould Penicillium notatum. While working with Staphylococci, he saw that the bacteria would not grow in a zone around a mould contaminating one of his culture plates. Ernest Chain and Howard Florey later established its full value as an antibiotic, it was used widely to treat soldiers wounded in the Second World War, and the three shared the Nobel Prize in 1945.
What does the word antibiotic mean?
Antibiotic literally means against life, but in practice it is a chemical made by one microbe that kills, or slows the growth of, other disease-causing microbes. Penicillin was the first to be discovered, and most antibiotics come from bacteria and fungi.
Chemicals, enzymes and bioactive molecules
Read this section in the notes →Which microbe produces which organic acid?
Aspergillus niger, a fungus, produces citric acid. Acetobacter aceti, a bacterium, produces acetic acid. Clostridium butylicum produces butyric acid. Lactobacillus produces lactic acid. Mixing up the fungus and the bacteria is the usual mistake, since only citric acid comes from a fungus.
What are streptokinase, cyclosporin A and statins used for?
Streptokinase, from the bacterium Streptococcus, is a clot buster used to remove clots from blood vessels of patients with myocardial infarction. Cyclosporin A, from the fungus Trichoderma polysporum, is an immunosuppressive agent for organ transplant patients. Statins, from the yeast Monascus purpureus, lower blood cholesterol by competitively inhibiting the enzyme for cholesterol synthesis.
How are enzymes from microbes used in everyday products?
Lipases are used in detergents to remove oily stains, and pectinases and proteases are used to clarify bottled fruit juices. Because the enzymes are produced by microbes in large quantities, they can be added to products. Streptokinase is a medical enzyme of the same sort.
Primary sewage treatment
Read this section in the notes →What happens in primary sewage treatment?
Primary treatment is a physical process. Large and small particles are removed by sequential filtration and sedimentation. Floating debris is removed first, then grit settles, and the solids that settle form the primary sludge, while the remaining liquid is the primary effluent. This effluent goes on to secondary treatment.
Secondary treatment and BOD
Read this section in the notes →What is BOD and what does a high value mean?
Biochemical oxygen demand is the amount of oxygen that bacteria would consume to break down all the organic matter in one litre of water. A higher BOD means more organic matter and more pollution. During secondary treatment the BOD of the effluent falls as microbes digest the organic matter.
What is activated sludge?
It is the sediment of flocs from the aeration tank, formed by masses of bacteria associated with fungal filaments. After the effluent settles, this sediment is called activated sludge. A small part is pumped back into the aeration tank as inoculum, and the rest goes to anaerobic sludge digesters.
Biogas
Read this section in the notes →Which gases make up biogas and which microbes produce it?
Biogas is mainly methane with carbon dioxide and some hydrogen sulphide. It is produced by methanogenic bacteria such as Methanobacterium, which grow anaerobically in sludge and in the rumen of cattle, where they help in digesting cellulose. The cattle dung is the usual feed of a biogas plant, and the gas serves as fuel.
Biocontrol agents
Read this section in the notes →What is biological control and what are some examples?
Biological control uses living organisms to manage pests and diseases, with less harm to the environment than chemicals. Ladybird beetles eat aphids and dragonflies eat mosquitoes. Bacillus thuringiensis, as Bt, kills butterfly caterpillars, and the fungus Trichoderma controls plant pathogens in the root ecosystem.
Why are baculoviruses suitable for integrated pest management?
Baculoviruses such as the genus Nucleopolyhedrovirus attack insects and other arthropods and are species-specific with a narrow spectrum. They have no negative impact on plants, mammals, birds or even non-target insects, which is desirable when beneficial species must be conserved, as in IPM.
Biofertilisers
Read this section in the notes →What are biofertilisers and which organisms are used?
Biofertilisers are living organisms that add to the nutrient quality of soil, mainly bacteria, fungi and cyanobacteria. Rhizobium is symbiotic in the root nodules of legumes. Azospirillum and Azotobacter live freely in soil and fix nitrogen, and cyanobacteria such as Anabaena and Nostoc do the same in paddy fields.
What is the role of mycorrhiza?
Mycorrhiza is a symbiotic association of a fungus with the roots of higher plants, such as the fungus Glomus. The fungus absorbs phosphorus from the soil and passes it to the plant, and these plants also show resistance to root pathogens and tolerance to salinity and drought, and grow better in general.
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