Exercises
Bacteria cannot be seen with the naked eyes, but these can be seen with the help of a microscope. If you have to carry a sample from your home to your biology laboratory to demonstrate the presence of microbes with the help of a microscope, which sample would you carry and why?
Answer
A small amount of curd (or fermented idli/dosa batter) is the best sample. Curd is formed by lactic acid bacteria (LAB) such as Lactobacillus, and even a tiny drop contains millions of these bacteria. A drop placed on a slide (and stained if possible) clearly shows the rod-shaped bacteria under the microscope. The sample is easy to obtain, safe to handle and non-pathogenic.
Give examples to prove that microbes release gases during metabolism.
Answer
(i) Dough for idli and dosa, fermented by bacteria, puffs up because of the CO₂ released.
(ii) Bread dough fermented by baker's yeast (Saccharomyces cerevisiae) rises due to CO₂ produced during fermentation.
(iii) The large holes in Swiss cheese are formed by the large amount of CO₂ produced by the bacterium Propionibacterium sharmanii.
(iv) Methanogens such as Methanobacterium growing anaerobically on cellulosic material (in cattle rumen, sludge digesters and biogas plants) release methane along with CO₂ and H₂.
(v) In anaerobic sludge digesters, bacteria release methane, hydrogen sulphide and CO₂ (biogas).
In which food would you find lactic acid bacteria? Mention some of their useful applications.
Answer
Lactic acid bacteria (e.g. Lactobacillus) are found in curd (and other fermented milk products such as buttermilk and yoghurt).
Useful applications: (i) They convert milk into curd by producing acids that coagulate and partially digest milk proteins. (ii) They improve the nutritional quality of curd by increasing its vitamin B₁₂ content. (iii) In our stomach they help check the growth of disease-causing microbes. (iv) Lactobacillus is used for the commercial production of lactic acid.
Name some traditional Indian foods made of wheat, rice and Bengal gram (or their products) which involve use of microbes.
Answer
Wheat: bread, bhatura, naan, jalebi (fermented batter). Rice: idli, dosa, appam (rice with black gram). Bengal gram: dhokla and khaman (fermented gram-flour batter). Rice is also used to make some traditional fermented drinks. In all these, bacteria and/or yeast ferment the dough or batter, which rises due to CO₂.
In which way have microbes played a major role in controlling diseases caused by harmful bacteria?
Answer
Many microbes (mostly fungi and some bacteria) produce antibiotics – chemical substances that kill or retard the growth of disease-causing microbes. Penicillin, the first antibiotic, was discovered by Alexander Fleming from the mould Penicillium notatum; Chain and Florey later established its value as a drug. Antibiotics have greatly improved our ability to treat deadly bacterial diseases such as plague, whooping cough, diphtheria and leprosy, which once killed millions. In addition, many vaccines are prepared from killed or weakened microbes.
Name any two species of fungus, which are used in the production of the antibiotics.
Answer
Penicillium notatum (penicillin) and Penicillium chrysogenum (penicillin, commercial production). Another example is Cephalosporium acremonium (cephalosporins).
What is sewage? In which way can sewage be harmful to us?
Answer
Sewage is the municipal waste water produced daily in cities and towns; human excreta is its major component. It contains large amounts of organic matter and microbes.
Harm: (i) Many of the microbes in sewage are pathogenic and spread water-borne diseases such as cholera, typhoid, dysentery and hepatitis. (ii) Its high organic load gives it a very high BOD; when discharged into rivers, microbes decomposing it use up dissolved oxygen, killing fish and other aquatic life. (iii) It pollutes water bodies, makes the water foul-smelling and unfit for drinking or bathing.
What is the key difference between primary and secondary sewage treatment?
Answer
Primary treatment is a physical process: floating debris, grit and other particles are removed by sequential filtration and sedimentation; the settled solids form primary sludge and the supernatant is the primary effluent.
Secondary treatment is a biological process: the primary effluent is aerated in tanks, where aerobic microbes grow as flocs and consume most of the organic matter, greatly reducing BOD. The flocs then settle as activated sludge, which is partly recycled as inoculum and partly digested anaerobically.
Do you think microbes can also be used as source of energy? If yes, how?
Answer
Yes. Methanogenic bacteria such as Methanobacterium, which grow anaerobically on cellulosic material, produce biogas – mainly methane along with CO₂ and H₂. In a gobar gas plant, a slurry of cattle dung (rich in methanogens) is fed into a 10–15 feet deep concrete tank; the gas collects under a floating cover and is piped to houses for cooking and lighting. Biogas is also produced in the anaerobic sludge digesters of sewage treatment plants. Yeast fermentation also yields ethanol, which can be used as a fuel.
Microbes can be used to decrease the use of chemical fertilisers and pesticides. Explain how this can be accomplished.
Answer
Biofertilisers (replace chemical fertilisers): (i) Rhizobium in root nodules of legumes fixes atmospheric nitrogen. (ii) Free-living bacteria Azospirillum and Azotobacter fix nitrogen in the soil. (iii) Mycorrhizal fungi (e.g. Glomus) absorb phosphorus from soil and pass it to the plant. (iv) Cyanobacteria such as Anabaena, Nostoc and Oscillatoria fix nitrogen and add organic matter in paddy fields.
Biocontrol agents (replace chemical pesticides): (i) Bacillus thuringiensis (Bt) spores sprayed on plants kill caterpillars when the toxin is released in their gut. (ii) The fungus Trichoderma controls several plant pathogens in the root zone. (iii) Baculoviruses (genus Nucleopolyhedrovirus) are species-specific, narrow-spectrum insecticides that do not harm non-target organisms.
Three water samples namely river water, untreated sewage water and secondary effluent discharged from a sewage treatment plant were subjected to BOD test. The samples were labelled A, B and C; but the laboratory attendant did not note which was which. The BOD values of the three samples A, B and C were recorded as 20mg/L, 8mg/L and 400mg/L, respectively. Which sample of the water is most polluted? Can you assign the correct label to each assuming the river water is relatively clean?
Answer
Higher BOD means more organic matter and more pollution. Sample C (400 mg/L) is the most polluted.
Labels: C (400 mg/L) – untreated sewage water; A (20 mg/L) – secondary effluent from the sewage treatment plant; B (8 mg/L) – river water (relatively clean).
Find out the name of the microbes from which Cyclosporin A (an immunosuppressive drug) and Statins (blood cholesterol lowering agents) are obtained.
Answer
Cyclosporin A is obtained from the fungus Trichoderma polysporum. Statins are obtained from the yeast Monascus purpureus.
Find out the role of microbes in the following and discuss it with your teacher.
(a) Single cell protein (SCP)
(b) Soil
Answer
(a) Single cell protein: Microbial biomass such as Spirulina (a cyanobacterium), yeasts and bacteria like Methylophilus methylotrophus can be grown cheaply on waste materials (straw, molasses, sewage, potato-processing waste) and used as a protein-rich food or feed. It is a possible solution to protein shortage and also reduces pollution.
(b) Soil: Microbes decompose dead organic matter and recycle nutrients; form humus; fix nitrogen (Rhizobium, Azotobacter, Azospirillum, cyanobacteria); help plants absorb phosphorus (mycorrhiza); carry out nitrification and other steps of nutrient cycles; and some (e.g. Trichoderma) protect plants from soil-borne pathogens.
Arrange the following in the decreasing order (most important first) of their importance, for the welfare of human society. Give reasons for your answer. Biogas, Citric acid, Penicillin and Curd
Answer
Penicillin > Biogas > Curd > Citric acid.
Penicillin – a life-saving antibiotic that has controlled many deadly bacterial infections. Biogas – a cheap, clean, renewable fuel for cooking and lighting in rural areas, and the spent slurry is a manure. Curd – a nutritious food rich in vitamin B₁₂ whose LAB also check harmful microbes in the stomach. Citric acid – an industrial chemical used mainly as a food preservative and flavouring, useful but least essential.
How do biofertilisers enrich the fertility of the soil?
Answer
Biofertilisers are organisms that enrich the nutrient quality of the soil. (i) Symbiotic Rhizobium in legume root nodules fixes atmospheric nitrogen into organic forms. (ii) Free-living Azotobacter and Azospirillum fix nitrogen in the soil. (iii) Cyanobacteria (Anabaena, Nostoc, Oscillatoria) fix nitrogen and add organic matter, especially in paddy fields. (iv) Mycorrhizal fungi such as Glomus absorb phosphorus and pass it to plants, and also give resistance to root pathogens and tolerance to salinity and drought. Thus they add nutrients naturally and reduce dependence on chemical fertilisers.
Important Extra Questions
Why are bottled fruit juices clearer than home-made ones?
Answer
Bottled juices are clarified using microbial enzymes – pectinases and proteases – which break down pectin and proteins that make juice cloudy.
What is streptokinase and what is its use?
Answer
Streptokinase is an enzyme produced by the bacterium Streptococcus and modified by genetic engineering. It is used as a "clot buster" to dissolve clots in blood vessels of patients with myocardial infarction.
Which beverages are produced with and without distillation?
Answer
Wine and beer are produced without distillation; whisky, brandy and rum are produced by distillation of the fermented broth. Brewer's yeast Saccharomyces cerevisiae is used.
What are flocs and activated sludge?
Answer
Flocs are masses of aerobic bacteria associated with fungal filaments forming mesh-like structures in the aeration tank. When they settle in the settling tank after BOD reduction, the sediment is called activated sludge.
Name the two organisations that developed biogas technology in India.
Answer
The Indian Agricultural Research Institute (IARI) and the Khadi and Village Industries Commission (KVIC).