Lesson 7: Introduction to Microbiology

Rule 1: Applied and industrial microbiology — Applied microbiology studies the enzymes of microbes, proteins, applied genetics and molecular biology, and is used to produce food and medicines on a large scale. Industrial microbiology deals with the commercial use of microbes: products made by fermentation (bread, cheese, wine, chemicals, enzymes, nutrients, medicines) as well as garbage management and pollution control.

• Easy : Making curd at home is a small example of fermentation by Lactobacillus.
• Medium : A bakery uses yeast, a dairy uses lactobacilli and a pharma factory uses moulds, all are industrial microbiology at different scales.
• Exam-type : Why is industrial microbiology useful for the environment? Microbes can also treat garbage and control pollution, not only make products.

Rule 2: Dairy products — Dairy products (cheese, butter, cream, kefir, yoghurt) preserve milk. Milk is first pasteurized (पाश्चरीकरण) to destroy unwanted microbes, then fermented by lactobacilli: lactose becomes lactic acid, which coagulates the milk proteins, and flavour compounds such as diacetyl (butter flavour) form.

In the figure : In the figure how dairy products are made.

• Easy : Raw milk is heated and cooled quickly (pasteurized) before curd is made in a dairy.
• Medium : Lactose (milk sugar) → lactic acid; the acid makes the milk sour and thick.
• Exam-type : Why is milk pasteurized before adding the culture? Unwanted microbes would spoil the product or compete with the useful ones.

Rule 3: Yoghurt, butter and cheese — Yoghurt: Streptococcus thermophilus and Lactobacillus delbrueckii are added in the ratio 1:1; Streptococcus makes lactic acid that gels the proteins, and the lactobacilli make acetaldehyde, which gives the taste. Butter: sweet cream and cultured (with microbes) types. Cheese: whey is removed; the enzyme rennet from the alimentary canal of cattle was used earlier, while protease from fungi gives vegetarian cheese. Fresh cheese is soft (cottage, cream, mozzarella); cheddar ripens for 3–12 months (semi-hard); parmesan for 12–18 months (very hard). Strict cleanliness is needed because viruses can attack the bacteria; virus-resistant and mutant strains are developed.

• Easy : In the 1:1 yoghurt culture, if 20 g of Streptococcus is used, then 20 g of Lactobacillus is used too.
• Medium : Cheddar ripens in 3 to 12 months, parmesan in 12 to 18 months, so parmesan takes at least 12 − 12 = 0 to 15 months longer; at the extremes, 18 − 3 = 15 months more.
• Exam-type : Vegetarian cheese is made with protease from fungi instead of rennet from cattle, which is why some cheeses suit vegetarians.

Spot the difference

Rule 1: Probiotics — Probiotics are milk and other products with active useful bacteria (Lactobacillus acidophilus, L. casei, Bifidobacterium bifidum). They increase the useful microbes of the intestine, decrease harmful ones (Clostridium), improve resistance and re-activate microbes made inactive by antibiotics. Forms: yoghurt, kefir, sauerkraut, miso soup, pickles, microalgae (Spirulina, Chlorella). Used for diarrhoea and in poultry.

• Easy : Curd with live cultures eaten after antibiotics helps restore the gut's useful bacteria.
• Medium : Spirulina and Chlorella are microalgae sold as probiotic food supplements.
• Exam-type : Heating curd to boiling would kill the live bacteria, so a probiotic curd should be eaten without boiling.

Rule 2: Bread, vinegar and soya sauce — Bread: baker's yeast Saccharomyces cerevisiae ferments sugar into CO₂ and ethanol; the CO₂ makes the dough rise and the bread spongy. Vinegar is 4% acetic acid (CH₃COOH): yeast makes ethanol from fruit juices, molasses or starch, and Acetobacter and Glucanobacter convert it into acetic acid. Soya sauce: fermentation of wheat or rice flour and soyabean by the fungus Aspergillus oryzae. Ajinomoto is also made by microbial fermentation.

In the figure : In the figure steps in making vinegar.

• Easy : Dough rises because yeast makes CO₂ gas bubbles, which are trapped in the dough.
• Medium : A 250 mL bottle of vinegar with 4% acetic acid has 250 × 4 ÷ 100 = 10 mL of acetic acid.
• Exam-type : Two-step vinegar making: sugar → (yeast) → ethanol → (Acetobacter) → acetic acid.

Rule 3: Coffee, cocoa and drinks — Beverages: coffee and cocoa seeds are separated from the fruit by microbes (Lactobacillus brevis; Candida, Hansenula, Pichia, Saccharomyces). Wine from grapes and cider from apples are made by Saccharomyces cerevisiae.

• Easy : Microbes help separate the beans of coffee and cocoa from the fruit pulp.
• Medium : The same yeast, Saccharomyces cerevisiae, is used for bread, wine and cider, though the food is different.
• Exam-type : Fermenting grape juice gives alcohol, but fermenting dough gives CO₂ for rising; the same process, different aim.

Spot the difference

Rule 1: Microbial enzymes — Microbial enzymes replace chemical catalysts in industry. They are active at low temperature, pH and pressure (saving energy), specific (no unwanted by-products), reusable and eco-friendly. Uses: detergents; glucose and fructose syrup from corn flour; cheese; textile, leather and paper industries.

• Easy : An enzyme that works at 40 °C saves the fuel that a chemical process at 200 °C would burn.
• Medium : An enzyme acts only on its own substrate, so it makes only the wanted product with no unwanted waste.
• Exam-type : Name two advantages of enzymes over chemicals: they work at low temperature and are specific; they can often be reused.

Rule 2: Acids, vitamins and other products — L-glutamic acid (Brevibacterium, Corynebacterium) is used for ajinomoto. Citric acid (Aspergillus niger) is used in drinks and toffees. Gluconic acid (A. niger) is used for calcium and iron supplements; lactic acid (Lactobacillus delbrueckii); itaconic acid (Aspergillus itaconius). Xanthan gum (Xanthomonas) makes ice creams thick. Citric, malic and lactic acids give acidity; nycin and natamycin restrict microbes; vitamins C, B₁₂ and B₂ are antioxidants; beta carotene and lycopene are edible colours; xylitol and aspartame are low-calorie sweeteners.

In the figure : In the figure useful products made by microbes.

• Easy : The sour taste of a lemon-flavoured drink comes from citric acid made by Aspergillus niger.
• Medium : Aspartame and xylitol let us make sweets with fewer calories.
• Exam-type : Xanthan gum from Xanthomonas gives ice cream its smooth, thick body, so it is a food additive made by a bacterium.

Rule 3: Antibiotics — Antibiotics (प्रतिजैविके) are made by bacteria and fungi: penicillin, streptomycin, tetracyclines and others. Rifamycin is effective against tuberculosis.

• Easy : Penicillin comes from a mould and kills many disease-causing bacteria.
• Medium : Rifamycin is used in the treatment of tuberculosis.
• Exam-type : Antibiotics work on bacteria, not on viruses, so they do not cure a common cold.

Spot the difference

Rule 1: Fuels from microbes — Methane is made by anaerobic microbial decomposition of urban, agricultural and industrial waste. Ethanol (clean, smokeless) is made by yeast fermentation of molasses. Hydrogen, the fuel of the future, is made by bio-photolysis of water. Biofuels are renewable and exist in solid, liquid and gaseous forms (gobar gas).

In the figure : In the figure fuels made with the help of microbes.

• Easy : Gobar gas is a biofuel: cow dung is decomposed by microbes without oxygen to give methane.
• Medium : Ethanol from molasses can be blended with petrol, so it reduces use of petroleum and gives a cleaner burn.
• Exam-type : Why is a biofuel renewable? The plants or waste it comes from can be grown or produced again, unlike coal and petrol.

Rule 2: Solid waste and sewage — In a modern landfill (भराव), pits are lined with plastic sheets to stop toxic material from leaching; waste covered with soil, saw dust and leaves is decomposed by microbes, and the best compost forms in a few days. In sewage treatment microbes decompose carbon compounds (releasing methane and CO₂) and destroy the pathogens of cholera and typhoid; phenol-oxidizing bacteria decompose xenobiotic chemicals. The sludge is used as fertilizer and the treated water is safe. This is bioremediation.

In the figure : In the figure microbes clean sewage.

• Easy : Covering waste with soil, saw dust and leaves keeps it moist and full of microbes, so it turns into compost.
• Medium : Why the plastic sheet at the bottom of a landfill? To stop poisonous liquid from seeping into the ground and the groundwater.
• Exam-type : After treatment, sludge goes to farms as fertilizer and the clean water is released safely, so sewage becomes useful instead of a hazard.

Spot the difference

Rule 1: Microbes against pollution — Some microbes remove sulphur from fuels; thiobacilli and sulphobacilli convert metals into compounds before leaching. Oil spills are cleared by hydrocarbonoclastic bacteria (Pseudomonas, Alcanivorax borkumensis), which convert hydrocarbons into CO₂ and water. Vibrio and Ideonella sakaiensis decompose PET plastic; Actinomycetes and Streptomyces decompose rubber. Acidophilium and Acidobacillus ferroxidens use the sulphuric acid of acid rain; Geobacter converts soluble uranium salts into insoluble ones.

In the figure : In the figure oil-eating bacteria clean the sea.

• Easy : After an oil spill, Pseudomonas bacteria break the oil into CO₂ and water.
• Medium : Ideonella sakaiensis decomposes PET plastic, the kind used in water bottles.
• Exam-type : Geobacter turns soluble uranium into insoluble uranium, which cannot spread in groundwater, so it protects drinking water.

Rule 2: Microbes in farming — Microbial inoculants sprayed on seeds or released into plants supply nutrients; Azotobacter with artificial nitrogenase is used in organic farming and prevents soil pollution by chemical fertilizers. Microbes can destroy harmful pesticides such as fluoroacetamide in the soil. Bioinsecticides: bacterial and fungal toxins toxic to insects are built into plants; some fungi and viruses act as pesticides; spinosad is a biopesticide. Biodegradable polylactic acid plastic can be used to store garbage.

• Easy : Azotobacter added to the soil supplies nitrogen to crops in organic farming.
• Medium : A fungus that infects insects can be sprayed on a crop in place of a chemical pesticide.
• Exam-type : Compare: chemical insecticide may harm birds and soil; spinosad, a biopesticide, is derived from a soil bacterium and breaks down faster.

Spot the difference