Lesson 8: Useful and Harmful Microbes

In the figure : In the figure lactobacilli turn milk into curd.

Rule 1: How curd forms — Lactobacilli are rod-shaped, anaerobic bacteria (they make energy without oxygen). They are seen blue in buttermilk stained with methylene blue. They convert lactose, the milk sugar, into lactic acid by fermentation. The pH of the milk falls, the milk proteins coagulate and the milk becomes yoghurt with a sour taste. The low pH destroys harmful microbes.

• Easy : Curd tastes sour because of lactic acid.
• Medium : More lactic acid means more acidity, and more acidity means a lower pH, so the pH of the milk keeps falling as curd sets.
• Exam-type : Why is the old curd added in a little amount only? A few lactobacilli multiply in warm milk and make all the milk curd.

Rule 2: Probiotics and uses — Probiotic foods contain useful microbes such as lactobacilli. They kill harmful bacteria like Clostridium in the alimentary canal and improve immunity. Uses: milk products (yoghurt, buttermilk, ghee, cheese, shrikhand), cider, cocoa, pickles, treatment of abdominal discomfort, leavened fodder for cattle, and wine and some breads.

• Easy : Curd, buttermilk and shrikhand are made with lactobacilli.
• Medium : A glass of buttermilk after a heavy meal helps because its useful bacteria help the gut.
• Exam-type : Why are probiotic foods called helpful? They carry useful microbes that kill harmful bacteria like Clostridium and improve immunity.

Spot the difference

In the figure : In the figure Rhizobium and the plant help each other.

Rule 1: Symbiosis — Rhizobium lives in the root nodules of leguminous plants such as beans, groundnut, soyabean and sweet pea. The rhizobia give the plant nitrates, nitrites and amino acids, and get energy as carbohydrates from the plant. This mutually beneficial relationship is symbiosis.

• Easy : The plant gives sugar; the bacteria give nitrogen compounds.
• Medium : Groundnut and soyabean both host rhizobia, so both are rich in protein.
• Exam-type : Is it symbiosis if only one partner gains? No. Symbiosis needs benefit to both partners.

Rule 2: Nitrogen fixation and inoculation — Rhizobia make nitrogen compounds from atmospheric nitrogen, which is nitrogen fixation; that is why beans and pulses are rich in proteins. They need leguminous plants as hosts. Leftover roots of a harvested leguminous crop are dumped in the field, and seeds are coated with rhizobial solution or powder, which is rhizobial inoculation. This reduces the use and the cost of chemical fertilizers.

• Easy : Coating bean seeds with rhizobial powder is rhizobial inoculation.
• Medium : A farmer saving the cost of chemical fertilizer on a 2-acre bean field: if the fertilizer costs 3000 rupees per acre, he saves 2 × 3000 = 6000 rupees.
• Exam-type : Why plough in the leftover roots? They still hold the nodules with nitrogen compounds, which enrich the soil.

Spot the difference

In the figure : In the figure fermentation by yeast.

Rule 1: About yeast — Yeast is a unicellular, eukaryotic, heterotrophic fungus with about 1500 species. It reproduces asexually by budding. In a sugar solution it multiplies quickly and converts carbohydrates into alcohol and carbon dioxide by fermentation. The gas makes dough rise and bread spongy.

• Easy : Bread has tiny holes because carbon dioxide bubbles filled the dough.
• Medium : Yeast needs sugar; with no sugar it cannot ferment and the dough will not rise.
• Exam-type : Yeast is eukaryotic and heterotrophic: it has a nucleus and cannot make its own food.

Rule 2: Ethanol, wine and bio-remediation — Molasses from sugarcane juice is fermented by the yeast Saccharomyces to give ethanol (C₂H₅OH) as the main product. Ethanol is a smokeless, high quality fuel, and in India 10% ethanol is mixed with petrol and diesel. Maize, barley and other grains are also used, and the glucose and fructose of grape juice are fermented by yeast to make wine. Bio-remediation: the yeast Yarrowia lipolytica absorbs toxins and heavy metals, Saccharomyces cerevisiae absorbs arsenic, and Alcanivorax bacteria clean oil spills in the ocean.

• Easy : Petrol with 10% ethanol: in 20 litres of mixture, ethanol is 20 × 10 ÷ 100 = 2 litres.
• Medium : In 50 litres of such fuel, petrol is 50 − 5 = 45 litres.
• Exam-type : Why is ethanol a "clean" fuel? It is smokeless and has high quality.

Spot the difference

In the figure : In the figure broad- and narrow-spectrum antibiotics.

Rule 1: Antibiotics — Antibiotics are carbon compounds obtained from some bacteria and fungi to destroy or stop the growth of harmful micro-organisms. They mainly act on bacteria, and some act on protozoa. They brought a revolution in medicine, and tuberculosis has almost been eradicated in some countries. Broad-spectrum antibiotics (ampicillin, amoxicillin, tetracycline) act on many bacteria and are used when the pathogen is unknown. Narrow-spectrum antibiotics (penicillin, gentamycin, erythromycin) are used when the pathogen is known.

• Easy : Amoxicillin is a broad-spectrum antibiotic; penicillin is narrow-spectrum.
• Medium : If the germ causing the infection is not known, a broad-spectrum antibiotic is chosen.
• Exam-type : Antibiotics do not work against viruses, because they act mainly on bacteria. So they are not the medicine for a common viral cold.

Rule 2: Penicillin and caution — Penicillin comes from the fungus Penicillium. It treats Staphylococci, Clostridia and Streptococci infections of the ear, nose, throat and skin, pneumonia and scarlet fever. Alexander Fleming discovered it accidentally on 3 September 1928, when he saw bacteria destroyed around fungal colonies in a Petri dish. Caution: take antibiotics only when a doctor prescribes them, complete the full course, do not buy them without a prescription and do not suggest them to others. The National Institute of Virology was set up in Pune in 1952.

• Easy : Fleming discovered penicillin in 1928 from the fungus Penicillium.
• Medium : A doctor prescribes tablets for 5 days. If you stop on day 3 because you feel better, some bacteria may survive. Always finish the course.
• Exam-type : From 1928 to 1952 (NIV Pune) is 1952 − 1928 = 24 years.

Spot the difference

In the figure : In the figure which microbes cause which diseases.

Rule 1: Fungi that spoil things — Fungal spores in the air germinate on cotton, gunny bags, leather and wood when moisture is present, and their hyphae weaken the material. On pickles, murabba, jam and sauces, fungi release poisonous mycotoxins, so such food must not be eaten.

• Easy : A mouldy jam jar must be thrown away.
• Medium : Leather shoes kept in a damp cupboard get a fuzzy coat because fungal spores germinate with moisture.
• Exam-type : To keep things safe from fungi, keep them dry. Moisture is what the spores need to germinate.

Rule 2: Clostridium — Clostridium bacteria spoil cooked food and cause food poisoning. They are rod-shaped and anaerobic, and form bottle-shaped endospores in adverse conditions. Van Ermengem proved that Clostridium botulinum causes food poisoning.

• Easy : Cooked food kept too long can have Clostridium and cause food poisoning.
• Medium : Spores let Clostridium survive hard times, like a seed does.
• Exam-type : Eat freshly cooked food and store leftovers properly to avoid food poisoning.

Rule 3: Diseases and prevention — Viral diseases: AIDS, hepatitis, influenza, measles, chicken pox, bird flu, swine flu and dengue. Bacterial diseases: pneumonia, leprosy and cholera. Protozoal disease: malaria. Fungal diseases: dandruff, ringworm and scabies. Prevention includes vaccination, clean water and food, personal hygiene and mosquito control.

• Easy : Malaria is protozoal; cholera is bacterial.
• Medium : Measles and dengue are viral, so antibiotics will not cure them.
• Exam-type : Mosquito control (drain stagnant water) helps to prevent malaria and dengue.

Spot the difference