Microorganisms: Friend and Foe – Case-based Questions with Answers
Class 8
Science
Chapter 2
Microorganisms: Friend and Foe — 20 Case-Based Questions (Topic-wise)
CBSE Class 8 Science – Chapter Wise Study Materials Based on NCERT
Content Bank — Chapter highlights
- Types of microbes: bacteria, fungi, viruses, protozoa and algae
- Roles: beneficial (food, medicine, environment) and harmful (diseases, spoilage)
- Control methods: sterilisation, pasteurisation, antiseptics, antibiotics, vaccines
- Practical activities: curd making, yeast fermentation, observing moulds
CBSE Board Examinations — Chapter Presentation (Systematic):
- Unit Focus: Apply microbe concepts to real-life situations — health, food, agriculture and environment.
- Question Style: Case-based scenarios test comprehension, application and reasoning. Answers are NCERT-aligned and exam-focused.
Topic A — Basics & Identification (Cases 1–5)
Case 1: A student leaves a slice of bread on a moist plate for a week and notices fuzzy green patches.
Q1: Identify the organism likely responsible and state why it grew.
A1: Mould (a fungus) likely grew because moulds thrive in warm, moist, nutrient-rich conditions on organic matter like bread.
Case 2: Two bottles contain boiled milk — one left uncovered, the other sealed. After a day the uncovered one sours.
Q2: Explain which microbes caused souring and one preventive measure.
A2: Lactic acid bacteria (e.g., Lactobacillus) cause milk souring by fermenting lactose to lactic acid. Preventive measure: keep milk refrigerated and cover it to reduce contamination.
Case 3: A microscope slide shows tiny rod-shaped cells dividing into two.
Q3: What process is observed and which group of microbes does it suggest?
A3: Binary fission (cell division) is observed, which suggests bacteria, as many bacteria reproduce by binary fission.
Case 4: A viral infection spreads rapidly at a school via sneezing and coughing.
Q4: State the likely mode of transmission and one control measure the school should adopt.
A4: Mode: airborne droplet transmission. Control: encourage respiratory hygiene — cover mouth while coughing, use masks when necessary, and ensure good ventilation.
Case 5: A pond shows greenish water with slimy surface in summer.
Q5: Which group of microorganisms is likely abundant and what role do they play?
A5: Algae (phytoplankton/cyanobacteria) are likely abundant; they photosynthesise, producing oxygen and supporting aquatic food chains but can cause algal blooms that deplete oxygen.
Topic B — Beneficial Microorganisms (Cases 6–10)
Case 6: A baker sets dough and notices it rises, becoming spongy.
Q6: Explain the microbiological process behind this and name the organism involved.
A6: Yeast ferments sugars in the dough producing CO₂ gas, which forms bubbles and makes the dough rise. Organism: Saccharomyces cerevisiae (yeast).
Case 7: A farmer grows legumes and notices nodules on roots.
Q7: Identify the microbes involved and their agricultural benefit.
A7: Rhizobium bacteria form symbiotic nodules and fix atmospheric nitrogen into forms usable by plants, enriching soil nitrogen and reducing fertilizer needs.
Case 8: A patient receives penicillin and recovers from a bacterial infection.
Q8: State the source of penicillin and its mode of action (brief).
A8: Penicillin is produced by the fungus Penicillium; it inhibits bacterial cell wall synthesis, causing bacterial cells to burst.
Case 9: A community sets up a compost pit for vegetable waste.
Q9: Explain how microbes help in composting and one benefit to soil.
A9: Decomposer microbes (bacteria and fungi) break down organic waste into humus, releasing nutrients and improving soil structure and fertility.
Case 10: A biotech firm uses bacteria to produce insulin.
Q10: Name the technique used and one advantage of microbial production of medicines.
A10: Technique: recombinant DNA technology (genetic engineering). Advantage: large-scale, cost-effective production of human proteins (e.g., insulin) with high purity.
Topic C — Harmful Microbes & Diseases (Cases 11–14)
Case 11: After a picnic several people develop watery diarrhoea the next day.
Q11: Suggest a likely cause and two immediate steps to manage the outbreak.
A11: Likely cause: food or water contaminated with bacteria (e.g., Vibrio, Salmonella) or toxins. Steps: ensure rehydration (oral rehydration salts), seek medical care and identify/remove contaminated food source.
Case 12: A crop field shows wilting and discolored leaves despite adequate water.
Q12: Propose a microbial cause and one control measure.
A12: Cause: a plant pathogen (fungus or bacteria) causing wilt or blight. Control: use resistant varieties, crop rotation and remove infected plants to limit spread.
Case 13: An influenza outbreak occurs in a town during winter.
Q13: Identify the type of pathogen and one public health action to reduce spread.
A13: Pathogen: virus (influenza virus). Action: vaccination campaigns, respiratory hygiene, isolation of sick individuals and promoting handwashing.
Case 14: Stored grains develop weevil infestation during monsoon.
Q14: Explain why infestation occurred and one traditional remedy to protect grains.
A14: High humidity and improper drying encourage insect infestations. Traditional remedy: dry grains properly and store with neem leaves as a natural repellent; use airtight containers.
Topic D — Controlling Microorganisms (Cases 15–17)
Case 15: A clinic needs to sterilise surgical instruments.
Q15: Recommend a sterilisation method and briefly explain why it is suitable.
A15: Autoclaving (steam under pressure) is recommended because it kills all microbes and spores effectively and is standard for surgical instruments.
Case 16: A small dairy intends to make milk safer without changing taste much.
Q16: Suggest a suitable process and its basic principle.
A16: Pasteurisation — mild heating (e.g., 72°C for 15 seconds) to kill most pathogens while preserving taste and nutrients.
Case 17: A laboratory worker spills a disinfectant and must clean the bench.
Q17: Explain the difference between disinfectant and antiseptic and which one to use here.
A17: Disinfectants are for inanimate surfaces (e.g., bleach) and should be used to clean the bench; antiseptics are milder and used on skin.
Topic E — Practicals & Classroom Observations (Cases 18–20)
Case 18: In a supervised lab, students set up yeast + sugar + warm water and fit balloons on flasks; balloons inflate over time.
Q18: Explain what students observed and one real-life application.
A18: Observation: CO₂ produced by yeast fermentation inflates the balloon. Application: explains bread leavening and alcohol fermentation in brewing.
Case 19: A class compares two milk samples — boiled then cooled vs raw — by adding a spoon of curd; only boiled-cooled milk sets into curd properly.
Q19: Why did this happen and what does it teach about microbes?
A19: Boiling reduced unwanted microbes; adding starter culture (curd) then allows desired lactic bacteria to ferment milk. Teaches importance of controlled microbial inoculation and hygiene.
Case 20: A community health worker demonstrates handwashing and shows reduced incidence of infections in follow-up visits.
Q20: Explain why handwashing is effective and mention two critical moments to wash hands.
A20: Handwashing physically removes microbes, preventing transmission. Critical moments: before eating/handling food and after using the toilet or caring for a sick person.
Note: All Case-Based Questions and Answers are organised topic-wise and strictly follow the NCERT syllabus, making them ideal for CBSE Class 8 board exams standard.
Exam tip: For case-based answers keep responses concise — identify the concept, state 2–3 key points and give a brief example.