Microorganisms: Friend and Foe – MCQs with Answers and Explanations
Class 8
Science
Chapter 2
Microorganisms: Friend and Foe — 50 Topic-wise MCQs with Answers & Explanations
CBSE Class 8 Science – Chapter Wise Study Materials Based on NCERT
CBSE Board Examinations — Chapter Presentation (Systematic):
- Unit focus: Types of microbes, their roles, diseases, control methods and practical applications.
- Question style: Multiple Choice Questions (MCQs) with detailed explanations for concept clarity.
- Instructions: These MCQs are strictly NCERT-aligned for CBSE Class 8 board exam standard. No countdown timer included.
Use these MCQs for classroom tests, self-assessment and revision. Each question includes the correct option and a concise explanation.
Topic 1: Basics & Types (Questions 1–9)
Q1. Microorganisms are usually measured in:
A. Centimetres
B. Metres
C. Micrometres (µm)
D. Kilometres
Answer: C. Micrometres (µm)
Explanation: Microbes such as bacteria and single-celled algae are microscopic and typically measured in micrometres (one-millionth of a metre).
Q2. Which of the following is NOT a microorganism?
A. Bacteria
B. Virus
C. Fungus (mould)
D. Earthworm
Answer: D. Earthworm
Explanation: Earthworms are multicellular macro-organisms visible to the naked eye; bacteria, viruses and moulds are microscopic.
Q3. Which microbe is an acellular particle requiring a host to reproduce?
A. Bacterium
B. Virus
C. Fungus
D. Alga
Answer: B. Virus
Explanation: Viruses are not cells; they invade host cells and use host machinery to replicate.
Q4. Which microorganism is most likely responsible for fermentation in bread-making?
A. Penicillium
B. Yeast
C. Amoeba
D. Algae
Answer: B. Yeast
Explanation: Yeasts (e.g., Saccharomyces cerevisiae) ferment sugars producing CO₂ that makes dough rise.
Q5. Which of the following microbes photosynthesises?
A. Fungi
B. Bacteria
C. Algae
D. Viruses
Answer: C. Algae
Explanation: Algae are photosynthetic organisms producing oxygen and serving as primary producers in aquatic ecosystems.
Q6. Which shape is commonly associated with cocci bacteria?
A. Rod-shaped
B. Spherical
C. Spiral
D. Filamentous
Answer: B. Spherical
Explanation: Cocci are spherical bacteria; bacilli are rod-shaped and spirilla are spiral-shaped.
Q7. Which group includes moulds used in antibiotic production?
A. Protozoa
B. Fungi
C. Bacteria
D. Algae
Answer: B. Fungi
Explanation: Moulds like Penicillium (a fungus) produce antibiotics such as penicillin.
Q8. Which of the following is a unicellular fungus?
A. Yeast
B. Mould
C. Alga
D. Virus
Answer: A. Yeast
Explanation: Yeasts are unicellular fungi; moulds are multicellular and form hyphae.
Q9. Which microbe is used for making curd?
A. Rhizobium
B. Lactobacillus
C. Penicillium
D. Cyanobacteria
Answer: B. Lactobacillus
Explanation: Lactobacillus bacteria ferment milk sugar into lactic acid, forming curd.
Topic 2: Habitats & Growth Conditions (Questions 10–18)
Q10. Which condition favours bacterial growth?
A. Dry and cold
B. Warm and moist
C. Extremely hot
D. Vacuum
Answer: B. Warm and moist
Explanation: Many bacteria grow rapidly in warm, moist environments with available nutrients.
Q11. Mould growth on bread is accelerated by:
A. Freezing
B. Drying
C. Moisture and warmth
D. Removing oxygen
Answer: C. Moisture and warmth
Explanation: Moulds need moisture and moderate temperatures to grow on food surfaces.
Q12. Extremophiles are microbes that can live in:
A. Only human body
B. Harsh extreme environments like hot springs or salt lakes
C. Only soil
D. Only freshwater
Answer: B. Harsh extreme environments like hot springs or salt lakes
Explanation: Extremophiles adapt to extreme temperatures, salinity, acidity or pressure.
Q13. Which factor does NOT directly affect microbial growth?
A. Temperature
B. Nutrient supply
C. Humidity
D. Moon phase
Answer: D. Moon phase
Explanation: Moon phase does not scientifically influence microbial growth; temperature, nutrients and moisture do.
Q14. Which habitat is rich in diverse microbes and used for composting?
A. Sterile glass bottle
B. Compost heap with decaying organic matter
C. Clean running tap water
D. Salt crystals
Answer: B. Compost heap with decaying organic matter
Explanation: Compost heaps provide nutrients, moisture and aeration ideal for decomposer microbes.
Q15. Why does refrigeration slow down food spoilage?
A. It kills all microbes instantly
B. It slows microbial metabolism and reproduction
C. It adds preservatives
D. It increases humidity
Answer: B. It slows microbial metabolism and reproduction
Explanation: Low temperatures reduce metabolic rates of microbes, delaying spoilage but not killing all microbes.
Q16. Which microbes are commonly found in human gut as normal flora?
A. Coliform bacteria like E. coli (non-pathogenic strains)
B. Viruses only
C. Large fungi only
D. None
Answer: A. Coliform bacteria like E. coli (non-pathogenic strains)
Explanation: Many non-pathogenic bacterial species are part of the gut microbiota aiding digestion and immunity.
Q17. Standing water with excess nutrients often leads to:
A. Decrease in algal growth
B. Algal bloom
C. Complete sterilization
D. No change
Answer: B. Algal bloom
Explanation: Nutrient pollution (e.g., nitrates, phosphates) leads to excessive algal growth which can harm aquatic life.
Q18. Which practice reduces microbial contamination of drinking water at home?
A. Storing water in open containers
B. Boiling water before drinking
C. Adding sugar to water
D. Leaving water in sunlight only
Answer: B. Boiling water before drinking
Explanation: Boiling kills many disease-causing microbes, making water safer for consumption.
Topic 3: Beneficial Microorganisms (Questions 19–30)
Q19. Which bacteria are involved in nitrogen fixation in legumes?
A. Rhizobium
B. Streptococcus
C. Staphylococcus
D. E. coli
Answer: A. Rhizobium
Explanation: Rhizobium forms root nodules on legumes and converts atmospheric nitrogen into usable forms for the plant.
Q20. Which process uses microbes to convert sugar into alcohol?
A. Pasteurisation
B. Fermentation
C. Sterilisation
D. Distillation
Answer: B. Fermentation
Explanation: Yeast ferments sugars anaerobically to form ethanol and carbon dioxide; used in brewing and baking.
Q21. Penicillin is derived from which organism?
A. Bacterium Bacillus
B. Fungus Penicillium
C. Alga
D. Virus
Answer: B. Fungus Penicillium
Explanation: Penicillin is an antibiotic produced by the mould Penicillium; it targets bacterial cell wall synthesis.
Q22. Which microbial application helps treat sewage and wastewater?
A. Composting
B. Sewage treatment using microbes
C. Baking
D. Freezing
Answer: B. Sewage treatment using microbes
Explanation: Microbial digestion breaks down organic waste in sewage plants, reducing pollutants before discharge.
Q23. Which microbes are commonly used to make curd?
A. Yeast
B. Lactobacillus
C. Moulds like Rhizopus
D. Cyanobacteria
Answer: B. Lactobacillus
Explanation: Lactobacillus ferments lactose into lactic acid, thickening milk into curd and preserving it.
Q24. Microbial enzymes are used in which industry?
A. Food and beverage
B. Textile
C. Detergent
D. All of the above
Answer: D. All of the above
Explanation: Enzymes from microbes are used across industries — food processing, textiles, detergents and more.
Q25. Which technology uses bacteria to produce human insulin?
A. Traditional fermentation
B. Recombinant DNA technology
C. Pasteurisation
D. Sterilisation
Answer: B. Recombinant DNA technology
Explanation: Human insulin genes are inserted into bacterial plasmids to produce insulin in large quantities.
Q26. Which microbial process helps decompose dead plants and animals?
A. Photosynthesis
B. Decomposition by decomposer microbes
C. Aeration
D. Compost burning
Answer: B. Decomposition by decomposer microbes
Explanation: Bacteria and fungi break down organic matter returning nutrients to the soil.
Q27. Which of the following is a benefit of microbial bioremediation?
A. Increased pollution
B. Degradation of pollutants like oil
C. Faster corrosion
D. None
Answer: B. Degradation of pollutants like oil
Explanation: Specific microbes can metabolize hydrocarbons and break down pollutants in contaminated sites.
Q28. Which microorganisms help in the production of alcohol?
A. Bacteria only
B. Yeasts
C. Moulds only
D. Protozoa
Answer: B. Yeasts
Explanation: Yeast ferments sugars to produce ethanol in brewing and wine-making.
Q29. Which term describes bacteria that live in the absence of oxygen?
A. Aerobic
B. Anaerobic
C. Phototrophic
D. Thermophilic
Answer: B. Anaerobic
Explanation: Anaerobic bacteria do not require oxygen and may even be harmed by it; some perform fermentation.
Q30. Which microbe is commonly used to produce curd?
A. Saccharomyces cerevisiae
B. Lactobacillus
C. Penicillium
D. Plasmodium
Answer: B. Lactobacillus
Explanation: Lactobacillus ferments lactose into lactic acid, transforming milk into curd.
Topic 4: Harmful Microorganisms & Diseases (Questions 31–40)
Q31. Which organism causes tuberculosis?
A. Mycobacterium tuberculosis
B. Influenza virus
C. Candida
D. Plasmodium
Answer: A. Mycobacterium tuberculosis
Explanation: TB is a bacterial disease caused by Mycobacterium tuberculosis affecting lungs and sometimes other organs.
Q32. Which pathogen causes malaria?
A. Bacterium
B. Protozoan (Plasmodium)
C. Fungus
D. Virus
Answer: B. Protozoan (Plasmodium)
Explanation: Malaria is caused by Plasmodium species transmitted by Anopheles mosquitoes (vector-borne).
Q33. Which of these is a viral disease?
A. Cholera
B. Influenza
C. Ringworm
D. Rust in plants
Answer: B. Influenza
Explanation: Influenza is caused by influenza viruses; cholera is bacterial and ringworm fungal.
Q34. Food poisoning can be caused by microbes or their:
A. Vitamins
B. Toxins
C. Oxygen
D. Light
Answer: B. Toxins
Explanation: Some microbes produce toxins (e.g., Staphylococcus aureus) that cause food poisoning even if microbes are dead.
Q35. Which measure helps prevent vector-borne diseases?
A. Increasing stagnant water
B. Eliminating mosquito breeding sites
C. Not using nets
D. Ignoring vaccination
Answer: B. Eliminating mosquito breeding sites
Explanation: Removing stagnant water, using larvicides and bed nets reduce mosquito populations and disease transmission.
Q36. Antibiotics are effective against:
A. Viruses
B. Bacteria
C. All microbes
D. Fungi only
Answer: B. Bacteria
Explanation: Antibiotics target bacterial structures/processes and are not effective for viral infections.
Q37. Which practice reduces spread of respiratory infections in schools?
A. Sharing handkerchiefs
B. Encouraging cough etiquette and handwashing
C. Not ventilating rooms
D. Ignoring sick students
Answer: B. Encouraging cough etiquette and handwashing
Explanation: Respiratory hygiene and hand hygiene reduce droplet and contact transmission of pathogens.
Q38. Which of the following is a symptom commonly caused by many infections?
A. Fever
B. Hair growth
C. Improved appetite
D. None
Answer: A. Fever
Explanation: Fever is a common immune response to infection, signalling the body is fighting pathogens.
Q39. Cholera is primarily spread through:
A. Airborne droplets
B. Contaminated water
C. Mosquito bites
D. Sexual contact
Answer: B. Contaminated water
Explanation: Vibrio cholerae spreads via contaminated water and food, leading to severe watery diarrhoea.
Q40. Which of the following is NOT a method to prevent food spoilage?
A. Refrigeration
B. Proper drying
C. Using clean containers
D. Leaving food in warm open air for long
Answer: D. Leaving food in warm open air for long
Explanation: Exposing food to warm open air speeds microbial growth and spoilage; the other options help preserve food.
Topic 5: Control Measures (Questions 41–46)
Q41. Sterilisation aims to:
A. Reduce microbes partially
B. Kill all microbes and spores
C. Increase microbial growth
D. Preserve food
Answer: B. Kill all microbes and spores
Explanation: Sterilisation (e.g., autoclaving) completely eliminates all forms of life including resistant spores.
Q42. Pasteurisation is mainly used to:
A. Sterilise instruments
B. Kill most pathogens in food like milk while retaining quality
C. Grow microbes
D. Remove water
Answer: B. Kill most pathogens in food like milk while retaining quality
Explanation: Pasteurisation applies mild heat to reduce pathogens without major changes to taste or nutrients.
Q43. An antiseptic is used to:
A. Clean surgical instruments only
B. Reduce microbes on living tissues like skin
C. Sterilise food
D. Feed microbes
Answer: B. Reduce microbes on living tissues like skin
Explanation: Antiseptics like iodine are safe for skin application to prevent infections.
Q44. Which practice helps reduce antibiotic resistance?
A. Completing prescribed antibiotic course
B. Using leftover antibiotics without advice
C. Giving antibiotics for viral infections
D. Overusing antibiotics in animals
Answer: A. Completing prescribed antibiotic course
Explanation: Proper use and completion prevent surviving bacteria from developing resistance; misuse promotes resistance.
Q45. Which water treatment step helps kill microbes?
A. Filtration only
B. Chlorination
C. Adding sugar
D. Heating then cooling immediately
Answer: B. Chlorination
Explanation: Chlorination disinfects water killing many pathogens; filtration removes solids and some microbes.
Q46. Which of the following prevents cross-contamination in food handling?
A. Using same cutting board for raw meat and vegetables
B. Washing hands and using separate utensils
C. Not washing hands
D. Keeping food at room temperature long
Answer: B. Washing hands and using separate utensils
Explanation: Hygiene and separation of raw and cooked foods reduce transfer of pathogens.
Topic 6: Practicals & Applications (Questions 47–50)
Q47. In a classroom yeast experiment, balloon inflation shows production of:
A. Oxygen
B. Carbon dioxide
C. Nitrogen
D. Hydrogen
Answer: B. Carbon dioxide
Explanation: Yeast ferments sugar producing CO₂ which inflates the balloon — same process causes dough to rise.
Q48. Adding a spoon of curd to warm milk and keeping it covered results in:
A. Milk turning sour into curd (controlled fermentation)
B. Milk becoming clear
C. Milk evaporating
D. No change
Answer: A. Milk turning sour into curd (controlled fermentation)
Explanation: Starter curd adds beneficial Lactobacillus that ferment milk into curd under warm conditions.
Q49. Observing mould on fruit is an example of which process?
A. Photosynthesis
B. Microbial spoilage
C. Nitrogen fixation
D. Sterilisation
Answer: B. Microbial spoilage
Explanation: Mould growth on fruit indicates spoilage due to fungal colonisation and decomposition.
Q50. Which simple natural method helps protect stored grains from insects?
A. Mixing grains with wet soil
B. Adding neem leaves
C. Keeping grains open in rain
D. Leaving grains warm and moist
Answer: B. Adding neem leaves
Explanation: Neem leaves act as natural insect repellents and are traditionally used to reduce pest attack in stored grains.
Note: These MCQs are topic-wise and strictly follow NCERT Class 8 Chapter 2 content, making them suitable for CBSE revision and classroom assessments.