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Question 1. What is the primary purpose of aseptic technique in microbiology laboratories? A) To enhance microbial growth B) To prevent contamination and ensure safety C) To stain bacteria more effectively D) To calibrate microscopes accurately Answer: B Explanation: Aseptic technique aims to prevent contamination of cultures and laboratory environments, ensuring safety and the integrity of experimental results.
Question 2. Which part of the microscope is responsible for adjusting the amount of light reaching the specimen? A) Ocular lens B) Objective lens C) Iris diaphragm D) Condenser Answer: C
Explanation: The iris diaphragm controls the amount of light passing through the condenser to illuminate the specimen properly.
Question 3. What is the significance of sterilizing inoculating loops until "cherry red"? A) It ensures the loop is cooled before use B) It indicates the loop is sterile and free of microbes C) It prevents microbial contamination during transfer D) It helps in staining procedures Answer: B Explanation: Heating the inoculating loop until cherry red disinfects it, ensuring it is sterile before transferring microbes, thus preventing contamination.
Question 4. How is total magnification calculated in light microscopy? A) Objective × Ocular B) Ocular + Objective
A) Calibrate the ocular micrometer with a stage micrometer B) Count the number of bacteria C) Adjust the condenser height D) Apply stain to the slide Answer: A Explanation: Calibration with a stage micrometer is necessary to determine the units of measurement for cell size.
Question 7. What is the primary purpose of heat-fixing a bacterial smear? A) To adhere bacteria to the slide and kill them B) To stain bacteria C) To increase bacterial motility D) To reduce bacterial size Answer: A
Explanation: Heat fixing adheres bacteria to the slide surface and kills them, preventing them from washing off during staining.
Question 8. Which stain is used as the primary stain in Gram staining? A) Safranin B) Crystal Violet C) Malachite Green D) Iodine Answer: B Explanation: Crystal Violet is the primary stain that initially colors all bacteria purple in Gram staining.
Question 9. What structural component in Gram-positive bacteria causes retention of crystal violet- iodine complex? A) Outer membrane B) Mycolic acids C) Thick peptidoglycan layer
A) Gram stain B) Acid-fast stain C) Endospore stain with Malachite Green D) Capsule stain with Nigrosin Answer: C Explanation: Malachite Green is used to stain endospores, allowing visualization of resistant spore structures.
Question 12. What is the purpose of a capsule stain? A) To visualize bacterial flagella B) To identify capsule formation by bacteria C) To stain bacterial DNA D) To detect acid-fast bacteria Answer: B Explanation: Capsule staining (negative stain) highlights the capsule as a clear halo around the bacterial cell.
Question 13. Which culture method is most suitable for isolating individual bacterial colonies? A) Pour plate B) Spread plate C) Quadrant streak plate D) Broth culture Answer: C Explanation: The quadrant streak plate technique dilutes bacteria across quadrants, leading to isolated colonies.
Question 14. Which type of media inhibits the growth of Gram-negative bacteria but allows Gram- positive bacteria to grow? A) Enriched media B) Selective media C) Differential media
B) Catalase test C) Urease test D) Indole test Answer: B Explanation: The catalase test detects enzyme that breaks down hydrogen peroxide into water and oxygen, producing bubbles.
Question 17. What does a positive oxidase test indicate? A) Presence of cytochrome c oxidase B) Ability to ferment glucose C) Production of urease D) Presence of catalase Answer: A Explanation: A positive oxidase test indicates the presence of cytochrome c oxidase, part of the electron transport chain.
Question 18. The IMViC series tests for bacteria primarily to differentiate among which group? A) Gram-negative enteric bacteria B) Gram-positive cocci C) Spore-forming bacteria D) Acid-fast bacteria Answer: A Explanation: IMViC tests are used to differentiate among Gram-negative enteric bacteria such as E. coli and Enterobacter.
Question 19. Which test evaluates the ability of bacteria to utilize citrate as the sole carbon source? A) Urease test B) Citrate utilization test C) MR test D) VP test
C) Heating at high temperature D) Osmotic pressure Answer: C Explanation: Raising temperature (e.g., boiling or incineration) inhibits or kills microbes by denaturing proteins.
Question 22. Which agent is commonly used to determine antibiotic susceptibility via disk diffusion? A) Disinfectant solution B) Kirby-Bauer disk C) Staining dye D) Enzyme substrate Answer: B Explanation: The Kirby-Bauer disk diffusion method uses antibiotic-impregnated disks to assess bacterial susceptibility.
Question 23. In the Kirby-Bauer test, an infected bacteria showing a large zone of inhibition is classified as: A) Resistant B) Susceptible C) Intermediate D) Inactive Answer: B Explanation: A large zone of inhibition indicates the bacteria are susceptible to the antibiotic.
Question 24. Which test is used to identify the presence of Salmonella by detecting hydrogen sulfide production? A) Urease test B) SIM media C) Indole test D) Catalase test Answer: B
C) Mannitol salt agar D) Chocolate agar Answer: B Explanation: Blood agar allows visualization of hemolysis patterns, helping identify hemolytic bacteria.
Question 27. What is the primary purpose of a motility test using SIM media? A) To detect carbohydrate fermentation B) To observe bacterial motility C) To identify spore formation D) To determine urease activity Answer: B Explanation: The SIM medium tests for bacterial motility by observing diffuse growth away from the stab line.
Question 28. Which biochemical test detects the ability of bacteria to produce indole from tryptophan?
A) MR test B) VP test C) Indole test D) Citrate test Answer: C Explanation: The indole test detects tryptophanase activity, which produces indole from tryptophan.
Question 29. What structural feature in bacteria causes a gram-negative cell to appear pink following Gram staining? A) Thick peptidoglycan layer B) Outer membrane and thin peptidoglycan C) Mycolic acids D) Flagella Answer: B Explanation: The outer membrane and thin peptidoglycan layer in gram-negative bacteria do not retain crystal violet, appearing pink after counterstaining.
Answer: A Explanation: Enriched media contain additional nutrients to support the growth of fastidious bacteria.
Question 32. What is the primary reason for performing serial dilutions in microbiology? A) To increase microbial concentration B) To isolate pure colonies C) To estimate microbial population in a sample D) To stain bacteria more effectively Answer: C Explanation: Serial dilutions reduce microbial concentration, enabling accurate enumeration through plate counts.
Question 33. Which temperature classification describes bacteria that grow optimally at temperatures below 20°C? A) Psychrophiles B) Mesophiles
C) Thermophiles D) Hyperthermophiles Answer: A Explanation: Psychrophiles thrive at low temperatures, typically below 20°C.
Question 34. How does high salt concentration affect microbial growth? A) It accelerates growth B) It inhibits most bacteria but favors halophiles C) It destroys bacterial cell walls D) It has no effect Answer: B Explanation: High osmotic pressure inhibits many microbes but supports halophiles adapted to saline environments.
Question 35. Which molecule is primarily responsible for UV-induced DNA damage in bacteria?
