Q.1 Which of the following is the primary stain used in Gram staining?
Crystal violet
Safranin
Methylene blue
Carbol fuchsin
Explanation - Crystal violet is the primary stain in Gram staining, which helps differentiate Gram-positive and Gram-negative bacteria based on cell wall properties.
Correct answer is: Crystal violet
Q.2 In acid-fast staining, what is the main purpose of using acid-alcohol?
To act as a mordant
To decolorize non-acid-fast cells
To counterstain cells
To fix the stain
Explanation - Acid-alcohol removes primary stain from non-acid-fast cells but not from acid-fast bacteria due to the presence of mycolic acids.
Correct answer is: To decolorize non-acid-fast cells
Q.3 Which technique is most commonly used for bacterial colony isolation?
Streak plate method
Spread plate method
Pour plate method
Slant culture
Explanation - The streak plate method allows for dilution of bacteria on agar surface to obtain isolated colonies.
Correct answer is: Streak plate method
Q.4 What is the role of iodine in Gram staining?
Primary stain
Decolorizer
Mordant
Counterstain
Explanation - Iodine acts as a mordant, forming a complex with crystal violet to enhance stain retention in Gram-positive cells.
Correct answer is: Mordant
Q.5 Which culture medium is selective for Gram-negative bacteria?
Nutrient agar
MacConkey agar
Blood agar
Chocolate agar
Explanation - MacConkey agar contains bile salts and crystal violet, which inhibit Gram-positive bacteria while allowing Gram-negative growth.
Correct answer is: MacConkey agar
Q.6 Which diagnostic test is used for identifying Mycobacterium tuberculosis?
Ziehl-Neelsen stain
Gram stain
Coagulase test
Catalase test
Explanation - Ziehl-Neelsen stain is used to detect acid-fast bacteria such as Mycobacterium tuberculosis.
Correct answer is: Ziehl-Neelsen stain
Q.7 Blood agar is primarily used to detect which bacterial property?
Motility
Hemolysis
Capsule formation
Antibiotic resistance
Explanation - Blood agar helps differentiate bacteria based on hemolytic activity (alpha, beta, or gamma hemolysis).
Correct answer is: Hemolysis
Q.8 Which enzyme is tested using the catalase test?
Catalase
Oxidase
Coagulase
Urease
Explanation - Catalase test detects the presence of the catalase enzyme, which breaks down hydrogen peroxide into water and oxygen.
Correct answer is: Catalase
Q.9 What is the principle of ELISA in diagnostic microbiology?
Detection of bacterial DNA
Antigen-antibody interaction
Protein denaturation
Lipid membrane staining
Explanation - ELISA is based on antigen-antibody binding and detection using an enzyme-linked indicator.
Correct answer is: Antigen-antibody interaction
Q.10 Which medium is enriched and used for fastidious organisms like Neisseria?
Chocolate agar
Nutrient agar
MacConkey agar
Sabouraud agar
Explanation - Chocolate agar provides essential growth factors for fastidious organisms such as Neisseria and Haemophilus.
Correct answer is: Chocolate agar
Q.11 Which staining technique is used to visualize bacterial capsules?
Negative staining
Gram staining
Acid-fast staining
Endospore staining
Explanation - Negative staining with India ink or nigrosin highlights capsules as clear halos around bacteria.
Correct answer is: Negative staining
Q.12 Which test differentiates Staphylococcus aureus from other Staphylococcus species?
Catalase test
Coagulase test
Oxidase test
Indole test
Explanation - Coagulase test is positive in Staphylococcus aureus, distinguishing it from other staphylococci.
Correct answer is: Coagulase test
Q.13 What does the oxidase test detect?
Presence of catalase
Presence of cytochrome c oxidase
Fermentation of glucose
Hemolytic activity
Explanation - The oxidase test identifies bacteria producing cytochrome c oxidase enzyme.
Correct answer is: Presence of cytochrome c oxidase
Q.14 Which medium is commonly used for fungal culture?
Sabouraud dextrose agar
MacConkey agar
Nutrient agar
Chocolate agar
Explanation - Sabouraud dextrose agar supports fungal growth due to its acidic pH and high dextrose content.
Correct answer is: Sabouraud dextrose agar
Q.15 Which microscopic technique uses fluorescent dyes for bacterial detection?
Phase contrast microscopy
Fluorescence microscopy
Electron microscopy
Bright-field microscopy
Explanation - Fluorescence microscopy uses fluorescent dyes or labeled antibodies to detect microorganisms.
Correct answer is: Fluorescence microscopy
Q.16 Endospore staining primarily uses which dye?
Malachite green
Crystal violet
Safranin
Carbol fuchsin
Explanation - Endospore staining involves malachite green to stain spores and safranin as a counterstain for vegetative cells.
Correct answer is: Malachite green
Q.17 What does the indole test detect?
Breakdown of urea
Production of hydrogen sulfide
Ability to produce indole from tryptophan
Fermentation of lactose
Explanation - Indole test detects bacteria that can break down tryptophan into indole, such as E. coli.
Correct answer is: Ability to produce indole from tryptophan
Q.18 Which technique is commonly used to determine bacterial motility?
Hanging drop method
Streak plate method
Catalase test
Coagulase test
Explanation - The hanging drop method allows observation of live, motile bacteria under the microscope.
Correct answer is: Hanging drop method
Q.19 Which diagnostic method is based on DNA amplification?
ELISA
PCR
Agglutination test
Western blot
Explanation - Polymerase Chain Reaction (PCR) amplifies DNA sequences to detect microbial pathogens.
Correct answer is: PCR
Q.20 Which medium differentiates lactose fermenters from non-fermenters?
MacConkey agar
Blood agar
Chocolate agar
Nutrient agar
Explanation - MacConkey agar differentiates bacteria based on lactose fermentation, showing pink colonies for fermenters.
Correct answer is: MacConkey agar
Q.21 What is the principle of the coagulase test?
Clotting of plasma
Breakdown of hydrogen peroxide
Oxidation of cytochromes
Fermentation of glucose
Explanation - Coagulase enzyme causes clotting of plasma, a key diagnostic for Staphylococcus aureus.
Correct answer is: Clotting of plasma
Q.22 Which molecular technique is commonly used for microbial identification?
Southern blot
PCR
Gram staining
ELISA
Explanation - PCR is widely used for rapid and sensitive detection of microbial DNA or RNA.
Correct answer is: PCR
Q.23 Which method is best for counting viable bacterial cells?
Direct microscopic count
Streak plate method
Colony-forming unit count
Spectrophotometry
Explanation - Colony-forming units (CFU) reflect viable cells capable of growing on culture media.
Correct answer is: Colony-forming unit count
Q.24 Which test is used to detect urease activity in bacteria?
Catalase test
Urease test
Indole test
Oxidase test
Explanation - The urease test detects bacterial urease enzyme, which hydrolyzes urea to ammonia and CO₂.
Correct answer is: Urease test
Q.25 Which diagnostic method involves antigen-antibody precipitation in gel?
Agglutination test
Immunodiffusion test
ELISA
PCR
Explanation - Immunodiffusion detects antigen-antibody interactions by visible precipitate formation in gel.
Correct answer is: Immunodiffusion test