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Question 1. Which scientist is credited with formulating the postulates that link specific microorganisms to specific diseases? A) Louis Pasteur B) Robert Koch C) Antonie van Leeuwenhoek D) Joseph Lister Answer: B Explanation: Koch’s postulates define criteria to prove a causal relationship between a microbe and a disease. Question 2. In the three‑domain system, which domain contains organisms that lack peptidoglycan in their cell walls? A) Bacteria B) Archaea C) Eukarya D) Cyanobacteria Answer: B Explanation: Archaea have pseudo‑peptidoglycan or other polymers, not true peptidoglycan. Question 3. Which staining technique differentiates Mycobacterium species from other bacteria by using a lipid‑soluble dye? A) Gram stain B) Acid‑fast stain (Ziehl‑Neelsen) C) Endospore stain D) Capsule stain Answer: B
Explanation: The acid‑fast stain retains carbol‑fuchsin after acid decolorization due to mycolic acids. Question 4. What is the primary structural component that gives Gram‑positive bacteria their thick violet appearance after Gram staining? A) Outer membrane lipopolysaccharide B) Thin peptidoglycan layer C) Thick peptidoglycan layer D) Mycolic acid‑rich cell wall Answer: C Explanation: A thick peptidoglycan layer retains crystal violet‑iodine complex. Question 5. Which of the following is NOT a function of bacterial pili? A) Conjugative DNA transfer B) Attachment to host cells C) Motility in liquid environments D) Formation of biofilms Answer: C Explanation: Flagella, not pili, provide swimming motility. Question 6. The enzyme that catalyzes the conversion of phosphoenolpyruvate to pyruvate in glycolysis is: A) Hexokinase B) Phosphofructokinase‑ 1 C) Pyruvate kinase D) Lactate dehydrogenase
D) Constant cell density Answer: C Explanation: Cells adjust metabolism and synthesize new proteins before exponential growth. Question 10. Which genetic recombination mechanism involves bacteriophage‑mediated transfer of DNA from one bacterium to another? A) Transformation B) Conjugation C) Transduction D) Binary fission Answer: C Explanation: Transduction uses a phage vector to move genetic material between cells. Question 11. The operon model that allows E. coli to metabolize lactose only when glucose is absent is called: A) Trp operon B) Lac operon C) Ara operon D) His operon Answer: B Explanation: The lac operon is inducible by lactose and catabolite‑repressed by glucose. Question 12. Which viral structure is responsible for attachment to host cell receptors? A) Capsid proteins only B) Envelope glycoproteins
C) Viral RNA genome D) Nucleocapsid Answer: B Explanation: Envelope glycoproteins recognize and bind specific host receptors. Question 13. A bacteriophage that integrates its DNA into the host chromosome is undergoing which cycle? A) Lytic only B) Lysogenic only C) Both lytic and lysogenic (temperate) D) Chronic Answer: C Explanation: Temperate phages can enter a lysogenic state before entering lytic replication. Question 14. Which of the following agents is composed solely of a short circular RNA molecule without protein coating? A) Virus B) Prion C) Viroid D) Bacteriophage Answer: C Explanation: Viroids are naked, circular, single‑stranded RNA pathogens. Question 15. The scrapie disease in sheep is an example of a disease caused by: A) Bacteria
B) Respiratory mucosa C) Skin abrasions D) Urinary tract Answer: B Explanation: N. meningitidis colonizes nasopharyngeal epithelium before invasive disease. Question 19. Which organism is the most common cause of community‑acquired bacterial pneumonia in adults? A) Streptococcus pneumoniae B) Haemophilus influenzae C) Mycoplasma pneumoniae D) Legionella pneumophila Answer: A Explanation: S. pneumoniae is the leading bacterial pathogen for typical pneumonia. Question 20. The bacterium that requires an acidic environment (pH < 5) for optimal growth and causes gastric ulcers is: A) Helicobacter pylori B) Campylobacter jejuni C) Clostridium difficile D) Vibrio cholerae Answer: A Explanation: H. pylori thrives in the stomach’s acidic milieu and damages mucosa. Question 21. Which protozoan parasite is transmitted by the Anopheles mosquito and causes malaria?
A) Giardia lamblia B) Entamoeba histolytica C) Plasmodium falciparum D) Trypanosoma cruzi Answer: C Explanation: Plasmodium species undergo sexual reproduction in Anopheles vectors. Question 22. The first line of innate immune defense that prevents microbial entry at mucosal surfaces is: A) Complement activation B) Phagocytosis by neutrophils C) Physical barrier of epithelial cells and mucus D) Antibody‑mediated opsonization Answer: C Explanation: Intact epithelium and mucus trap and exclude pathogens. Question 23. Which complement pathway is initiated by antigen‑antibody complexes? A) Classical pathway B) Lectin pathway C) Alternative pathway D) Terminal pathway Answer: A Explanation: The classical pathway is activated when C1q binds to IgG or IgM complexes.
Question 27. Autoclaving sterilizes media by using: A) Dry heat at 160 °C for 2 h B) Moist heat (steam) at 121 °C under pressure for 15 min C) Ethylene oxide gas D) UV radiation Answer: B Explanation: Pressurized steam denatures proteins and nucleic acids, achieving sterility. Question 28. Which disinfectant works by oxidizing cellular components and is effective against most bacteria, viruses, and fungi? A) Ethanol (70 %) B) Chlorine (hypochlorite) C) Iodine tincture D) Hydrogen peroxide (3 %) Answer: B Explanation: Hypochlorite releases chlorine, a strong oxidizing agent. Question 29. The bacterial cell wall component that provides rigidity to Gram‑negative organisms is: A) Peptidoglycan only B) Lipoteichoic acid C) Outer membrane lipopolysaccharide D) Mycolic acids Answer: C
Explanation: LPS contributes to structural integrity and barrier function. Question 30. Which of the following best describes a “capsule” in bacterial cells? A) A rigid protein layer inside the cytoplasm B) A gelatinous polysaccharide layer external to the cell wall C) A thin peptidoglycan sheath D) A lipid bilayer containing porins Answer: B Explanation: Capsules are thick polysaccharide layers that protect against phagocytosis. Question 31. The enzyme that confers resistance to aminoglycosides by phosphorylating the drug is: A) β‑lactamase B) Acetyltransferase C) Phosphotransferase (APH) D) Methyltransferase Answer: C Explanation: APH enzymes add a phosphate group, inactivating aminoglycosides. Question 32. Which of the following viruses has a segmented, negative‑sense RNA genome? A) Influenza A virus B) Adenovirus C) Herpes simplex virus D. Poliovirus Answer: A
Answer: B Explanation: Gram‑negative cells lose crystal violet during decolorization and retain safranin. Question 36. Which of the following best describes the function of bacterial flagella? A) DNA transfer during conjugation B) Attachment to surfaces via pili C) Propulsion through liquid environments D) Production of extracellular toxins Answer: C Explanation: Flagella rotate to provide motility. Question 37. The “burst size” in a bacteriophage infection refers to: A) Number of phage particles released per lysed cell B) Time required for phage adsorption C) Length of the latent period D) Size of the phage capsid Answer: A Explanation: Burst size is the average count of progeny phage released after lysis. Question 38. Which bacterial genus is known for producing endospores that can survive extreme heat and desiccation? A) Clostridium B) Bacillus C) Streptococcus D) Listeria
Answer: B Explanation: Bacillus species form highly resistant endospores. Question 39. In the context of antimicrobial therapy, “bactericidal” agents: A) Inhibit bacterial protein synthesis only B) Kill bacteria outright, reducing viable counts C) Inhibit bacterial growth without killing D) Only affect Gram‑positive organisms Answer: B Explanation: Bactericidal drugs lead to bacterial death, unlike bacteriostatic agents. Question 40. Which of the following is an example of a live‑attenuated vaccine? A) Inactivated polio vaccine (IPV) B) Hepatitis B subunit vaccine C) Measles‑mumps‑rubella (MMR) vaccine D) Tetanus toxoid vaccine Answer: C Explanation: MMR contains weakened viruses that stimulate immunity without causing disease. Question 41. The “quorum sensing” mechanism in bacteria primarily regulates: A) Binary fission rate B) Gene expression in response to cell‑density signals C) Antibiotic resistance mutation frequency D) Endospore formation only
C) Limulus Amebocyte Lysate (LAL) assay D) Catalase test Answer: C Explanation: The LAL assay uses horseshoe crab blood cells to clot in response to LPS. Question 45. The term “facultative anaerobe” refers to a microorganism that: A) Can only grow in the presence of oxygen B) Grows only in the absence of oxygen C) Can grow with or without oxygen, using fermentation when anaerobic D) Requires nitrate as an electron acceptor Answer: C Explanation: Facultative anaerobes switch between aerobic respiration and fermentation. Question 46. Which of the following fungal pathogens produces a dimorphic transition from mold in the environment to yeast in human tissue? A) Candida albicans B) Histoplasma capsulatum C) Aspergillus fumigatus D) Cryptococcus neoformans Answer: B Explanation: H. capsulatum grows as a mold in soil and as a yeast in the host. Question 47. The bacterial enzyme that modifies penicillin‑binding proteins to confer resistance to β‑lactam antibiotics is: A) β‑lactamase
B) PBP2a (encoded by mecA) C) MurA transferase D) DNA gyrase Answer: B Explanation: Altered PBPs reduce binding affinity for β‑lactams, leading to resistance. Question 48. Which of the following is a primary mechanism of viral entry into host cells? A) Passive diffusion through the cell wall B) Endocytosis mediated by viral envelope proteins C) Direct injection of DNA via a pilus D) Conjugative transfer Answer: B Explanation: Enveloped viruses bind receptors and are internalized by endocytosis. Question 49. The bacterial toxin that cleaves 28S rRNA, halting protein synthesis, is produced by: A) Clostridium botulinum B) Staphylococcus aureus C) Shigella dysenteriae D) Bacillus anthracis Answer: C Explanation: Shiga toxin inactivates the 60S ribosomal subunit. Question 50. Which type of immunity involves memory B cells that produce high‑affinity antibodies upon re‑exposure to antigen? A) Innate immunity
B) Tetanus toxin (tetanospasmin) C) Exotoxin A (ADP‑ribosylating) D) Capsule polysaccharide Answer: B Explanation: Tetanospasmin blocks inhibitory neurotransmitter release, causing spastic paralysis. Question 54. The presence of a “double‑cortin” band on an agarose gel after restriction digestion indicates: A) Successful ligation of DNA fragments B) Linearization of a circular plasmid C) Incomplete digestion D) Plasmid supercoiling Answer: B Explanation: A double‑cortin (two bands) often represents linearized plasmid and a fragment. Question 55. Which antibiotic class inhibits bacterial cell wall synthesis by binding to the D‑Ala‑D‑Ala terminus of peptidoglycan precursors? A) Aminoglycosides B) Tetracyclines C) Glycopeptides (e.g., vancomycin) D) Fluoroquinolones Answer: C Explanation: Vancomycin blocks transglycosylation by binding D‑Ala‑D‑Ala. Question 56. Which of the following microbes is the leading cause of neonatal sepsis worldwide?
A) Streptococcus agalactiae (Group B Strep) B) Escherichia coli C) Listeria monocytogenes D) Neisseria meningitidis Answer: A Explanation: Group B Streptococcus colonizes maternal genital tract and can infect newborns. Question 57. The “catalase test” is used to differentiate which two genera? A) Staphylococcus (catalase‑positive) vs. Streptococcus (catalase‑negative) B) Enterococcus vs. Enterobacter C) Clostridium vs. Bacillus D) Pseudomonas vs. Klebsiella Answer: A Explanation: Catalase production distinguishes staphylococci (bubble formation) from streptococci. Question 58. In a Gram stain, which step follows crystal violet application? A) Decolorization with alcohol/acetone B) Counterstaining with safranin C) Application of iodine (mordant) D) Fixation with heat Answer: C Explanation: Iodine forms a crystal violet‑iodine complex that is later decolorized. Question 59. The bacterial enzyme that confers resistance to macrolides by methylating the 23S rRNA is:
