General Microbiology (Bacterial Genetics, Culture Media, Stains, Sterilization) MCQs

Microbiology · 64 free questions with answers & explanations.

1. A laboratory needs to sterilize serum-containing culture media. Autoclaving will denature the serum proteins. The best method of sterilization is:
2. In an experiment, an F+ bacterium donates a plasmid to an F- bacterium through a conjugation pilus, converting the F- cell to F+. This horizontal gene transfer requires:
3. During conjugation in Gram-negative bacteria, the transfer of chromosomal genes (Hfr × F- cross) is almost always incomplete. The main reason is:
4. Malachite green spore stain uses heat to drive the primary stain into endospores. After decolorization with water, the counterstain safranin colors vegetative bacterial cells red. The endospores appear:
5. Membrane filtration (0.22 µm pore size) is used to sterilize heat-labile solutions. However, it CANNOT sterilize solutions contaminated with:
6. MacConkey agar is a selective and differential medium. The selective components that inhibit Gram-positive organisms are:
7. In bacterial conjugation, the mechanism by which F+ bacteria transfer DNA to F- bacteria involves a relaxase enzyme. The relaxase function in conjugative transfer is:
8. A microbiologist using phenol coefficient test to compare disinfectant efficacy must maintain a critical condition. The phenol coefficient represents the ratio of the dilution of test disinfectant to phenol required to kill bacteria in a specified time under which conditions?
9. During autoclaving at 121°C/15 psi for 15 minutes, the killing of bacterial endospores depends on moist heat achieving which specific lethal effect that dry heat does NOT achieve at the same temperature?
10. MacConkey agar is a selective and differential medium. The component responsible for selecting against Gram-positive organisms while allowing growth of Gram-negatives is:
11. A researcher is performing whole genome sequencing of a multidrug-resistant Enterobacter cloacae isolate. She finds a resistance gene on a 100 kb self-transmissible plasmid with tra operon genes and ori region. This plasmid can transfer resistance to a sensitive strain via:
12. Moist heat sterilisation by autoclaving is superior to dry heat for sterilising culture media containing dextrose because:
13. In the Gram staining technique, the decolourisation step with acetone-alcohol is critical. Gram-negative bacteria lose the crystal violet-iodine complex and appear red (safranin-stained) because:
14. Bacterial conjugation mediated by the F plasmid transfers genetic material. In an Hfr (high frequency recombination) × F− cross, what is the expected outcome regarding fertility factor transfer?
15. MacConkey agar is a selective and differential medium. What is the basis of its selectivity and how does it differentiate lactose-fermenting from non-fermenting organisms?
16. During heat sterilization in an autoclave, the D-value (decimal reduction time) concept is used to validate sterilization efficacy. The standard autoclaving cycle is 121°C for 15 minutes at 15 psi pressure. Which organism is used as the biological indicator (BI) to validate autoclave performance?
17. Transduction is the transfer of bacterial genes by bacteriophages. In generalized transduction, which phage and which DNA is packaged in the phage head?
18. Bacterial transformation involves uptake of naked exogenous DNA. For a bacterium to be 'competent' for natural transformation, it must express a type IV pilus-like apparatus (in Gram-negative organisms like Neisseria) or a ComFA-like system (in Gram-positive organisms). The key feature of transformation that distinguishes it from transduction is:
19. Regarding disc diffusion (Kirby-Bauer) antibiotic susceptibility testing, which statement about inoculum preparation is most critical for accurate results?
20. In autoclaving (moist heat sterilization), the standard cycle of 121°C for 15 minutes at 15 psi (103 kPa) achieves a sterility assurance level of 10⁻⁶. The mechanism of sterilization is primarily through:
21. During conjugation in bacteria, a high-frequency recombination (Hfr) strain transfers chromosomal genes to an F– strain. After mating, the recipient rarely becomes fully Hfr. Which characteristic of Hfr × F– mating explains this observation?
22. Ziehl-Neelsen (ZN) stain uses carbol fuchsin with heat as the primary stain for acid-fast organisms. Which component of the mycobacterial cell wall confers acid fastness and retains the red dye against decolourisation with acid-alcohol?
23. Autoclave sterilisation uses moist heat at 121°C for 15 minutes at 15 psi. The fundamental reason moist heat is more effective than dry heat at equivalent temperatures is:
24. A microbiology lab uses an autoclave set at 121°C and 15 psi for 20 minutes. A batch of culture media is prepared and a Bacillus stearothermophilus biological indicator strip is included. After autoclaving, the strip shows no growth on incubation at 55–60°C. What does this confirm?
25. A plasmid carrying the bla gene (encoding TEM-type beta-lactamase) and a conjugation gene is transferred between two strains of E. coli. Which genetic transfer mechanism is operative, and which step requires direct cell-to-cell contact?
26. In the Gram stain procedure, which chemical step is responsible for the differential staining of gram-positive versus gram-negative bacteria, and what structural feature of gram-positive bacteria accounts for crystal violet retention?
27. A laboratory needs to select Neisseria gonorrhoeae from a genital swab that is expected to be contaminated with normal flora. Which selective medium is used, and what components make it selective?
28. A clinical microbiology lab uses a selective medium containing bile salts and sodium deoxycholate to selectively isolate Gram-negative enteric bacilli from stool. Lactose fermenters appear pink while non-fermenters are colourless. This medium is:
29. In bacterial genetics, the transfer of DNA between bacteria via a bacteriophage vector is called:
30. Prions (proteinaceous infectious particles) causing variant Creutzfeldt-Jakob disease (vCJD) cannot be destroyed by which standard sterilisation method?
31. Transformation in bacteria refers to the uptake of naked DNA from the environment. Among pathogenic bacteria, natural competence for genetic transformation is a key mechanism of antibiotic resistance dissemination. Which organism is classically used to demonstrate transformation and is naturally competent?
32. Autoclave sterilization at 121°C for 15 minutes is the standard cycle for moist heat sterilization. Monitoring of autoclave efficacy using a biological indicator uses:
33. Acridine orange fluorescent stain is used in clinical microbiology to detect bacteria in blood culture broths or CSF. The basis of its selectivity is that acridine orange:
34. A clinical lab uses an autoclave to sterilize surgical dressings. The standard recommended parameters are 121°C at 15 psi for 15 minutes. The indicator used to validate sterilization cycle completion is:
35. A hospital investigates a nosocomial outbreak of Pseudomonas aeruginosa in the ICU. Isolates from multiple patients share identical antibiogram patterns. The most discriminatory molecular epidemiological typing method is:
36. A microbiology quality control officer finds that a batch of culture media for selective isolation of Neisseria gonorrhoeae (Modified Thayer-Martin medium) is contaminated with Proteus mirabilis (swarming over the plates). Which component of the medium is responsible for inhibiting Proteus, and at what concentration is it included?
37. Autoclave sterilization fails in a central sterile services unit. Biological indicators (Geobacillus stearothermophilus spore strips) placed inside the load show growth after incubation. Which parameter most likely failed?
38. An autoclave operator validates a cycle at 121°C for 15 minutes using a biological indicator (BI). The BI contains spores of Geobacillus stearothermophilus with a D-value of 1.5 minutes. How many D-values are achieved in this cycle, and what is the theoretical sterility assurance level (SAL) concept this demonstrates?
39. An R-plasmid (resistance plasmid) carrying blaZ (beta-lactamase), tetM, aacA, and ermB genes is transferred from a methicillin-susceptible S. aureus to a new clinical isolate during horizontal gene transfer. Which mechanism of transfer is most likely in the clinical S. aureus context?
40. Tyndallization (intermittent sterilization) is used to sterilize heat-labile materials that cannot withstand autoclaving. It involves heating at 100°C for 30 minutes on three consecutive days with incubation at 37°C between sessions. This is specifically effective because:
41. The phenomenon of bacterial transduction involves transfer of bacterial DNA via:
42. In a hospital, an outbreak of carbapenem-resistant Klebsiella pneumoniae occurs in the ICU. Molecular typing shows all isolates share an identical pulsed-field gel electrophoresis (PFGE) pattern. This indicates:
43. Which statement about autoclave sterilization validation using Bacillus stearothermophilus (Geobacillus stearothermophilus) spores is correct?
44. A microbiology laboratory uses glutaraldehyde 2% for disinfecting flexible endoscopes. This method kills vegetative bacteria, fungi, enveloped viruses, non-enveloped viruses, and mycobacteria but does NOT destroy spores within a 20-minute contact time. What level of disinfection does this represent?
45. During horizontal gene transfer, a bacteriophage accidentally packages a fragment of the donor bacterium's chromosomal DNA (not phage DNA) and injects it into a recipient bacterium. This process is called:
46. Autoclaving is used to sterilise culture media and surgical instruments. Which biological indicator is used to monitor the efficacy of steam autoclaving (121°C/15 psi for 15 minutes)?
47. During phenotypic characterisation of a Gram-negative aerobic bacillus in the lab, the following results are noted: oxidase positive, catalase positive, glucose oxidised but not fermented, non-motile by hanging drop, mucoid blue-green pigment on blood agar. Which organism is most likely, and which pigment is pathognomonic?
48. Bacterial transformation (uptake of naked DNA from the environment) requires the recipient bacteria to be in a state of 'competence.' The Gram-positive organism classically used to discover transformation (Griffith, 1928) was:
49. Mannitol salt agar (MSA) is a selective and differential medium. The selective agent and the differentiating property are respectively:
50. Ziehl-Neelsen (ZN) staining for acid-fast bacilli uses hot carbol-fuchsin as the primary stain. The acid-fastness of mycobacteria is due to:
51. In bacterial transformation, naked DNA from a lysed donor bacterium enters a competent recipient cell. Which organism was used in Griffith's classic experiment demonstrating transformation, and what was the transforming principle identified by Avery, MacLeod, and McCarty?
52. For the selective isolation of Bordetella pertussis, which culture medium and incubation conditions are used?
53. Acridine orange (AO) stain is used in microbiology primarily to detect:
54. A plasmid carrying resistance genes is transferred between bacteria via physical cell-to-cell contact requiring a sex pilus. This mode of genetic transfer is:
55. TCBS (thiosulfate citrate bile salts sucrose) agar is selective for Vibrio species. V. cholerae O1/O139 produces which colony appearance on TCBS?
56. A 28-year-old man presents with recurrent sino-pulmonary infections since childhood and is found to have very low serum IgG but normal T-cell counts. Nitroblue tetrazolium (NBT) test is normal. Which immunoglobulin class is MOST important in providing long-term immunological memory after vaccination?
57. A child is brought to the emergency department with severe bacterial sepsis. The resident notes that the pathogen produces a toxin that acts as a superantigen. Which of the following BEST describes the mechanism of action of superantigens?
58. A laboratory test measures the lowest concentration of an antibiotic that completely inhibits visible bacterial growth after 18–24 hours of incubation. This concentration is termed the:
59. During sterilization quality control, a biological indicator using Geobacillus stearothermophilus spores is used for which method?
60. A 32-year-old HIV-positive man on no medications has a CD4 count of 180 cells/μL. Which complement pathway is classically activated by mannose-binding lectin (MBL) on the surface of microorganisms?
61. A 4-year-old child presents with recurrent pyogenic infections, partial albinism, and peripheral blood smear showing giant cytoplasmic granules in neutrophils. Which primary immunodeficiency does this describe?
62. A hospital has an outbreak of carbapenem-resistant Klebsiella pneumoniae (CRKP). The organism produces NDM-1 (New Delhi Metallo-beta-lactamase). Metallo-beta-lactamases like NDM-1 are inhibited by which agent used in clinical susceptibility testing to detect MBL production?
63. Hot-air oven (dry heat) sterilisation at 160°C for 1 hour is used for which category of materials and is NOT suitable for which of the following?
64. In H. pylori infection, which non-invasive test is the gold standard for confirming eradication after completing treatment, and why is it preferred over serology for this purpose?