AP-Micro Microbiology: Laboratory Operations and Safety 3 — Questions and Answers
Question 1: Which autoclave parameter is most critical for ensuring sterilization of a dense load of laboratory media?
- Exposure temperature only
- Exposure time only
- Correct combination of temperature, time, and steam penetration (Correct answer)
- Chamber pressure alone
Correct answer: Correct combination of temperature, time, and steam penetration
Effective sterilization requires moist heat (steam) to penetrate the entire load at the correct temperature (121°C) for sufficient time (≥15 min), as failure of any parameter can leave viable spores.
Question 2: A microbiologist observes unexpected growth on a control plate that received no inoculum. This finding most likely indicates:
- Contamination during preparation or plating (Correct answer)
- The organism is a fast-growing mutant
- The incubator temperature was too low
- The media contained a selective agent
Correct answer: Contamination during preparation or plating
Growth on an uninoculated control plate indicates contamination occurred during media preparation, pouring, or plate handling, compromising experimental validity.
Question 3: What is the purpose of a splash guard or shield when working with concentrated acids used in microbiological staining procedures?
- To prevent aerosol and splash exposure to face and eyes (Correct answer)
- To keep staining reagents from evaporating
- To maintain the pH of the staining solution
- To prevent contamination of the stain by airborne organisms
Correct answer: To prevent aerosol and splash exposure to face and eyes
Splash guards and face shields protect mucous membranes from chemical splashes and aerosols generated when handling corrosive reagents like concentrated sulfuric or hydrochloric acid.
Question 4: Which sterilization method is most appropriate for heat-labile solutions such as antibiotic stocks used in microbiology?
- Autoclaving at 121°C
- Incineration
- Filtration through a 0.22 μm membrane (Correct answer)
- UV irradiation for 30 minutes
Correct answer: Filtration through a 0.22 μm membrane
Filtration through a 0.22 μm membrane physically removes bacteria and larger particles without heat, preserving the activity of heat-sensitive compounds like antibiotics.
Question 5: In a Gram stain procedure, what is the role of iodine (Gram's iodine)?
- Primary stain that colors all cells purple
- Mordant that fixes crystal violet into the peptidoglycan layer (Correct answer)
- Decolorizer that removes crystal violet from Gram-negative cells
- Counterstain that colors Gram-negative cells pink
Correct answer: Mordant that fixes crystal violet into the peptidoglycan layer
Gram's iodine acts as a mordant, forming a crystal violet–iodine complex that becomes trapped within the thick peptidoglycan layer of Gram-positive cell walls.
Question 6: When pipetting a potentially infectious sample, the safest technique to avoid aerosol generation is:
- Pipetting rapidly to minimize exposure time
- Using mouth pipetting for greater control
- Using mechanical pipette aids and working inside a BSC (Correct answer)
- Centrifuging the sample first to reduce volume
Correct answer: Using mechanical pipette aids and working inside a BSC
Mechanical pipetting devices (never mouth pipetting) combined with working inside a biosafety cabinet minimize aerosol generation and exposure to infectious materials.
Question 7: A researcher is working with a human pathogen classified at BSL-3. Which additional safety measure distinguishes BSL-3 from BSL-2 laboratories?
- Use of standard PPE such as gloves and lab coat
- All procedures performed in a certified Class II or Class III BSC with directional airflow and controlled access (Correct answer)
- Avoidance of eating and drinking in the lab
- Handwashing before leaving the lab
Correct answer: All procedures performed in a certified Class II or Class III BSC with directional airflow and controlled access
BSL-3 requires all manipulations of infectious materials inside certified BSCs, sustained directional airflow into the lab, and controlled access — requirements not mandatory at BSL-2.
AP-Micro Microbiology: Laboratory Operations and Safety 3
Which autoclave parameter is most critical for ensuring sterilization of a dense load of laboratory media?