Microbiology: Laboratory Operations and Safety Flashcards
7 cards from real AP practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Microbiology: Laboratory Operations and Safety flashcards as text
Which indicator is used to confirm that an autoclave reached sterilizing conditions during a given cycle?
Answer: Biological indicator using Geobacillus stearothermophilus spores
Biological indicators containing G. stearothermophilus spores are the gold standard for autoclave validation because they directly confirm that the most resistant organisms were killed.
What is the primary reason that laboratory centrifuge rotors must be balanced before each run?
Answer: To avoid rotor failure and potential aerosol release from broken tubes
An unbalanced rotor generates extreme vibration that can cause catastrophic rotor failure, shattering tubes and releasing infectious aerosols — a critical safety hazard.
MacConkey agar is both selective and differential. What property makes it selective?
Answer: Crystal violet and bile salts inhibit the growth of Gram-positive organisms
Crystal violet and bile salts in MacConkey agar inhibit Gram-positive bacteria, making it selective for Gram-negative enteric organisms.
Which disinfectant is most appropriate for decontaminating stainless steel laboratory bench surfaces that will also contact living cells in the next experiment?
Answer: 70% isopropanol or ethanol
70% ethanol or isopropanol is effective against vegetative bacteria and viruses on bench surfaces and evaporates completely, leaving no toxic residue that could harm subsequent cell cultures.
During a streak plate procedure for isolation of pure colonies, what is the purpose of each successive streak region?
Answer: To dilute the inoculum progressively, separating individual cells that grow into isolated colonies
Each successive streak dilutes the number of organisms carried forward from the previous zone, ultimately separating individual cells far enough apart that each grows into a distinct isolated colony.
Which of the following laboratory practices violates standard biosafety guidelines at any BSL level?
Answer: Mouth pipetting to obtain precise volumes
Mouth pipetting is absolutely prohibited in any microbiological laboratory because it creates a direct route for ingestion or inhalation of infectious agents and toxic chemicals.
A scientist needs to count the number of viable bacteria in a sample using the pour plate method. The final plates show 52 colonies after diluting the sample 1:1,000,000. What is the estimated CFU/mL in the original sample?
Answer: 5.2 × 10⁷ CFU/mL
CFU/mL = colonies ÷ (dilution factor × volume plated); with 52 colonies at 10⁻⁶ dilution and 1 mL plated: 52 ÷ 10⁻⁶ = 5.2 × 10⁷ CFU/mL.