Microbiology Lab 1 – Expanded Quiz

1. Broth & Slant Cultures

Q: Why do we use both broth and slant cultures for microorganisms?

A: Broth shows growth patterns (turbidity, sediment, pellicle), while slants show colony morphology (color, texture, elevation).

Q: Which organism produced bright yellow, glossy colonies on a slant?

A: Kocuria rhizophila.

Q: Which organism showed pale pink colonies with weak turbidity?

A: Rhodospirillum rubrum.

Q: Why is it important to note colony texture (moist, dry, glossy) on slants?

A: Colony texture is a useful diagnostic feature for distinguishing different bacteria.

2. Carbohydrate Fermentation

Q: What indicator is used in the BCP (Bromcresol Purple) fermentation test?

A: Bromcresol purple (yellow = acid, purple = alkaline).

Q: What does a bubble in the Durham tube mean?

A: Gas was produced during fermentation.

Q: E. coli showed yellow broth with bubbles in glucose broth. What does this indicate?

A: Acid and gas production from glucose fermentation.

Q: Staphylococcus xylosus produced brownish broth with no bubble. What does this indicate?

A: No fermentation of glucose or sucrose \u2192 negative test.

3. Streak Plate Technique

Q: Why is the loop flamed between streak zones?

A: To reduce bacterial load, diluting the culture so isolated colonies can form.

Q: What is the difference between a 3-zone and a 4-zone streak plate?

A: A 4-zone streak achieves greater dilution and better separation of colonies compared to a 3-zone streak.

Q: What does the appearance of single, well-isolated colonies tell you?

A: A pure culture has likely been isolated.

4. Staining Principles

Q: Why don\u2019t bacteria take up acidic dyes in an indirect (negative) stain?

A: Because both bacteria and acidic dyes carry a negative charge \u2192 repulsion.

Q: In a negative stain, what gets stained \u2014 the cell or the background?

A: The background gets stained, leaving the bacterial cells clear.

5. Safety & Lab Policies

Q: Why must test tubes never be picked up by their caps?

A: The cap can come off, causing spills and contamination.

Q: What is the first thing you do if you spill a bacterial culture?

A: Cover spill with paper towels, soak with disinfectant for 10 minutes, notify instructor.

Q: Why are lab coats and goggles required, even with \u201csafe\u201d organisms?

A: To protect against accidental exposure, spills, or aerosols.

Q: Where should all contaminated materials (pipettes, gloves, plates) be disposed of?

A: In the biohazard waste container.

Q: Why do plates need to be labeled with your initials, organism name, and date?

A: For safety, accurate identification, and tracking of incubation time.

6. Application / Critical Thinking

Q: If your glucose fermentation broth is still purple at 24 hours, what should you do?

A: Continue incubation up to 48 hours (slow fermenters); if still purple, record as negative.

Q: Which organism from your results likely had the longest generation time? Why?

A: Rhodospirillum rubrum, because it showed the weakest growth and turbidity.

Q: If you observed colonies spreading across the agar and merging, what could be the cause?

A: Poor streaking technique or insufficient flaming between zones \u2192 overgrowth.