Lecture 1: Microbiology Basics Recap

Question 1

What are the key differences between Gram-positive and Gram-negative bacteria in a Gram stain?

Answer 1

Gram-positive: Purple (thick peptidoglycan layer).

Gram-negative: Pink (thin peptidoglycan + outer lipid membrane).

Question 2

Define alpha, beta, and gamma hemolysis on blood agar.

Answer 2

Alpha: Partial lysis (green/brown discoloration).

Beta: Complete lysis (clear zones).

Gamma: No hemolysis.

Question 3

What is the purpose of selective and differential agars? Give one example of each.

Answer 3

Selective: Inhibits unwanted organisms (e.g., MAC for Gram-negatives).

Differential: Distinguishes organisms by biochemical reactions (e.g., BPA for Staphylococcus).

Question 4

Describe the quadrant streaking method for isolation.

Answer 4

Initial streak in one direction; flame loop.

Turn 90°, streak once; flame loop.

Repeat for 3rd quadrant without touching prior streaks.

Final streak into the center.

Question 5

Name 4 common biochemical tests used in bacterial identification.

Answer 5

Catalase, oxidase, urease, indole.

Question 6

How does the API 20E system identify bacteria?

Answer 6

Uses 20 biochemical tests to generate a 7-digit code matched to a database.

Question 7

Why is antibiotic testing often performed alongside identification?

Answer 7

To expedite treatment decisions due to growth time constraints.