Exercise 9

Question 1

Contrast the difference between an isolated pure colony and an isolated mixed colony.

Answer 1

An isolated colony may not have formed from a single cell and may still be mixed (isolated mixed colony). An isolated pure colony will contain only one kind (genus and species) of organism since it arises from a single cell or CFU.

Question 2

State what is meant by a “pure” culture. Why is it important to study only pure cultures?

Answer 2

A “pure” culture is one that contains only one type (genus and species) of bacteria and one that arises from a single cell or CFU. A pure culture allows us to study its morphological and chemical features.

Question 3

List four aseptic techniques which should be performed during the streak plate procedure.

Answer 3

(1) Sterilize the loop before and after use; (2) Cover the plate each time; (3) Streak the plate at an angle to avoid airborne bacteria; (4) Sterilize the lip of the culture tubes.

Question 4

Discuss the desirability and composition of agar stock slants.

Answer 4

Agar stock slants contain tryptone soy agar, which is an all purpose medium and provides a large protected surface area for growth. If refrigerated, it can maintain the culture for several weeks or months. Slants are also easier to store and have less contamination than plates.

Question 5

List and describe three techniques of pure culturing.

Answer 5

1. Pour plate method - requires the test specimen to be pipetted into a sterile dish; 2. Spread plate method - requires the test specimen to be spread over the surface of an agar plate by means of a bent glass rod; 3. Streak plate method - requires the test specimen to be streaked onto the surface of an agar plate by means of a sterile inoculating loop.