subcultures of organisms grown from clinical specimens

Week 2 – Review Sheet

Exercise 4: Pure bacterial colonies

1. When an agar plate is inoculated, why is the loop sterilized after the initial inoculum is put on?

2. Distinguish between a pure culture and a mixed culture.

3. Define a bacterial colony. List four characteristics by which bacterial colonies may

be distinguished. 4. Why should a Petri dish not be left open for any extended period?

5. Why does the streaking method you used to inoculate your plates result in isolated colonies?

Exercise 5: Pour plate and streaking technique to obtain pure cultures

1. Discuss the relative convenience of pour- and streak-plate techniques in culturing clinical


2. How do you decide which colonies should be picked from a plate culture of a mixed flora?

3. Why is it necessary to make pure subcultures of organisms grown from clinical specimens?

4. What kinds of clinical specimens may yield a mixed flora in bacterial cultures?

5. When more than one colony type appears in pure culture, what are the most likely sources of extraneous contamination?

Exercise 3: Primary media for isolation of microorganisms

1. Define a differential medium and discuss its purpose.

2. Define a selective medium and describe its uses.

3. Why is MacConkey agar selective as well as differential?

4. Why is blood agar useful as a primary isolation medium?

5. What is the major difference between Modified Thayer-Martin (MTM) and chocolate agar? When would you use MTM rather than chocolate agar?