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?