Microbiology Lab Review: Chapter 10

Lab review

Chapter 10 - Obtaining Pure Cultures

1.    [CH-10-01] Explain term “pure culture”. Why is it important to grow bacteria as pure cultures?

Pure culture means that the growth media on the get/liquid contains the sole bacteria desired. Other species are not present. Obtaining a pure culture is important for experiments and to narrow down the focus of possible antibiotics.

2.    [CH-10-02] How could you determine whether a liquid bacterial culture is pure?

Take a sample of the liquid and make a streak plate. Incubate the streak plate and look at the colonies that form. If the individual colonies look identical it is highly likely that the bacterial culture is pure.

3.    [CH-10-03] Most colonies on streak plate grow from single isolated colony-forming unit (CFU) or cells. On rare occasion, however, a colony can be a mixture of two different organisms. How could you confirm the purity of a colony?

Take a sample of the colony and make another streak plate. Wide streaks with big spreads should allow for easy identification of colonies. Incubate the plate and compare the colonies that growth. If the individual colonies throughout the plate look identical, it is highly likely that the bacterial culture is pure.

4.    [CH-10-04] If you found the colony to be a mixture of organisms, what could you do to grow them as pure culture?

After making a second streak plate and discovering the colony to be a mixture of organisms, take a sample of the colony desired and place into another solid media plate or liquid broth. The growth should be uncontaminated this time addition streak plates can be done to confirm that.

5.    [CH-10-05] What is a purpose of incubating the unopened plates? Be specific

A few unopened plates are incubated because they serve as a control group. If they have any growth on them, the culture media being used was contaminated prior to the experiment.

6.    [CH-10-06] If growth appears on unopened plates, what are some likely explanations?

The culture media was contaminated during the preparation of the TSA plates, likely after the mixture was heated and cooled to make the solid media.

7.    [CH-10-07] The plates you are using for the lab will be autoclaved to completely sterilize them. The measures taken to disinfect the tabletops are not as extreme. Why?

The tabletop, unlike the plates does not come into direct contact with the culture. Therefore, it does not need to be cleaned as thoroughly since it is less likely to contaminate the media.

8.    [CH-10-08] Describe bacterial morphology, oxygen and temperature requirements of Escherichia coli – No grouping, Gram negative, rod shaped, facultative anaerobe, mesophilic

9.    [CH-10-09] What are the likely sources (reservoir) of microbes that grow best at 37^OC?

Humans and animals as there body temperature is closely within the 37ºC range (36.5-37.5ºC).

10. [CH-10-10] Colony size, color, and shape are critical of bacterial growth description. At least three other principal factors – not description of the organism itself – typically included when describing bacterial growth. What are they and why are they important? (REPEAT from Chapt 9?)

Oxygen requirements → Determine bacteria’s oxygen requirements in the environment important when trying to culture the bacterial colonies.

Optimal temperature → Determines bacteria’s growth temperature range (Optimal temperature for rapid growth).

Tonicity → Bacterial growth environment, amount of solutes determines suitable environment.