You expose a tube of bacteria to ultraviolet radiation to induce mutations. You then spread the bacteria on a plate containing media with all known required nutrients and wait until separate colonies form. Finally, you use replicate plating to transfer a print of this plate to an additional sterile plate containing minimal media plus a known required nutrient for the bacteria. Based on this protocol, colonies growing on the plate with minimal media would include what type of cells?

Question

A.  only wild-type bacteria
B.  only mutant bacteria
C.  mutants that only require the supplied nutrient
D.  Both wild-type and all mutants
E.  wild-type bacteria and the mutants that only require the supplied nutrient

Clarify Question
· What is the key concept addressed by the question?
· What type of thinking is required?

Gather Content
· What do you already know about growth media? How does it relate to the question?
· What other information is related to the question? Which information is most useful?

  Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
 
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
 

Answer 1

E.  wild-type bacteria and the mutants that only require the supplied nutrient

Clarify Question
· What is the key concept addressed by the question?
        o What types of bacterial strains would you expect to grow on the second petri dish, the one containing minimal media plus one required nutrient?
· What type of thinking is required?
        o This question is an “apply” question. You need to use your knowledge of bacterial metabolism and growth media to answer a question about a novel situation.

Gather Content
· What do you already know about growth media? How does it relate to the question?
        o Growth media can be liquid, like broth, or solid, like agar. Bacteria growing on solid media in a petri dish will form spots called colonies.
        o You can make media with and without nutrients. There are some nutrients that all strains of a bacterial species need, similar to the essential amino acids all humans need to consume in order to survive. Media with only the essential nutrients is called “minimal media”. A nutrient that a bacterial strain needs to grow is called a “required nutrient”.
· What other information is related to the question? Which information is most useful?
        o Some mutations make a bacterial strain unable to produce a nutrient that the wild-type strain easily produces on its own. This means that, for example, mutant strain S needs 13 essential nutrients to grow, while the wild-type strain needs only 12 of those nutrients. Other mutants could develop the ability to produce a nutrient on their own, meaning that they could survive on only 11 essential nutrients when the wild-type strain needs 12.

  Choose Answer
· Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o The options give your three types of bacteria:
            · wild-type
            · mutants
            · mutants that only require the supplied nutrient
        o Read through the question and try to answer it for just one of the strains. Start with wild-type. Will wild-type bacteria grow in the minimal media that has only one additional required nutrient?
        o Once you’ve determine which of the three types of bacteria will grow in the petri dish, find an option that lists just those bacteria.
 
Reflect on Process
· Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o This question asked you to determine which types of bacteria will be able to grow in a petri dish with minimal media that has one additional required nutrient.
        o If you got the correct answer, great job!
        o If you got an incorrect answer, where did you get stuck?
            · Questions like this can be difficult because there are so many options. To make it easier, look at the options and try to break them down into easier choices. For this question, all options are some combination of three types of bacteria (wild-type, mutant, or mutant that only requires the supplied nutrient). You can then answer the question using these simplified options. Once you’ve done this, you’ll be able to pick the correct answer.