A microbiologist suspects a strain of Staphylococcus aureus is resistant to methicillin. What would be the best approach to test this suspicion?

Question

A.  Perform a Gram stain.
B.  Sequence the chromosome looking for resistance genes.
C.  Calculate the doubling time for the bacteria.
D.  Inject the bacteria into a host animal and look for disease symptoms.
E.  Add methicillin to the growth plates.

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 antibiotic resistance? 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.  Add methicillin to the growth plates.

Clarify Question
· What is the key concept addressed by the question?
        o You are a doctor, and your patient has a Staphylococcus aureus infection. You would normally treat this patient with methicillin, but you suspect that this strain is resistant to methicillin. You get some of the bacteria using a sterile Q-tip. What do you do now?
· What type of thinking is required?
        o This question is an “evaluate” question. You should consider the background information, then determine which of the five options would give you the answer you need.

  Gather Content
· What do you already know about antibiotic resistance? How does it relate to the question?
        o Generally, a bacterium has a gene that codes for particular proteins that make it resistant to an antibiotic. For example, if the antibiotic stops a cellular enzyme from functioning, the resistance gene could code for a slightly different version of the enzyme, one that is not affected by the antibiotic.
        o Resistance genes generally occur on R plasmids. A single bacterium could have multiple resistance genes, giving it resistance to multiple antibiotics.
· What other information is related to the question? Which information is most useful?
        o It is important for scientists to design experiments that will actually answer their question. Which of the options will answer the question you are asking?
 
Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?
        o Look at the answers and assess them one-by-one. Which one will tell you whether the strain from your patient is resistant to methicillin?
        o Remember, you want a method that is cheap, reliable, and fast. You need to treat this patient as soon as possible.
 
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 pick an experiment that would quickly tell you whether your patient’s strain of S. aureus is resistant to methicillin.
        o If you got the correct answer, great job!
        o If you got an incorrect answer, where did you get stuck?
            · Did you remember that R plasmids are small circles of DNA in the cell? They can integrate into the bacterial chromosome, but they aren’t always there. Sequencing the bacterial chromosome will not necessarily find an R plasmid.             · The fastest way to tell whether the strain is resistant is to expose it to methicillin and see if the bacteria can grow. If you put a sample in a tube with growth medium and methicillin, and the tube becomes cloudy with bacteria, you know that the strain is resistant. This procedure is frequently done in hospitals.