Student Question

# You want to do an experiment to determine which type of antibiotic is best for treating Staphylococcus aureus bacteria. You obtain a TSA agar plate, and streak S. aureus over the surface. Then you place small disks of different antibiotics on top of the bacterial surface. Next, you incubate all the plates for 24 hours at 37 degrees Celsius so the bacteria can grow. Afterwards, you measure whether the bacteria were able to grow next to each antibiotic disk.What is the biggest problem with this experiment? There are not multiple replicates for each antibiotic There are not multiple antibiotics to compare to each other There are no controlled variables There is no dependent variable

The goal is to perform an experiment to find out which antibiotics will be most effective against bacteria Staphylococcus aureus. You have used TSA as growth medium and streaked with S.aureus bacteria, and after that you have placed different antibiotics disks on agar plate and incubate it for 24 hrs at 36 C.

First, we have to think what kind of result should we expect after 24 hrs.

You need to think in terms of antibiotic resistant or antibiotic susceptible. Antibiotic resistance means bacteria are able to grow and survive in the presence of antibiotics, and antibiotic susceptible means bacteria are not able to grow in the presence of particular antibiotics.

So, what is the biggest problem with this experiment?

If we go through the 1st choice (There are not multiple replicates for each antibiotic.) then ust consider if it will make a difference to the growth of the bacteria if you use a replicate of each antibiotics. It will not make difference to the growth of the bacteria.

The second choice states, There are not multiple antibiotics to compare to each other.

Do you think this is correct? We are using different antibiotic disks, whichIt means this is not the biggest problem.

For the 3rd choice (There are no controlled variables.) think about this option: in an experiment nowhere it is mentioned that we are using control groups or controlled variables. So,you have to think why we need a control group or control variables and how useful it is.

Now the last option: There is no dependent variable.

Since we have streaked the TSA plate with S.aureus bacteria, we will be able to see bacterial growth. We will be either able to see growth around antibiotic disks or we will be able to notice the zone of inhibition.

The biggest problem with this experiment is we are not using any controlled variables.

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