Question

Assume that:
• you are provided with a 10% solution of the antibiotic, penicillin, that is produced by this
fungus
• you have some Petri dishes containing a culture medium on which bacteria are growing,
forming a cloudy layer
• when the bacteria are killed by the penicillin a clear area shows on the plate
• the usual laboratory instruments are also available.

Make an outline plan of an investigation to determine the minimum concentration of this
penicillin needed to kill the bacteria.

Answer 1

In questions like this they expect our understanding on variables that has to be constant, an invariables that have to be changed.
For this type of experiment according to my knowledge of practicals:
1. Make sure the same species of bacteria is used in the experiment
2. Inoculate the bacteria in the petri dish(es) and allow them to grow in an incubator with a temperature(suitable and constant) and same pH conditions.
3. Wait for about 1 day, to see their growth.
4. Remove the cover of petri dish(es) and add *DIFFERENT CONCENTRATIONS* of the antibiotic pennicillin ( concentration can be maintained by diluting)
5. Keep them again at a constant temp, and then examine their results after several hours
Observation : Check the areas which is more clear with different concentration

Conclusion: The one with more clear with less concentration is supossed to be the best conc of antibiotic to be used

Answer 2

Ok. But what concentrations should we use?

Answer 3

They have told to find the "minimum concentration " that is supposed to be more "effective", so gradually u can decrease the conc and find if that one clears alot of bacteria moulds.