The MIC of penicillin for Staphylococcus epidermis was determined to be in tube #5

The minimum inhibitory concentration (MIC) of an antibacterial agent such as penicillin is defined as the maximum dilution of the product that will still inhibit the growth of a test microorganism.

In this case, the MIC concentration was a 5-fold serial dilution of a 1000 ug/ml starting solution.

I'll assume that you used a 1:10 dilution each time.

Therefore, tube #1 was 1000 ug/ml.

Tube #2 was 100 ug/ml.

Tube #3 was 10.0 ug/ml.

Tube #4 was 1.00 ug/ml.

Tube #5 was 0.100 ug/ml.

Therefore, 0.100 ug/ml appears to be the MIC for penicillin acting against Staphylococcus epidermis.

Note: If for some reason you did not do a 1:10 dilution each time, but rather did a 1:100 dilution, then adjust each dilution accordingly. In other words, Tube #2 would end up as 10.0 ug/ml and Tube #3 wold be 0.100 ug/ml, etc.