Answer:
1) Today Lousie(,) we know the value of penicillin.
2) By 1940(,) however(,) this mold became a new medicine.
3) Alexander Flemming(,) I believe(,) was the man who discovered the mold used in making penicillin.
4) To tell the truth(,) I don't know why he even noticed it.
5) However(,) the mold had ruined one of Flemming's experiments.
6) Frank(,) many scientists would have thrown out and experiment like this.
7) In fact(,) the mold had killed the bacteria in his dish.
8) Nevertheless(,) Flemming took a second look.
9) The second look(,) in my opinion(,) showed his true genius.
10) On the other hand(,) perhaps any hard-working scientist would have done the same.
Explanation: I added the commas where they are needed.