How was penicillin discovered ?

Arrange the following in the decreasing order (most important first) of their importance, for the welfare of human society. Give reasons for your answer.

Biogas, Citric acid, Penicillin and Curd

Vitamin $B_{12}$ is produced directly during the course of fermentation by

Name any two species of fungus, which are used in the production of the antibiotics.

Dough kept overnight in warm weather becomes soft and spongy because of