In cattle,these are helpful in the digestion of cellulose.

Which organisms have been used for human welfare for centuries?

Mushrooms are a rich source of . . . . . . .

Consider the following statements:
$I.$ Antibiotics are chemical substances produced by some microorganisms which can kill or retard the growth of other disease-causing microorganisms.
$II.$ Penicillin is the first antibiotic discovered by Alexander Fleming $(1928)$,while working on the bacterium $Staphylococcus$ $aureus$.
$III.$ The function of penicillin as an antibiotic was established by Ernst Chain and Howard Florey.
Which of the statements given above are correct?

The richest sources of vitamin $B_{12}$ are

Identify the bacterium from the given options.