Which products synthesized by microorganisms are useful to mankind?

The chemical substances which are produced by some microbes and can kill or retard the growth of other microbes are known as:

Which of the following processes is performed only by bacteria?

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?

Who was the first discoverer of antibiotics?

Give scientific reasons: In the rumen of cattle,components having high cellulose content get digested easily.