What are organisms that cannot be seen with the naked eye called?

Who coined the term 'antibiotic'?

What is the role of bacteria present in the rumen of cattle?

Which of the following can be the use of micro-organisms?

The drug quinine is obtained from which of the following?

From the given below,which one increases vitamin $B_{12}$ in our stomach and also plays a very beneficial role in checking disease-causing microbes?