On which bacteria was Alexander Fleming working during the discovery of penicillin?

What is the meaning of the term 'antibiotic' for humans and microbes,respectively?

Probiotics are

Microbes like $Spirulina$ can be grown on materials like:
$I$. Waste water from potato processing plants
$II$. Straw
$III$. Animal manure and sewage
$IV$. Molasses
Choose the correct option.

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

The chemotherapeutic importance of penicillin was given by .....