Theory:

Not all microorganisms are harmful; some are useful in day to day life, for example in the preparation of bread, curd, and cakes. Microorganisms are really useful in many industries such as the food, agriculture and pharmaceutical. It is also helpful in cleaning the environment.
  • Food industry
  • Agriculture industry
  • Pharmaceutical industry
While preparing curd, you could have witnessed your mother putting one spoon of curd in milk, which will turn the milk to curd. By doing so, there will be the formation of Lactobacillus, which will turn milk into curd. Yeast is another bacteria that we use to add softness and fluffiness to bakery items such as cake and pastries and bread. Yeast is used for fermentation.
Fermentation
Yeast performs fermentation to obtain energy by converting sugar into alcohol. With yeast, a large scale alcohol is produced for the commercial purposes.
 
Even while making batter for idlis and parottas, bacteria play a key role in adding softness and taste to the food. A batter that was only half of the vessel on the previous night occupies the full space of the vessel in next morning. How did that happen? It is because of the leavening action of the lactic acid bacterium, L. mesenteroides.
 
Yeast is used in the process of converting maida batter (used for parottas) into the more soft and fluffy. 
 
Microorganisms can live anywhere starting from plants, animals, inside a cell, etc.,. We are surrounded by microorganisms.