We have this science experiment on how to grow a bacteria, our teacher told us to use gelatin, beef stock, and sugar as an alternative for agar. I am confused on why we need gelatin, beef stock, and sugar in growing a bacteria. What ingredient is in those 3 materials that can aid in growing bacteria?
Everything that lives needs food to grow. That food may be as simple as nutrients needed to build parts to more complex tasks like storing and/or converting energy.
Different bacteria require different things. A nutrient source like beef stock contains lots of amino acids and proteins which can feed a large spectrum of bacteria.
Sugar is a densely packed energy store that many bacteria can use for powering biologic processes.
Gelatin is a mixture of peptides and proteins which adds even more nutrients to the system. More importantly, its thick gelatinous structure acts as a binding agent/ substrate which keeps everything from sloshing around in the dish (which can be very inconvenient when moving it around).
In short the simple petri dish is about providing a wide variety of nutrients so that it can support the growth of a wide variety of microbes.