bacteria can multiply fast if they environment has these factors
warm temperature
food
moisture
oxygen
Agar(jelly-like substance) or broth is used to grow bacteria since they have the nutrients the bacteria needs
when we grow bacteria, we prevent contamination so for this we follow aseptic techniques like disinfecting the surface, washing hands, sterilizing equipments
first you heat the inoculating loop by heating it then you dip it in the bacterial culture
the petri dish lid is opened a little and then zig-zag streaks are made with the inoculating loop close the lid and place it in an incubator at 25 degrees
sterilise all equipment
you can also use it to investigate different disinfectants by dipping a filter paper in it and placing in the agar jelly and the one with no zone of inhibition is the most effective