[header] How to grow bacteria in a petri dish [title] Prepare the agar. [step] Agar is the jelly-like substance used to culture bacteria. It is made from a type of red algae, which provides an ideal growing surface for many different types of bacteria. 
Some types of agar contain added nutrients (such as sheep's blood) which help to promote more vigorous bacterial growth. [substeps] The easiest type of agar to use for this experiment is a nutrient agar which comes in powder form.