A Petri dish filler is a person who fills Petri dishes with AGAR or GEL. AGAR is a jelly-like substance made from seaweed, and GEL is a thick, jelly-like substance made from a variety of substances. Both are used to culture bacteria and other microorganisms in the laboratory. Petri dish fillers are responsible for ensuring that the AGAR or GEL is evenly distributed in the dish and that the dish is properly sealed.