if plates are poured with agar that is too hot, then the agar would kill the bacteria. Agar is the support medium for bacterial and fungal culture, as nutrients, antibiotics, salts and various growth enhancers. It is a dried hydrophilic, colloidal extracted from various species of red algae. Mainly used in cultures for bacteria and other microorganism. Too hot agar would kill the bacteria as the bacteria would get denatured.