Why is propanoic acid often added to the diet of C. elegans?
Propanoic acid is toxic against many types of mold, fungi and bacteria (wiki), but same time has almost no effect to C. elegans ("fivefold less", reference). This is why propanoic acids and (more commmon) its salts are used as conservants for C.elegans cultures (as well as for other animals, like Drosophila).