Yeast extract is an excellent and widely used source of B-complex vitamins (B1/thiamine, B2/riboflavin, B3/niacin, B5/pantothenic acid, B6/pyridoxine, B12, folate, biotin) in microbiological culture media. It is a water-soluble autolysate of Saccharomyces cerevisiae. Peptone and tryptone are protein hydrolysates providing amino acids and nitrogen. Agar is a polysaccharide solidifying agent with no nutritional significance.