Sulfur is an essential element for all life. Three amino acids, cysteine, cystine, and methionine, and two vitamins, biotin and thiamine are organosulfur compounds. Many cofactors also contain sulfur including glutathione, thioredoxin, and iron-sulfur proteins. Disulfides confer the mechanical strength and insolubility of the protein keratin. Sulfur is used in pharmaceutical skin preparations for the treatment of acne and other conditions mainly because of its action in killing Propionibacterium acnes.