text: A variety or organosulfur compounds occur in nature.  Most abundant are the amino acids methionine, cysteine, and cystine.  The vitamins biotin and thiamine, as well as lipoic acid contain sulfur heterocycles.  Glutathione is the primary intracellular antioxidant. Penicillin and cephalosporin are life-saving antibiotics, derived from fungi. Gliotoxin is a sulfur-containing mycotoxin produced by several species of fungi under investigation as an antiviral agent.