The L-cysteine derivatives (R)-2-amino-3-(methyldisulfanyl)propanoic acid (S-methylthio-L-cysteine), (R)-2-amino-3-(propyldisulfanyl)propanoic acid (S-propylthio-L-cysteine), (R)-2-amino-3-(1-propenyldisulfanyl)propanoic acid (S-(1-propenylthio)-L-cysteine), and (R)-2-amino-3-(2-propenyldisulfanyl)propanoic acid (S-allylthio-L-cysteine) were prepared from 3-[(methoxycarbonyl)dithio]-L-alanine, obtained from the reaction of L-cysteine with methoxycarbonylsulfenyl chloride. The occurrence of these S-(+)-alk(en)ylthio-L-cysteine derivatives in onion (Allium cepa L.) was proven by using UPLC-MS-ESI(+) in SRM mode. Their concentrations in fresh onion were estimated to be 0.19 mg/kg S-methylthio-L-cysteine, 0.01 mg/kg S-propylthio-L-cysteine, and 0.56 mg/kg (S-(1-propenyllthio)-L-cysteine, concentrations that are about 3000 times lower than that of isoalliin (S-(1-propenyl-S-oxo-L-cysteine). These compounds were treated with Fusobacterium nucleatum, a microorganism responsible for the formation of mouth malodor. These L-cysteine disulfides were demonstrated to predominantly produce tri- and tetrasulfides. Isoalliin is almost entirely consumed by the plant enzyme alliin lyase (EC 4.4.1.4 S-alk(en)yl-S-oxo-L-cysteine lyase) in a few seconds, but it is not transformed by F. nucleatum. This example of flavor modulation shows that the plant produces different precursors, leading to the formation of the same types of volatile sulfur compounds. Whereas the plant enzyme efficiently transforms S-alk(en)yl-S-oxo-L-cysteine, mouth bacteria are responsible for the transformation of S-alk(en)ylthio-L-cysteine.