Acid induction of citrate transport, citrate metabolism, and diacetyl/acetoin pathways in L. lactis. Proposed pathway for citrate fermentation and C4 compound production. CitP, citrate transporter; CitDEF; citrate lyase complex; AldB and AldC, α-acetolactate decarboxylases; ButA and ButB, diacetyl acetoin reductases (also named butanediol dehydrogenases).