Inhibition of the sodium-translocating NADH-ubiquinone oxidoreductase [Na+-NQR] decreases cholera toxin production in Vibrio cholerae O1 at the late exponential growth phase

Abstract Two virulence factors produced by Vibrio cholerae , cholera toxin (CT) and toxin-corregulated pilus (TCP), are indispensable for cholera infection. ToxT is the central regulatory protein involved in activation of CT and TCP expression. We previously reported that lack of a respiration-linked sodium-translocating NADH–ubiquinone oxidoreductase (Na + -NQR) significantly increases toxT transcription. In this study, we further characterized this link and found that Na + -NQR affects toxT expression only at the early-log growth phase, whereas lack of Na + -NQR decreases CT production after the mid-log growth phase. Such decreased CT production was independent of toxT and ctxB transcription. Supplementing a respiratory substrate, l -lactate, into the growth media restored CT production in the nqrA-F mutant, suggesting that decreased CT production in the Na + -NQR mutant is dependent on electron transport chain (ETC) activity. This notion was supported by the observations that two chemical inhibitors, a Na + -NQR specific inhibitor 2-n-Heptyl-4-hydroxyquinoline N-oxide (HQNO) and a succinate dehydrogenase (SDH) inhibitor, thenoyltrifluoroacetone (TTFA), strongly inhibited CT production in both classical and El Tor biotype strains of V. cholerae . Accordingly, we propose the main respiratory enzyme of V. cholerae , as a potential drug target to treat cholera because human mitochondria do not contain Na + -NQR orthologs.