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. 2020 Jul 28;13:2593–2599. doi: 10.2147/IDR.S263225

Figure 2.

Figure 2

Exogenous cysteine promotes bacterial respiration and production of reactive oxygen species (ROS). (A) Exogenous cysteine (10 mM) decreases the ratio of NAD+/NADH in the absence or presence of ciprofloxacin. (B) Enhanced bacterial respiration by exogenous cysteine (10 mM) during 60 minutes, determined by iodonitrotetrazolium chloride, a reduction-sensitive dye. (C) Addition of cysteine significantly triggers the production of ROS with or without ciprofloxacin, probed by 2′,7′-dichlorodihydrofluorescein diacetate (DCFH-DA). (D) ROS scavenger NAC counteracts the potentiation activity of cysteine with ciprofloxacin against E. coli B2 persisters. NAC, N-acetylcysteine. (E) Scheme of potentiation mechanisms of cysteine. Cysteine might be metabolized to pyruvate and further entry into the TCA cycle, then promotes the NADH production and bacterial respiration, triggers ROS production (partly from oxidation of cysteine), and thereby improves bactericidal lethality against bacterial persisters. All data in (A–D) were obtained in three independent experiments and areshown as mean±SD. *P<0.05, **P<0.01, ***P<0.001, determined by unpaired t-test.