Charge regulation effect, could ultimately lead to a reducing environment within

Charge regulation effect, could ultimately lead to a reducing environment within the cytoplasm, with a subsequent increase in reactive oxygen species through electron leakage [27]. This was suggested in the current study from the observation of significantly increased abundance of lmo1407 (pyruvate formate lyase; Table S1). This protein, generally associated with anaerobic metabolism, has been observed to increase under oxidative stress in the presence of increased reactive oxygen species [28]. Similarly, Listeria adhesion protein (lmo1634) was significantly increased at pH 9.0 and induction of this protein under anaerobicconditions has been described previously [29]. Given the evidence generated from the combined proteomics dataset we proposed that an energy generation shift towards fermentation was occurring during alkaline adaptation.Uncoupling of Oxidative Phosphorylation and Relative Lag Time after an Abrupt Shift to Low Oxygen TensionProteomic analysis indicated 25331948 that an energy shift induced in L. monocytogenes by prolonged exposure to alkaline culture conditions could support anaerobiosis and involved down-regulation of oxidative phosphorylation. To test whether oxidative phosphory-Alkaline Induced Anaerobiosis in L. monocytogenesFigure 5. Proteins associated with substrate level phosphorylation observed to be significantly increased (red font = increased, green font = decreased) following adaptation to growth at pH9.0. Sections of pathways where no proteins were identified are indicated with a double forward slash. KEGG enzyme classification numbers are shown. **Transport intermediate. doi:10.1371/journal.pone.0054157.glation was reduced alkaline adapted and non-adapted cells were exposed to carbonyl m-chlorophenyl hydrazone (CCCP). CCCP is a chemical inhibitor of oxidative phosphorylation, achieved by uncoupling the proton gradient and consequently, interfering with ATP synthase’s ability to generate ATP [30]. Should alkaline adapted L. monocytogenes be more reliant on substrate-level rather than oxidative phosphorylation increased survival when exposed to CCCP would be expected. Addition of CCCP inhibited growth at pH7.3, while growth continued for the pH 9.0 grown cells (Figure 6A). This is consistent with a shift to predominantly substrate-level phosphorylation from oxidative phosphorylation and, when coupled with our proteomic findings, the transition to anaerobiosis. This conclusion was further supported by a significant decrease in expression of acetolactate decarboxylase (lmo1992; Figures 4 and 5), the finalenzyme in the acetoin biosynthesis pathway and a metabolic indicator of anaerobic growth in L. monocytogenes [31]. Furthermore, acetoin was not detected in culture fluids of alkaline adapted L. monocytogenes EGD-e cells using a Voges-Proskauer method [32] but was at pH 7.3 (data not shown). Importantly, relative lag time (RLT) following an abrupt shift to low oxygen tension (1 60.5) supported a transition to anaerobiosis during alkaline adaptation, with reduced RLT for alkaline adapted L. monocytogenes EGD-e relative to non-adapted (pH 7.3) (Figure 6B). This is an important finding given that removal of air is a commonly applied food preservation hurdle, and indicates that alkaline adapted L. monocytogenes EGD-e may be capable of reaching dangerous numbers under such conditions faster than non-alkaline adapted cells.Alkaline Induced Anaerobiosis in L. monocytogenesFigure 6. Ionophore and atmospheric challenge after ad.