Abstract
Degradative amino acid decarboxylation pathways in bacteria generate secondary metabolic energy and provide resistance against acid stress. The histidine decarboxylation pathway of Streptococcus thermophilus CHCC1524 was functionally expressed in the heterologous host Lactococcus lactis NZ9000, and the benefits of the newly acquired pathway for the host were analyzed. During growth in M17 medium in the pH range of 5-6.5, a small positive effect was observed on the biomass yield in batch culture, whereas no growth rate enhancement was evident. In contrast, a strong benefit for the engineered L. lactis strain was observed in acid stress survival. In the presence of histidine, the pathway enabled cells to survive at pH values as low as 3 for at least 2 h, conditions under which the host cells were rapidly dying. The flux through the histidine decarboxylation pathway in cells grown at physiological pH was under strict control of the electrochemical proton gradient (pmf) across the membrane. Ionophores that dissipated the membrane potential (ΔΨ) and/or the pH gradient (ΔpH) strongly increased the flux, whereas the presence of glucose almost completely inhibited the flux. Control of the pmf over the flux was exerted by both ΔΨ and ΔpH and was distributed over the transporter HdcP and the decarboxylase HdcA. The control allowed for a synergistic effect between the histidine decarboxylation and glycolytic pathways in acid stress survival. In a narrow pH range around 2.5 the synergism resulted in a 10-fold higher survival rate.
Highlights
Degradative amino acid decarboxylation pathways in bacteria have diverse physiological functions
Proton Motive Force Inhibits the Flux through the Histidine Decarboxylation Pathway—The histidine decarboxylation gene cluster of S. thermophilus CHCC1524, encoding histidine decarboxylase HDC pathway (HdcA), the histidine/histamine exchanger HdcP, and the HdcA maturation enzyme HdcB, was functionally expressed in L. lactis NZ9000
In the presence of 25 M of the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) that completely dissipated both components of the pmf, a 3- to 4-fold increase in histamine production rate was observed (Fig. 1A)
Summary
Degradative amino acid decarboxylation pathways in bacteria have diverse physiological functions. Degradative amino acid decarboxylation pathways in bacteria generate secondary metabolic energy and provide resistance against acid stress. The coupled reactions of decarboxylation and electrogenic exchange in the pathways generate a proton motive force consisting of both a transmembrane pH gradient (inside alkaline) and electrical potential (inside negative) and provide the cell with metabolic energy [16]. We mimic the horizontal gene transfer event, the first step in the successful acquisition of new functionality in this evolutionary process, by introducing the histidine decarboxylation pathway of the yoghurt bacterium Streptococcus thermophilus CHCC1524 [16, 26] in the cheese bacterium Lactococcus lactis and analyzing the beneficial effects for the receiving organism. This study reveals a major benefit of the pathway for L. lactis in survival at low pH and uncovers a synergistic effect of the histidine decarboxylation pathway and the glycolytic pathway in acid stress survival
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