Abstract
The effect of pH on bacterial cell-growth and the evolution of extracellular pH triggered by bacterial growth has been monitored for three bacterial strains, Escherichia coli ATCC 25922 and Pseudomonas putida KT2440 as reference strains, and Pseudomonas pseudoalcaligenes CECT 5344 because of its capacity to assimilate cyanide as the sole nitrogen source under alkaline conditions. In a first instance, the influence of the initial pH in the growth curve has been texted in LB-medium adjusted to pH 6, 7 and 8, for E. coli and P. putida, and 7.5, 8.25 and 9 for P. pseudoalcaligenes. Although the initial pH were different, the pH of the extracellular medium at the end of the stationary phase converged to a certain pH that is specific for each bacterium. Similar experiments were carried out in minimal medium with glucose as the carbon source. In this case, the pHs of the culture of both Pseudomonadaceae strains were almost constant, whereas it suddenly dropped during the exponential growth phase of E. coli. When the initial pH was 6 the extracellular pH fell sharply to 4.5, which irreversibly prevented further cellular growth. Nevertheless, at higher initial pH values subsequent cellular growth of E. coli restored the medium to the initial pHs values. Finally, in all cases the evolution of the pH has been shown to depend on the carbon source used. Among the sources used, cellular growth with glucose or glycerol did not affect the extracellular pH, whereas citrate caused the alkalinization of the media. This phenotype is in concordance with computational predictions, at least in the case of the genome-scale metabolic model of Pseudomonas putida KT2440.
Highlights
The pH homeostasis in metabolism is critical for several reasons
As a proof of concept, we have demonstrated that the “in silico” prediction of the pH evolution using a well-curated genome-scale metabolic model of Pseudomonas putida KT2440 [4] agrees to the experimental pH
KT2440, were used as neutralophilic reference organisms, whereas P. pseudoalcaligenes CECT 5344 was choose as an alkaliphilic bacterium with a great biotechnological potential [6]
Summary
The pH homeostasis in metabolism is critical for several reasons. Because the structure/function of biological macromolecules, especially proteins, depends on the pH. Because pH, as any other cellular metabolite concentration, may affects the kinetic and thermodynamic force of chemical reactions involving protons as metabolites. Because pH changes severely affect the energetic metabolism, provided that the proton motive force use to be the main source of electrochemical potential for ATP synthesis. There are molecular mechanism to maintain intracellular pHs under certain values in the different subcellular compartments. Mitochondria and chloroplast are surrounded by the cytoplasm, which exhibit strict pH homeostasis [1]. Bacteria thrive in different habitats, and the pH of the environment affect the lifestyle and is the basis for classifying them into acidophiles (pH 1–3), alkalophiles (pH 10–13)
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