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Two-dimensional gel electrophoretic analysis of the proteome of Streptococcus mutans grown at a steady state in a glucose-limited anaerobic continuous culture revealed a number of proteins that were differentially expressed when the growth pH was lowered from pH 7.0 to pH 5.0. Changes in the expression of metabolic proteins were generally limited to three biochemical pathways: glycolysis, alternative acid production and branched-chain amino acid biosynthesis. The relative level of expression of protein spots representing all of the enzymes associated with the Embden-Meyerhof-Parnas pathway, and all but one of the enzymes involved in the major alternative acid fermentation pathways of S. mutans, was identified and measured. Proteome data, in conjunction with end-product and cell-yield analyses, were consistent with a phenotypic change that allowed S. mutans to proliferate at low pH by expending energy to extrude excess H(+) from the cell, while minimizing the detrimental effects that result from the uncoupling of carbon flux from catabolism and the consequent imbalance in NADH and pyruvate production. The changes in enzyme levels were consistent with a reduction in the formation of the strongest acid, formic acid, which was a consequence of the diversion of pyruvate to both lactate and branched-chain amino acid production when S. mutans was cultivated in an acidic environment.
Len et al. (Sat,) studied this question.