Multiple stress responses in Streptococcus mutans and the induction of general and stress-specific proteins

The authors have previously demonstrated that Streptococcus mutans shows an exponential-phase acid-tolerance response following an acid shock from pH 7.5 to 5.5 that enhances survival at pH 3.0. In this study the response of S. mutans H7 to acid shock was compared with the responses generated by salt, heat, oxidation and starvation. Prior induction of the acid-tolerance response did not cross-protect the cells from a subsequent challenge by the other stresses; however, prior adaptation to the other stresses, except heat (42 degrees C), protected the cells during a subsequent acid challenge at pH 3.5. Starvation by fivefold dilution of the basal medium (BM) plus fivefold reduction of its glucose content increased the numbers of survivors 12-fold, whereas elimination of glucose from fivefold-diluted BM led to a sevenfold enhancement compared to the control cells; this indicated a relationship between the acid and starvation responses. The stress responses were further characterized by comparing the 2D electrophoretic protein profiles of exponential-phase cells subjected to the various stress conditions. Cells were grown to exponential phase at pH 7.5 (37 degrees C) and then incubated for 30 min under the various stress conditions in the presence of 14C-labelled amino acids followed by cell extraction, protein separation by 2D gel electrophoresis and image analysis of the resulting autoradiograms. Using consistent twofold or greater changes in IOD % as a measure, oxidative stress resulted in the upregulation of 69 proteins, 15 of which were oxidation-specific, and in the downregulation of 24 proteins, when compared to the control cells. An acid shock from pH 7.5 to 5.5 enhanced synthesis of 64 proteins, 25 of them acid-specific, while 49 proteins exhibited diminished synthesis. The dilution of BM resulted in the increased formation of 58 proteins, with 11 starvation-specific proteins and 20 showing decreased synthesis. Some 52 and 40 proteins were enhanced by salt and heat stress, with 10 and 6 of these proteins, respectively, specific to the stress. The synthesis of a significant number of proteins was increased by more than one, but not all stress conditions; six proteins were enhanced by all five stress conditions and could be classified as general stress proteins. Clearly, the response of S. mutans to adverse environmental conditions results in complex and diverse alterations in protein synthesis to further cell survival.