Effect of carbon starvation and proteolytic activity on stationary-phase acid tolerance of Streptococcus mutans

Previous research with Streptococcus mutans and other oral streptococci has demonstrated that the acid shock of exponential-phase cells (pH 7.5 to 5.5 ) resulted in the induction of an acid tolerance response (ATR) increasing survival at low pH (3.5-3.0). The current study was designed to determine w hether two fresh isolates, H7 and BM71, and two laboratory strains, Ingbrit t and LT11, were capable of a stationary-phase ATR as estimated by a surviv al test at pH 3.5 for 3 h. All four strains were unable to generate a stati onary-phase ATR under control conditions at pH 7.5, with the exception of a burst of survivors in the transition between the exponential and stationar y phases when the carbon source (glucose) was depleted. Adaptation at pH 5. 5 resulted in the expected pH-dependent exponential-phase ATR, but only the fresh isolates exhibited a stationary-phase ATR at this pH. Glucose starva tion of cells in complex medium was shown to enhance acid tolerance for the fresh isolates, but not the laboratory strains. This tolerance was, howeve r, greatly diminished for all strains in a defined medium with a low concen tration of amino acids. Growth of strain H7 in complex medium resulted in t he formation of at least 56 extracellular proteins, nine of which were degr aded in the early stationary phase following the induction of proteolytic a ctivity during the transition period. No proteolytic activity was observed with strain LT11 and only 19 extracellular proteins/peptides were apparent in the medium with only one being degraded in the early stationary phase. S train H7 was also shown to have two- to fourfold higher levels of intracell ular glycogen in the stationary phase than strain LT11. These results sugge st that S. mutans H7 possessed the required endogenous metabolism to suppor t amino acid/peptide uptake in the early-stationary phase, which resulted i n the formation of basic end products that, in turn, contributed to enhance d intracellular pH homeostasis.