The effects of sorbate on oral streptococci grown in continuous culture

The antimicrobial effect of potassium sorbate on Streptococcus mutans and S. milleri grown in continuous culture was determined at pH values of 7.0, 5.5, and 5.0. Organisms were grown glucose-limited at a dilution rate of D = 0.1 h-1, corresponding to a doubling time of ca. seven hours, in a chemically defined medium under an atmosphere of 5% CO2 in N2. After equilibration at the appropriate pH level, the culture vessel was pulsed with potassium sorbate to a final concentration of 20 mmol L-1 or with sorbate followed almost immediately by a pulse of glucose (to 40 mmol L-1); that is, the cultures were exposed to transient excesses of sorbate with and without glucose--such as might occur during meals. At pH 7.0, sorbate had virtually no effect on glucose uptake and on the, albeit transient, increase in growth rate and acid production by both organisms. At pH 5.5 and 5.0, both cell viability and acid production in S. milleri were depressed following a pulse of sorbate, the effect being even more marked after a sorbate/glucose pulse. At low pH, the uptake of glucose in the presence of sorbate was drastically reduced. The effect on S. mutans was less marked, but growth and acid production were greatly depressed at pH 5.0 following consecutive pulses of sorbate. The antimicrobial effect of the widely used preservative potassium sorbate, at low pH, is confirmed for oral streptococci, and it is suggested that continuous culture offers a relevant system for testing potential antimicrobial agents.