MIC and MBC of nisin in combination with D-amino acids for S. mutans.

The asterisk indicates that the addition of free amino acids at 10 mM generated significant effect on antibacterial activities of nisin, and decreased the MICs and MBCs of nisin against S. mutans. D-tyrosine was prepared at a concentration of 0.8 mM due to its low solubility. The combinations of D-Asp, D-Cys, and D-Glu (3D-AA) or the combinations of L-Asp, L-Cys, and L-Glu (3L-AA) were more effective than the amino acids alone in the improvement of the antibacterial activities of nisin, and even the mixture of the three amino acids at their respective concentration of 10 mM inhibited S. mutans in the absence of nisin (MIC of nisin = 0). Each test was repeated five times on different days, and means and standard errors were given.