Inhibition of acid production by Streptococcus mutans NCTC 10449 by zinc and the effect of metal speciation.

Ionic zinc as zinc sulphate was strongly inhibitory to acid production from glucose by washed cell suspensions of Streptococcus mutans NCTC 10449 in a pH-stat assay (50% inhibition at 0.1 mM zinc). Zinc was adsorbed to the cells (up to 8 micrograms/mg cells). Several other zinc salts that dissociated to free zinc ions were also strongly inhibitory. Zinc in a partially complexed form as zinc citrate was less strongly inhibitory (50% inhibition at 0.2 mM zinc), and adsorption of zinc was lower (up to 3 micrograms/mg cells). Stoichiometric addition of ethylenediaminetetra-acetic acid (EDTA) to assays containing zinc sulphate completely removed inhibition and adsorption of zinc. A good correlation between inhibition of acid production and adsorption of zinc was found, and zinc adsorption was correlated with the levels of free zinc ion in the assay mixtures. The strongly anionic complexes of zinc, Zn(CIT)-, Zn(CIT)4-(2), and Zn(EDTA)2-, were not inhibitory and did not adsorb to the bacterial cells.