Multi-target antimicrobial actions of zinc against oral anaerobes

Zinc is used in oral care products as an antiplaque/antigingivitis agent. Our objective was to assess the antimicrobial actions of zinc against oral anaerobes associated with gingivitis, specifically Fusobacterium nucleatum and Prevotella intermedia, with focus on catabolism and oxidative metabolism.

The oral anaerobes were grown in complex medium in an anaerobic chamber, harvested by centrifugation and used directly for experiments with suspensions. Biofilm growth involved super-infection by F. nucleatum of an initial biofilm formed by Streptococcus sanguis.

Zn(2+) inhibited catabolism of glutamate, glutamyl-glutamate, glucose and fructose by F. nucleatum cells in suspensions with ID(50) values, respectively, of 0.05, 0.005, 0.01 and 0.01 mM. The ID(50) value for inhibition of glutamate catabolism by biofilms was 0.10 mM. Inhibition of glutamate catabolism could be related to inhibition of substrate uptake and of 2-oxoglutarate reductase. Zn(2+) also inhibited catabolism of aspartate or aspartyl-aspartate by P. intermedia with ID(50) values of 0.07 and about 0.03 mM, respectively. Respiration of intact cells of F. nucleatum and NADH oxidase in cell extracts were sensitive to zinc with ID(50) values, respectively, of about 1.0 and 1.4 mM. Zinc also inhibited production of hydrogen peroxide by F. nucleatum (ID(50) = ca. 0.04 mM.) but at high concentrations acted to potentiate and enhance peroxide killing of the anaerobe.

Zn(2+) is a potent inhibitor of catabolism by F. nucleatum and P. intermedia, including catabolism of peptides, which can be degraded to yield inflammatory metabolic end products. Zn(2+) also inhibits O(2) metabolism of F. nucleatum by about 50% and hydrogen peroxide production nearly completely but also enhances killing by peroxide added to cells.