Zinc Effects on Oxidative Physiology of Oral Bacteria

Abstract

Zinc is used as an anti-gingivitis agent in oral care products and affects multiple targets in oral bacteria. We found that it acts also to suppress respiration of oral streptococci and of Fusobacterium nucleatum, an organism associated with development of gingivitis. Zinc as either the sulfate or citrate chelate was a potent inhibitor of respiration of the oral streptococci Streptococus mutans GS-5 or UA 159 and S. sanguis NCTC 10904. 50% inhibitory concentrations for intact cells in suspensions were below 0.3 mM with nearly complete inhibition of O2 metabolism at higher zinc levels. 1.0 mM Zn2+ inhibited O2 utilization by F. nucleatum ATCC 25586 by some 50%, but almost 40% of O2 metabolism by the anaerobe was not Zn sensitive. NADH oxidase plays the major role on O2 utilization by oral streptococci and is also important for F. nucleatum. In cell extracts of streptococci, the 50% inhibitory concentration (IC50) of Zn2+ for NADH oxidase activity ranged from ca. 2.1 mM for S. sanguis NCTC 10904 to 3.0 mM for the GS-5 strains of S. mutans. The enzyme in extracts of F. nucleatum was somewhat more Zn sensitive with an IC50 value of 1.4 mM. S. sanguis is a major generator of H2O2 in dental plaque. Zn2+acted to reduce peroxide production assessed with use of horseradish-peroxidase and leuco-crystal violet. 0.1 mM ZnSO4 was more than 50% inhibitory for cells in suspensions or mono-organism biofilms. Zinc is primarily bacteriostatic rather than bactericidal, and inhibition was at least partially reversible after zinc removal. Zinc was inhibitory also for the protective enzymes thiolperoxidase (IC50= 0.1 mM), hypothiocyanite reductase (IC50 = 0.1 mM) and glutathione reductase (IC50 = 0.4 mM). Zinc acted mainly as a pro-oxidant for oral bacteria inhibiting NADH oxidase, considered to be protective against oxidative stress, and also other protective enzymes. This view is supported by findings of zinc enhanced peroxide killing of the organisms. Abbreviations. Reactive oxygen species (ROS), NADH oxidase (NOX), NADH hypothiocyanite reductase (NHOR), glutathione reductase (GR), thiolperoxidase (TSA), catalase (CAT), pseudocatalase (PCAT), superoxide dismutase (SOD), 50% inhibitory concentration (IC50).