Zinc effects on oxidative physiology of oral bacteria

Advances in Natural Sciences, Vol. 7, No. 1& 2 (2006) (131– 138) Biotechnology ZINC EFFECTS ON OXIDATIVE PHYSIOLOGY OF ORAL BACTERIA Nguyen Thi Mai Phuong Institute of Biotechnology, VAST Phan Tuan Nghia Vietnam National University Robert E. Marquis University of Rochester Medical Center, New York, USA Abstract. Zinc is used as an anti-gingivitis agent in oral care products and affects multiple tar- gets 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 Strep- tococus 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 Zn 2+ 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. Zn 2+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), pseudo- catalase (PCAT), superoxide dismutase (SOD), 50% inhibitory concentration (IC50). 1. INTRODUCTION Bacteria in dental plaque are subjected to oxidative stress caused by reactive oxy- gen species (ROS), including hydrogen peroxide, produced as a result of plaque oxygen metabolism or contained in oral care products (Marquis, 1995). Oxygen itself has only minimal capacity directly to cause oxidative damage to cells, although it can be damaging for iron-sulfur clusters in some organisms (Imlay, 2003). However, metabolism of oxygen generally results in production of a variety of toxic oxygen species such as superoxide radical (O−2 ) and hydrogen peroxide (H2O2). Hydrogen peroxide and superoxide radical 132 Nguyen Thi Mai Phuong, Phan Tuan Nghia, and Robert E. Marquis can take part in the Fenton reactions to yield the highly toxic hydroxyl radical (OH·). The ROS can react with proteins, nucleic acids and lipids with resulting oxidative dam- age. A variety of protective enzymes act during oxidative stress to eliminate ROS or to repair oxidative damage, including superoxide dismutase, NADH peroxidase, glutathione reductase, thiolperoxidase and NADH hypothiocyanide reductase. NADH oxidase also is considered to be a protective enzyme if water rather than H2O2 is the end product. Zinc is used in oral healthcare, mainly as zinc citrate, as an anti-gingivitis agent against bacteria such as Fusobacterium nucleatum and Prevotella intermedia, and also against caries caused by mutans streptococci. Zinc also has been found to be effective against oral anaerobes involved in periodontal diseases. Levels of zinc in dental plaque have been found to be about 2.5 mmole/g or 2.5 mmole/ml after use of zinc-containing toothpaste (Creeth, 1993). Moreover, zinc is considered to be a major antioxidant for an- imal cells (Powell, 2000; Zago and Oteiza, 2001). Mechanisms for its antioxidant actions include interactions with sulfhydryl groups to protect them against oxidation and compe- tition between zinc and reduced transition metal cations resulting in reduced production of hydroxyl radicals. Zinc can act as an antioxidant also for bacterial cells. For example, Gaballa and Helmann (2002) found that increased intracellular levels of zinc protected cells of Bacillus subtilis against agents such as thiol-oxidizing diamide. Zinc also pro- tected the cells against damage by hydrogen peroxide but protection depended very much on catalase, which was upregulated in association with a peroxide-induced PerR regulon. However, an antioxidant role of zinc in oral bacteria has not been identified . The focus of our study is on zinc effects on the oxidative metabolism of oral strep- tococci associated with dental caries and of Fusobacterium nucleatum associated with gingivitis. 2. MATERIALS AND METHODS 2.1. Zinc uptake by S. mutans UA 159 in suspension Cells were harvested from early stationary-phase cultures and washed once with salt solution containing 50 mM KCl and 1mM MgCl2. The cells were resuspended in 20 mM Tris-HCl or acetate buffer at the indicated pH values. At intervals, 1 ml of cell suspension was mixed with silicone oil and centrifuged. Zinc in the pellets were extracted with 0.3 M HCl solution and assayed by means of atomic absorption spectrophotometry. 2.2. Oxygen metabolism by oral bacteria For suspensions, cells were prepared as for zinc uptake experiments and resuspended in 20 mM Tris-HCl buffer, pH 7.0, containing 0.1% glucose. Oxygen levels were measured with a standard oxygen electrode (VWR) as described by Caldwell and Marquis (1999). Biofilms of S. mutans UA 159 were formed on glass slides in medium containing 3% tryptone, 0.5% yeast extract and 1% sucrose. The intact biofilms were washed with salt solution and placed in 20 mM Tris-HCl buffer, pH 7.0, plus 0.2% glucose. Oxygen metabolism was measured with a standard oxygen electrode as for cells in suspensions. Zinc Effects on Oxidative Physiology of Oral Bacteria 133 2.3. H2O2 production by S. sanguis ATCC 10904 Cells in suspensions and biofilms were prepared as for respiration assays in 20 mM Tris-HCl buffer, pH 7.0. H2O2 levels were assessed with use of horseradish peroxidase and leuco-crystal violet (Motolla et al, 1970). 2.4. Killing of S. mutans UA 159 Cells for the killing assay were suspended in 1% peptone at pH value of 7.0. The treated cells were sampled at intervals and diluted immediately in 1% peptone broth before plating on Difco tryptic soy agar as described by Dunning et al. (1998). 2.5. Enzymes assays NADH oxidase (Pool and Claiborne., 1986.), SOD (McCord and Fridovich., 1969), CAT and PCAT (Thibodeau and Keef, 1990), NHOR (Courtois and Pourtois, 1996), GR (Bazzichi et al., 2002), TSA (Park., 2000). 3. RESULT AND DISCUSSION 3.1. Uptake of zinc by oral bacteria It is noted that zinc is transported into cell cytoplasm where it reacts with multiple targets and consequently, results in the damage of the cells. Zinc is usually taken up into the cells by vehicles like chelators or transport systems. We have studied the uptake of zinc by S. mutans UA159 using AAS. Initial experiments were performed with cells of S. mutans UA159 grown in suspension cultures with excess glucose. The data in Fig.1A, showed that zinc was taken up by the organism by a saturable process. In the concentration range from 0 to about 1 mmolar, the rate of uptake at pH 7 increased linearly with increasing initial concentration of zinc sulfate. Then, at higher concentrations, the uptake diminished relative to further increases in initial concentration of zinc sulfate to level off at about 2.5 µmol Zn2+ taken up per minute per mg cell dry weight. The uptake of zinc from zinc citrate did not differ appreciably from that of zinc sulfate (data not shown). The uptake was pH sensitive with greater uptake at pH 7.0, compared with that at pH values of 6.0, 5.0 or 4.0 (Fig. 1B). Zinc has generally been found to have greatest antimicrobial activity at pH values close to neutrality and to have somewhat diminished activity at more acid pH values. At least part of this decline in antimicrobial activity is likely to be due to reduced uptake of the metal. 3.2. Zinc inhibition of respiration by oral bacteria Oral streptococci are capable of high levels of respiration (oxygen metabolism) but do not have electron transport systems in their membranes to be able to carry out oxidative phosphorylation. The need for respiration unconnected to oxidative phosphorylation is still somewhat of a mystery, although it may have to do with cell needs for oxidized NAD+ to act as an electron acceptor, for example, in glycolysis. Other oral bacteria would be expected to have at least moderate respiratory capacities, including obligate anaerobes. As shown by the graphs presented in Fig. 2, zinc sulfate inhibited respiration of intact cells of S. mutans UA159, S. sanguis NCTC 10904, S. mutans GS-5 or F. nucleatum 134 Nguyen Thi Mai Phuong, Phan Tuan Nghia, and Robert E. Marquis (a) (b) Fig. 1. Zinc uptake from zinc sulfate by S. mutans UA159 in suspension. (a): Up- take is a saturable process. (b): Uptake is sensitive to pH. ATCC 25586. The mutans streptococci (Fig. 2A) appeared to be the most sensitive with 50% inhibitory concentrations of about 0.3 mM and with only a low level of respiration largely insensitive to zinc. Respiration of S. sanguis showed a more complex response to zinc with part of respiration as sensitive as that of the mutans streptococci, part with intermediate sensitivity and a significant component with low zinc sensitivity. Respiration of F. nucleatum (Fig. 2B) showed both zinc sensitive and zinc insensitive components. About 60% of O2 utilization by this organism was inhibited by Zn while the remaining 40% was not. This may account for the higher IC50 (ca.1 mM) for respiration rate of this organism. The data show that mutans streptococci are the most sensitive with 50% inhibitory concentration (IC50) of ca. 0.3 mM, while IC50 of F. nucleatum was 1.0 mM. The same patterns for inhibition of respiration were obtained with zinc citrate (data not shown). The cells on biofilms were also sensitive to zinc sulfate with IC50 of about 75 mM, much higher compared to suspension cells (Fig. 2C). NADH oxidase plays the major role in O2 utilization by oral streptococci (Higuchi et al, 1999) and also is important for F. nucleatum. This enzyme is very active in this anaerobe and thought to be the key enzyme for the growth of F. nucleatum in oxygen conditions as well as for producing a reducing environment for growth of obligately anaer- obic organisms like P. gingivalis and P. intermedia (Diaz et al, 2002). The data (Fig 3.) showed that NADH oxidase of oral bacteria was sensitive to zinc with IC50 of about 1.4 mM for F. nucleatum, 2.1 mM for S. sanguis and 3.0 mM for S. mutans. The cells in biofilms were also sensitive to zinc, but higher zinc concentrations were required for the same level of inhibition compared to cells in suspensions (data not shown). The results suggest that the repressed respiration of cells in suspensions and biofilms is due mainly to inhibition of NADH oxidase. Respiration is source of toxic products such as ROS. ROS is considered to be toxic for host cells. The finding that zinc inhibits the respiration of oral bactreria indicated that zinc reduced ROS and therefore is an anti-oxidant in terms of in- hibition of respiration. However, respiration is in part dependent on zinc-sensitive NADH Zinc Effects on Oxidative Physiology of Oral Bacteria 135 (a) (b) (c) Fig. 2. Zinc inhibition of respiration of oral bacteria. (a): Streptococci; (b): F. nucleatum ATCC 25586; (c): Biofilms of S. mutans UA159. 100% respiration = 6.4, 3.7, 2.1, 1.7 0.66 nmoles O2 utilized/min/mg cell dry weight, respectively for S. mutans GS-5, S. mutans UA 159, S. sanguis NCTC 10904 and F. nucleatum ATCC 25586 and biofilms of S. mutans UA159. oxidase, which function in recycling NADH back into metabolism as NAD+, which can then serve as a receptor for reducing equivalents from glycolysis. Therefore, the sensitivity of NADH oxidase could cause metabolic problems. Fig. 3. Zinc inhibition of NADH oxidase. 100% activity = 0.056, 0.052, 0.032, 0.041 units/mg proteins, respectively for S. mutans GS-5, S. mutans UA 159, S. sanguis and F. nucleatum 3.3. Zinc inhibition of H2O2 production by oral bacteria Streptococci possess two types of NADH oxidase: H2O2-yielding NADH oxidase (is denoted NOX-1) and H2O yielding NADH oxidase (is denoted NOX-2). In S. mutans GS- 5, NOX-2 is a dominant enzyme and NOX-1 is an AhpCF (alkylhydroperoxide reductase) meaning that it can both produce and degrade H2O2. H2O2 production offers a competitive 136 Nguyen Thi Mai Phuong, Phan Tuan Nghia, and Robert E. Marquis advantage for bacteria in dental plaque and net H2O2 producers such as S. sanguis, S. gordonii, S. mitis. We found that Zn reduced the H2O2 production by S. sanguis 10904 in both planktonic and biofilm cells (Fig 4A). S. sanguis is a major H2O2 generator in dental plaque, and 0.1 mM zinc was more than 50% inhibitory for H2O2 production by cells in suspension. Biofilms were more resistant to zinc with IC50 value of ca. 0.5 mM (Fig. 4B). The inhibition of NADH oxidase of S. sanguis, as shown in Fig. 3, is likely to be the basis for reduced H2O2 production by this organism. (a) (b) Fig. 4. Zinc inhibition of H2O2 production by S. sanguis ATCC 10904. (a): Suspensions; (b): Biofilms 3.4. Zinc inhibition of protective enzymes in oral bacteria During oxygen metabolism, oral streptococci release reactive oxygen species includ- ing superoxide radical and hydrogen peroxide, which can cause oxidative stress for the bacteria. To protect the cells from the oxidative damage, a variety of protective enzymes are synthesized. Table 1 presented major protective enzymes in oral bacteria. We exam- ined the effect of zinc on the main enzymes including NHOR, GR, TSA, CAT, PCAT and SOD. NOX also considered to be a protective enzyme against O2 toxicity because the final product is water. The data in Table 1 indicated that not all protective enzymes including two important enzymes CAT and SOD, were targets of zinc. 3.5. Zinc enhancement of H2O2 killing of oral bacteria Fig. 5 showed that zinc enhanced H2O2 killing. However, very high levels of zinc were required for enhancement, and there are some questions regarding the physiological significance of this type of enhancement. When cells in suspensions were challenged with H2O2 in presence of Zn, the killing was somewhat enhanced compared to those that were exposed to H2O2 alone. The potential in enhancing toxicity of H2O2suggest that zinc is a pro-oxidant and enhance oxidative stress in the bacteria. Data on inhibition of protective enzyme activity presented above supported this finding. Zinc Effects on Oxidative Physiology of Oral Bacteria 137 Table 1. Zinc inhibition of protective enzymes against oxidative damage Enzymes Organisms Inhibition level IC50(mM) NADH oxidase NADH hypothiocyanite reductase (NHOR) Glutathione reductase (GR) Thiolperoxidase (TSA) Catalase (CAT) Pseudocatalase (PCAT) Superoxide dismutase (SOD) Streptococci F. nucleatum S. sanguis S. mutans S. mutans A.viscosus L. plantarum Streptococci + + + + + - - - 2.1 - 3.0 1.4 0.1 0.4 0.1 Fig. 5. Zinc enhancement of oxidative killing of S. mutans UA 159 in suspension. 4. CONCLUSIONS 1. Zinc is a potent inhibitor of respiration, especially that involving NADH oxidase in oral streptococci and is also an inhibitor of oxygen metabolism of the oral anaerobe F. nucleatum. 2. Zinc is an inhibitor for a number of enzymes considered to be protective against oxidative damage and appears to have an overall pro-oxidant action on oral bacteria. 3. 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