10 E.F. Sato et al. Fig. 1. Morphological features of oral neutrophils.Electron mi- crographs: A: OPMN at 0 h; B: OPMN at 4 h spontaneously generate superoxide radical and nitric oxide in the absence of any stimuli [9, 25]. The oral neutrophils perhaps remain functional at a short distance from the gin- gival margin by the flow of gingival fluid along the tooth surface, but once neutrophils are in saliva, they rapidly dead, due to osmotic shock [26]. Therefore, oral neutrophils im- mediately showed morphological changes which are charac- teristic of those of apoptosis (Fig. 1) [27]. In oral cavity, ox- idative stress and/or redox regulation, dependent pathway(s) activated caspase-3 in oral neutrophils, thereby inducing their apoptosis. Bacterial ROS Prodution and Their Defense Mecha- nism The oral bacteria of humans are composed of more than 300 bacterial species. Streptococcus mutans, S. sobrinus, S. cricetus, and S. rattus and S. sanguis are found in larger numbers on teeth, while S. salivarius is isolated mainly from the tongue [28]. On teeth, microorganisms colonize in a dental plaque. The microbial composition of saliva is similar to that of the tongue. Helicobacter pylori (H. pylori) and Enterococcus faecalis (E. faecalis) are also isolated as minor bacteria in oral cavity. In addition to production by neutro- phils and macrophage, bacteria can also produce superoxide anion and hydrogen peroxide. We previously reported that H. pylori generates the superoxide radical from respiratory chain and that the sensitivity of its respiration to NO is sig- nificantly lower than that of Escherichia coli (E. coli) and mammalian mitochondria [29–33]. Because superoxide is extremely reactive with NO, this reaction may contribute, at least in part, to the mechanism that allows H. pylori to be re- sistant to gastric NO [29]. However, peroxynitrite generated from NO and superoxide is also toxic and impairs DNA and cellular proteins [34]. Thus, oxidative stress elicited by NO, superoxide, and related metabolites may also injure H. pylori. To further elucidate the mechanism for the strong re- sistance of H. pylori to NO and related metabolites, we com- pared the effects of NO on the respiration of H. pylori and E. Fig. 2. NADH oxidase of Streptococcus mutans coli and the fate of this gaseous radical in and around these bacteria. A kinetic analysis revealed that the strong resis- tance of the H. pylori respiration was due, at least in part, to the low affinity of the respiratory chain terminal oxidase for NO in the membrane/lipid bilayers of intact cells [35]. E. faecalis, an intestinal commensal of humans and ani- mals, generates large amounts of extracellular superoxide [36]. Extracellular superoxide is produced by E. faecalis through the respiratory chain due to rapid nonenzymatic re- action of semiquinone radicals of demethylmenaquinone with oxygen [37]. NADH oxidases are found in several microorganisms [38]. There are two types of NADH oxidase, H2O forming and hydrogen peroxide forming. The hydrogen peroxide- forming NADH oxidases found in Streptococcus mutans, which are facultatively anaerobic bacteria that lack a heme and a respiratory chain (Fig. 2) [39, 40]. The enzymes cata- lyze the reduction of oxygen by NADH to form hydrogen peroxide. However, in the presence of AhpC, the NADH ox- idases also showed extremely high reductase activity for both hydrogen peroxide and alkyl hydroperoxides [39, 41]. These NADH oxidases are involved not only in the regener- ation of NAD but also in the removal of peroxides. Thus, in spite of lacking a respiratory chain and peroxide-scavenging enzymes such as catalase and heme-containing peroxidases, Streptococcus mutans can grow as well under aerobic condi- tions as do conventional aerobic bacteria [39, 42]. The Entero-Salivary Circulation of Nitric Oxide Metabolites and Their Defense System Because NO is potent bactericidal compound [43], its generation in gastrointestinal tracts is of critical importance in host defense (Fig. 3). In this context, Benjamin et al. [44] reported that Candida albicans retained their viability in acidic medium but died in the presence of nitrite. Dykhuizen et al. [45] also reported that H. pylori died in nitrite- containing acidic medium but survived in nitrite-containing J. Clin. Biochem. Nutr.