To further verify that Na + depletion induces an oxidative stress response of P. gingivalis, we carried out comparative analyses of small metabolites formed in parent and rprY mutant cells grown (2 hr) in the presence and absence of Na + . The **data** was analyzed in several ways including differences between parent and mutants strains in normal medium; in Na + -depleted medium; and metabolites that responded to Na + stress in parent or mutant. The p-values obtained from Two-Way ANOVA statistical analyses indicated whether differences were due to the effect of the rprY mutation or to Na + stress or both. Accordingly, as seen in Table 3, certain significant effects appeared to relate to the sodium status, e.g., in Na + stress, higher methionine sulfoxide, lower ketosphingosine, and more of certain dipeptides. Other effects appeared more related to RprY function than to Na + , e.g., in the mutant, higher undecanoate and lower thiamine. While it is difficult to make close connections of these changes with physiology (see Discussion), we here observe only that this initial analysis of a large number of metabolites from biosynthesis, catabolism and central metabolism reveals perturbations in relatively few of them in the four situations examined. Finally, we compared growth of a P. gingivalis oxyR mutant with that of the parent strain in Na + -depleted and normal media. As observed previously, the parent strain was able to grow in the Na +depleted medium while the oxyR mutant could not (Fig. 5), similar to the rprY mutant (Fig. 2). Since OxyR is a master regulator of genes involved in oxidative stress in Gram-negative bacteria [18], we infer that Na + depletion causes oxidative stress in the P. gingivalis cells.