Oxygen uptake and its relation to pH in a human salivary system during fermentation of glucose

Oxygen consumption by the mixed bacteria in salivary sediment was examined in relation to the decrease in pH that occurs when glucose at different concentrations (2.8 mM-1.68 M) was fermented in 4 h incubations at 37 degrees C. These experiments demonstrated that (i) the use of oxygen was extremely rapid, resulting in all cases in the PO2 decreasing within 1-2 min from atmospheric PO2 (approx. 20 kPa) to levels at or near zero; (ii) a period of about 30 min of reduced oxygen uptake consistently occurred after the initial PO2 drop, so long as salivary supernatant was present and the pH was allowed to fall; (iii) except for 11.2 mM glucose, the PO2 was kept at or near zero throughout each incubation with all glucose concentrations tested because of rapid oxygen consumption by the sediment bacteria--oxidizable substrates in the sediment and in added salivary supernatant contributed significantly to the prolonged oxygen depletion; (iv) the pH was important for determining the relative contributions of glucose and supernatant to the uptake of oxygen by the sediment bacteria and for observations (ii) and (iii). When the acids produced during aerobic degradation of glucose were tested for stimulation of oxygen uptake, L(+)lactic stimulated more rapid uptake than did D(-)lactic acid, whereas acetic and propionic acids showed none. These findings were in agreement with a metabolic scheme proposed earlier for aerobic degradation of glucose by the sediment microflora, and indicated where and how oxygen utilization might be involved in glucose fermentation.