The effect of glucose concentration on the interrelation between glucose utilization, pH and carbohydrate storage in a salivary system

A mixed-bacteria ecological approach to understanding the role of the oral bacteria in dental caries causation: an alternative to Streptococcus mutans and the specific-plaque hypothesis

For more than 100 years, investigators have tried to identify the bacteria responsible for dental caries formation and to determine whether their role is one of specificity. Frequent association of Lactobacillus acidophilus and Streptococcus mutans with caries activity gave credence to their being specific cariogens. However, dental caries occurrence in their absence, and the presence of other bacteria able to produce substantial amounts of acid from fermentable carbohydrate, provided arguments for non-specificity. In the 1940s, Stephan found that the mixed bacteria in dental plaque produced a rapid drop in pH following a sugar rinse and a slow pH return toward baseline. This response became a cornerstone of plaque and mixed-bacterial involvement in dental caries causation when Stephan showed that the pH decrease was inversely and clearly related to caries activity. Detailed examination of the pH (acid-base) metabolisms of oral pure cultures, dental plaque, and salivary sediment identified the main bacteria and metabolic processes responsible for the pH metabolism of dental plaque. It was discovered that this metabolism in different individuals, in plaque in different dentition locations within individuals, and in individuals of different levels of caries activity could be described in terms of a relatively small number of acid-base metabolic processes. This led to an overall bacterial metabolic vector concept for dental plaque, and helped unravel the bacterial involvement in the degradation of the carbohydrate and nitrogenous substrates that produce the acids and alkali that affect the pH and favor and inhibit dental caries production, respectively. A central role of oral arginolytic and non-arginolytic acidogens in the production of the Stephan pH curve was discovered. The non-arginolytics could produce only the pH fall part of this curve, whereas the arginolytics could produce both the fall and the rise. The net result of the latter was a less acidic Stephan pH curve. Both kinds of bacteria are numerous in dental plaque. By varying their ratios, we were easily able to produce Stephan pH curves indicative of different levels of caries activity. This and substantial related metabolic and microbial data indicated that it is the proportions and numbers of acid-base-producing bacteria that are at the core of dental caries activity. The elimination of S. mutans, as with a vaccine, was considered to have little chance of success in preventing dental caries in humans, since, in most cases, this would simply make more room for one or more of the many acidogens remaining. An understanding of mixed-bacterial metabolism, knowledge of how to manipulate and work with mixed bacteria, and the use of a bacterial metabolic vector approach as described in this article have led to (1) a more ecological focus for dealing with dental caries, and (2) new means of developing and evaluating anti-caries agents directed toward microbial mixtures that counter excess acid accumulation and tooth demineralization.

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.

Constituents of salivary supernatant responsible for stimulation of oxygen uptake by the bacteria in human salivary sediment

The 10,000 g supernatant of wax-stimulated whole saliva was fractionated by gel filtration and its components were tested along with amino acids, small peptides and urea for their ability to stimulate this oxygen uptake, and for their effects on pH. Fractions containing the larger components, the proteins and large peptides, stimulated much less oxygen uptake than unfractionated supernatant, and caused a small decrease in pH. Analysis with anthrone indicated that both these effects were due mainly to the carbohydrate associated with these constituents. In contrast, fractions containing the remaining lower molecular-weight components stimulated substantial oxygen uptake and a rise in pH; both effects were like those seen with whole saliva supernatant. The oxygen effects were attributed mainly to certain amino acids and small peptides in the small molecular-weight fractions. Ornithine, arginine, proline and glutamic acid consistently stimulated oxygen uptake by the oral microflora in a test of 23 amino acids with the sediments of 13 subjects. Ornithine and arginine at the same time stimulated a significant rise in pH, whereas the other two amino acids showed no such effect. Variable and sometimes significant oxygen uptake was seen with alanine, aspartic acid, asparagine, glutamine and cysteine in 4-7 of the subjects; infrequent or no effects were seen with the remainder of the amino acids tested. There was some evidence to suggest that amino acid stimulation of oxygen uptake may be inducible. Urea had no effect on uptake but did contribute significantly to the pH rise. Small peptides containing those amino acids that could stimulate oxygen uptake also stimulated such uptake; peptides without such acids did not.(ABSTRACT TRUNCATED AT 250 WORDS)

pH changes during simultaneous metabolism of urea and carbohydrate by human salivary bacteria in vitro

The effect of the wide natural variation in oral ureolysis rates on the pH changes resulting from simultaneous metabolism of 25 mM urea and 2.8 mM glucose in salivary-sediment bacteria were investigated. The pH curves were complex, and included distinctive plateaux indicative of balanced acid and base production. These neutralization plateaux occurred at different pHs, which were a function (r2 = 0.98) of the ureolytic rate as measured by the log of the initial pH-change rate in the urea-only reaction. In the simplest case, the pH curve was characterized by a rise or fall to the neutralization plateau, a variable period of time at the plateau (up to 1 h), then a pH rise. The pattern of pH changes induced by glucose alone varied between different sediments: in some cases, the pH decreased smoothly to an end-point; in others, the curve was more complex, and these features became superimposed on the urea/glucose curve. The rate of ureolytic ammonia release was almost constant and unaffected by simultaneous carbohydrate metabolism. Concomitant metabolism of endogenous carbohydrate present in sediments prepared 1-2 h following a meal was of sufficient magnitude to affect ureolytic pH curves. If the ureolytic activity was high, this effect was negligible; if it was low, metabolism of the endogenous carbohydrates could completely suppress the ureolytic pH rise. Soluble salivary components had little effect on ureolysis but pH changes were modified by buffering, and the presence of urea, ammonia, N-catabolic and acidogenic substrates in the saliva.

In-vitro urea-dependent pH-changes by human salivary bacteria and dispersed, artificial-mouth, bacterial plaques

The pH effects of urea metabolism were studied in washed salivary-sediment bacteria from subjects that had up to 10-fold variation in oral ureolytic activity, and in dispersed artificial-mouth plaques. Adequate evaluation required analysis of the [OH-] as well as the pH curve. An initial constant rate of pH-change, lasting until pH 7.8, was derived from the pH curve; this gave the best correlation (r = 0.95) with the ureolysis rate. From the [OH-]-curve, between pH 7.8 and 8.3 (approx.), a constant and maximal rate of change in [OH-] was determined. Although theoretically this was directly related to the rate of ammonia release, it was 10(-2) to 10(-3) times its value and correlated less well (r = 0.83) with ureolysis. Together with the initial and final pH, these two rates largely described urea-induced pH changes. After 12.5-fold dilution of the cells, changes in the pH curve were minor. Although the rate of ureolytic ammonia release was proportional to cell-protein concentration, the reduction in ureolytic activity was compensated by a corresponding reduction in cell pH-buffering. Consequently, in order to relate pH and [OH-] changes to ureolysis, it was necessary to control, or correct for, variations in the cell mass present. Buffering capacity in plaques was greater than in sediments. The 10-fold range in oral ureolytic activity by salivary bacteria gave a 10-20-fold range in base changes.

Kinetics and product stoichiometry of ureolysis by human salivary bacteria and artificial mouth plaques

Ureolysis was investigated in salivary bacteria from persons with widely-differing oral ureolytic activities. Rate curves and product stoichiometry were established for urea disappearance, ammonia appearance and conversion of [14C]-urea to 14CO2. Ammonia, released stoichiometrically from urea, was best measured by a direct phenate-hypochlorite reaction. About 80 per cent of the urea-C was liberated as free CO2. Slight deviations from ammonia stoichiometry and most of the CO2 loss occurred in the first 5-10 min of reaction, when the rate of urea disappearance was constant and up to 2-fold higher than subsequently. This rate-change suggests that flux in the ureolysis pathway may be under feedback control. Ureolysis by salivary-sediment bacteria followed Michaelis-Menten kinetics with a Km of 2.5 mM; rates of end-product formation were independent of urea concentration between 25 and 500 mM. Ureolysis was inhibited 98 per cent by 5 mM acetohydroxamic acid, a urease inhibitor, and could be partly solubilized by sonication to give an enzyme preparation which, without cofactor supplementation, quantitatively hydrolysed urea. Thus urea metabolism by oral bacteria may principally involve urease-catalysed hydrolysis, rather than non-urease pathways.

Catabolism of arginine by the mixed bacteria in human salivary sediment under conditions of low and high glucose concentration

The catabolism of glucose by the oral mixed bacteria results in a lowering of the pH whereas arginine degradation favours a rise. In the mouth, low and high levels of glucose cause different plaque pH conditions which, in turn, might affect the rate and mode of degradation of arginine. This possibility was examined in the suspended salivary-sediment system where these pH conditions can be simulated. With the pH, the metabolic parameters examined were arginine utilization, ammonia, carbon dioxide and putrescine formation, utilization of glucose and changes in levels of L(+)- and D(-)-lactic acid. At the lower glucose concentration, the pH rapidly fell and then slowly rose whereas, with the higher glucose level, the pH showed a greater fall and no subsequent rise. The more acidic pH conditions favoured by the higher glucose level inhibited arginine degradation and the appearance of its various end-products and intermediates. Arginine degradation with arginine-[U-14C] and paper chromatography-autoradiography showed successive appearance of citrulline, ornithine and putrescine and, depending upon the pH, some succinate. When the pH was held constant at several different values, arginine degradation was optimal when the pH was near neutrality. In supplementary experiments, arginine had little effect on the ability of the oral mixed bacteria to utilize glucose and produce and utilize lactic acid, whereas the arginine peptide, arginylisoleucine and saliva supernatant stimulated these processes. Thus glycolysis enhancement and a more rapid clearance of fermentable carbohydrate by the oral bacteria would accompany pH-rise activity with arginine peptide and saliva but would not accompany pH-rise activity with arginine.

Effect of urea on pH, ammonia, amino acids and lactic acid in the human salivary sediment system incubated with varying levels of glucose

With concentrations of urea of 0, 0.17, 0.85 or 1.7 per cent (w/v) in salivary sediment (16.7 per cent, v/v), the concentration of glucose varied between 0 and 30 per cent (w/v). The pH of the salivary sediment mixtures remained constant. As glucose was utilized by the salivary sediment, the pH curve of this system was characterized by a rapid fall, followed by a slow rise. In the presence of urea, however, the fall in pH was considerably inhibited and an early pH rise was favoured. Glucose suppressed the formation of NH3 from endogenous sources to an extent almost proportional to its concentration. Glucose also suppressed NH3 formation when urea was present. The effect was optimum near physiologic pH range. Urea favoured the formation of alanine perhaps by transamination or by direct amination of pyruvate involving different pathways. The findings suggest that the inhibition of pH-fall was the result, not only of the interactions between acid and base produced from glucose and urea, respectively, but was largely due to the buffering effect of the products of the metabolism of urea. There appeared to be some metabolic relationship in the formation of alanine and lactate but this did not control pH changes substantially.

Continuous culture studies on the growth and physiology of Streptococcus mutans

We examined the effects of nutritional limitations on the production of lactic acid by Streptococcus mutans grown at low growth rates in continuous culture. Lactic acid production was greater in nitrogen- and phosphate-limited continuous cultures than in glucose-limited conditions. These results are correlated with the release of calcium from enamel in cellfree broths from various fermentations.