Contribution of lactic and other acids to the pH of a human salivary sediment system during glucose catabolism

Identifying low pH active and lactate-utilizing taxa within oral microbiome communities from healthy children using stable isotope probing techniques

Background

Many human microbial infectious diseases including dental caries are polymicrobial in nature. How these complex multi-species communities evolve from a healthy to a diseased state is not well understood. Although many health- or disease-associated oral bacteria have been characterized in vitro, their physiology within the complex oral microbiome is difficult to determine with current approaches. In addition, about half of these species remain uncultivated to date with little known besides their 16S rRNA sequence. Lacking culture-based physiological analyses, the functional roles of uncultivated species will remain enigmatic despite their apparent disease correlation. To start addressing these knowledge gaps, we applied a combination of Magnetic Resonance Spectroscopy (MRS) with RNA and DNA based Stable Isotope Probing (SIP) to oral plaque communities from healthy children for in vitro temporal monitoring of metabolites and identification of metabolically active and inactive bacterial species.

Methodology/Principal Findings

Supragingival plaque samples from caries-free children incubated with ¹³C-substrates under imposed healthy (buffered, pH 7) and diseased states (pH 5.5 and pH 4.5) produced lactate as the dominant organic acid from glucose metabolism. Rapid lactate utilization upon glucose depletion was observed under pH 7 conditions. SIP analyses revealed a number of genera containing cultured and uncultivated taxa with metabolic capabilities at pH 5.5. The diversity of active species decreased significantly at pH 4.5 and was dominated by Lactobacillus and Propionibacterium species, both of which have been previously found within carious lesions from children.

Conclusions/Significance

Our approach allowed for identification of species that metabolize carbohydrates under different pH conditions and supports the importance of Lactobacilli and Propionibacterium in the development of childhood caries. Identification of species within healthy subjects that are active at low pH can lead to a better understanding of oral caries onset and generate appropriate targets for preventative measures in the early stages.

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.

Bacteria in human mouths involved in the production and utilization of hydrogen peroxide

Earlier studies have demonstrated that pure cultures of oral streptococci produce hydrogen peroxide but none has found any free peroxide in dental plaque or salivary sediment despite streptococci being major components of their mixed bacterial populations. The absence of peroxide in plaque and sediment could be due to the dominance of its destruction over its formation by bacterial constituents. To identify which of the oral bacteria might be involved in such a possibility, pure cultures of 27 different oral bacteria were surveyed (as well as dental plaque and sediment) for their peroxide-forming and peroxide-removing capabilities. Peroxide production was measured for each of the pure cultures by incubation with glucose at low and high substrate concentrations (2.8 and 28.0 mM) for 4 h and with the pH kept at 7.0 by a pH-stat. Removal of hydrogen peroxide was assessed in similar experiments where peroxide at 0, 29.4, 147.2 or 294.4 mM [0, 0.1, 0.5 and 1% (w/v)] replaced the glucose. Hydrogen peroxide formation was seen with only three of the bacteria tested, Streptococcus sanguis I and II (sanguis and oralis), and Strep. mitior (mitis biotype I); levels of hydrogen peroxide between 2.2 and 9.8 mM were produced when these micro-organisms were grown aerobically and 1.1 and 3.9 mM when grown anaerobically. Earlier reports indicate that such levels were usually sufficient to inhibit the growth of many plaque bacteria. The amounts formed were similar at the two glucose levels tested, suggesting that maximum peroxide production is reached at low glucose concentration. None of the three peroxide-producing organisms was able to utilize hydrogen peroxide but five of the other 24 tested, Neisseria sicca, Haemophilus segnis, H. parainfluenzae, Actinomyces viscosus and Staphylococcus epidermidis, could readily do so, as could the mixed bacteria in salivary sediment and dental plaque, both of which contain relatively high numbers of these peroxide-utilizing micro-organisms. The ability of the bacteria in plaque and sediment to degrade hydrogen peroxide was considerable and extremely rapid; peroxide removal and usually complete within the first 15 min of the incubation even when its initial level was as high as 294.4 mM. This almost overwhelming ability to remove peroxide was confirmed when peroxide-producing and -using cultures were mixed and when each of eight salivary sediments was incubated with glucose and with peroxide at concentrations up to 294.4 mM. In the glucose incubations, no hydrogen peroxide was observed, indicating dominance of microbial peroxide removers over hydrogen peroxide producers.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Arginolytic and ureolytic activities of pure cultures of human oral bacteria and their effects on the pH response of salivary sediment and dental plaque in vitro

Thirty-nine different microorganisms commonly found in supragingival plaque and salivary sediment were screened for their ability to raise the pH by producing base from arginine, lysylarginine and urea. Only Actinomyces naeslundii and Staphylococcus epidermidis showed significant pH-rise activity with all three compounds. Eleven bacteria demonstrated such activity with arginine and lysylarginine but not with urea. Only one, Actinomyces viscosus, produced a pH-rise with urea but not with the two arginine compounds. The remaining 26 bacteria showed little or no base-forming activity with any of the three test substrates. The ability of the different oral bacteria to produce base (especially from urea) was a less universal function than their ability to produce acid from fermentable carbohydrate. Substituting pure cultures of arginolytic or non-arginolytic bacteria for portions of the mixed bacterial populations of plaque or sediment in test incubations containing glucose and arginine altered their ability to produce pH-fall-pH-rise responses shaped like those of the Stephen curve in vivo. In general, addition of arginolytic bacteria made these in vitro pH responses less acidic, whereas addition of non-arginolytic bacteria made the responses more acidic. Because of the relatively high arginolytic activity of the plaque harvested in this study, the effect of adding non-arginolytic bacteria was more readily seen than the converse. Similar changes in levels of ureolytic microorganisms and incubation with glucose and urea had little effect on sediment or plaque being able to produce a pH-fall-pH-rise type of response. When increasing proportions of the mixed bacteria in salivary sediment were replaced with the highly cariogenic Lactobacillus casei or Streptococcus mutans, the pH minimum became slightly more acidic and then slightly more alkaline, whereas the pH-rise became progressively and significantly less. Thus arginolytic bacteria have a different and greater effect on shaping the pH response of plaque or sediment than ureolytic bacteria. A large change in the proportions of arginolytic or non-arginolytic microorganisms may be needed to make a plaque microflora potentially non-cariogenic or cariogenic, respectively.

In-vitro acid production by the oral bacterium Streptococcus mutans 10449 in various concentrations of glucose, fructose and sucrose

At intermediate and high concentrations, the results with the sugars were similar, with lactic acid as the main end product. Over 4 h, the pH fell from approx. 7 to 4. At low monosaccharide concentrations (2 mM glucose, 2 and 5 mM fructose), after an initial pH drop and period of lactic-acid production, evidence of pH rise and lactic-acid consumption were noted. This did not happen when sucrose was added to the bacteria. There was evidence of a heterolactic-acid fermentation pattern at low-sugar concentrations, lactic, acetic and formic acids being produced in similar amounts. The results suggest that, when low-sugar concentrations are present in dental plaque, Strep. mutans is capable of consuming previously-formed lactic acid.

The free amino acids in human dental plaque

Analysis of plaques from maxillary and mandibular incisors for free amino acids showed that the dicarboxylic amino acids, glutamic and aspartic, were present in largest amounts, with glutamic acid comprising at least 50 per cent of the total pool. Other amino acids in decreasing order of prominence included proline, ornithine, alanine, lysine, glycine, threonine and serine. This pattern was basically the same in the plaques from the different incisor sites but was clearly different from those of hydrolysates of either the plaque bacteria or the plaque matrix. The results were consistent with the most prominent plaque-free amino acids being associated mainly with the intermediary metabolism of the plaque bacteria. Urea and glucose were then applied to plaque in vivo in the form of rinses to determine if during their metabolism any of the plaque amino acids are affected. Glutamic- and aspartic-acid concentrations both rose after plaque exposure to urea accompanied by a small rise in alanine. After glucose exposure, aspartic- and glutamic-acid concentrations both showed large decreases and alanine showed a small increase. With glucose plus urea, glutamic acid rose and fell, aspartic acid decreased slightly and alanine increased several fold. In each case, the other free amino acids showed little or no change. Thus glutamic and aspartic acids are major components of the intra-cellular pool of amino acids and probably play an important role in alanine synthesis, presumably by facilitating transamination of pyruvate.

Quantitative assessment of urea, glucose and ammonia changes in human dental plaque and saliva following rinsing with urea and glucose

The rates of three processes associated with the rise and fall in plaque pH, that normally occur following a urea rinse, were determined: (i) disappearance of urea from plaque, (ii) disappearance of urea from saliva and (iii) formation and disappearance from plaque of the ammonia produced by the plaque bacteria from the urea. Also examined were two processes associated with the fall and rise in pH following a glucose rinse: the disappearance of glucose from plaque and from saliva. Entry into plaque of either urea or glucose during rinsing was immediate; the subsequent disappearance of both from the plaque was slow and followed first-order kinetics. The ammonia formation and urea-disappearance results suggested that clearance of urea from the plaque occurred mainly by bacterial degradation and not by diffusion out of the plaque. The rate constants for ammonia formation and for its subsequent disappearance from the plaque made it clear why a rapid rise and a slow subsequent fall in the pH occurs after urea rinsing. The rate constants enabled calculation of the ammonia produced as a percentage of the urea utilized. Only 16-26 per cent of the urea was recovered as ammonia and the remainder of the urea-N was stored probably as NH2 moieties of certain amino acids. Such storage may enable the plaque bacteria to maintain the pH at an elevated level for an extended period of time by bacterial production of ammonia from these stored compounds after the urea ceases to be available as a source of substrate.

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.

Effects of a single application of sodium fluoride gel on dental plaque acidogenesis

Quantitative comparisons of the lactate and acetate produced by human dental plaque in vitro and in situ were made before and after five-minute exposures to 1% NaF gel. Assays included total ionic plaque fluoride by a fluoride electrode, L(+)- and D(-)-lactate by an enzyme method, and acetate by a standard GLC procedure. A single topical application of NaF gel increased plaque fluoride about eight-fold. Six hours after gel use, plaque fluoride had declined to about 20% above pretreatment levels. Plaque fluoride baseline levels and variation between and within subjects were greater than expected. This may have been due largely to non-standardized oral hygiene practice and/or the routine use of fluoride dentifrices and the wide variation in the natural fluoride content of drinking water. Fluoride gel use significantly reduced L(+)-lactate in vitro, but D(-)-lactate and acetate were virtually unaffected. Conversely, gel use significantly inhibited the in situ production of each of these acids. The findings of this study indicate that topically applied fluoride gel impairs plaque acidogenesis to an extent that could be meaningful in preventing dental caries.