Effect of glucose concentration on carbon dioxide production in a human salivary sediment system

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.

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)

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.