Bacterial Acid Production Research Articles

Effect of fluoride-releasing restorative materials on bacteria-induced pH fall at the bacteria–material interface: An in vitro model study

ObjectivesInhibition of bacterial acid production by dental restorative materials is one of the strategies for secondary caries prevention. This study aimed to evaluate the effect of fluoride-releasing restorative materials on bacteria-induced pH fall at the bacteria–material interface. MethodsFour fluoride-releasing restorative materials, glass-ionomer cement (GIC), resin-modified glass-ionomer cement (RMGIC), resin composite (RC) and flowable resin composite (FRC) were used. Each specimen was immersed in potassium phosphate buffer at pH 7.0 for 10min and 4 weeks, and in potassium acetate buffer at pH 5.5 for 4 weeks. An experimental apparatus was made of polymethyl methacrylate and had a well with restorative materials or polymethyl methacrylate (control) at the bottom. The well was packed with cells of Streptococcus mutans, and the pH at the interface between cells and materials was monitored using a miniature pH electrode after the addition of 1% glucose for 90min, and the fluoride released into the well was quantified using a fluoride ion electrode. ResultsThe pH of GIC (4.98–5.18), RMGIC (4.77–4.99), RC (4.62–4.75) and FRC (4.54–4.84) at 90min were higher than that of control (4.31–4.49). The fluoride amounts released from GIC were the highest, followed by RMGIC, RC and FRC, irrespective of immersion conditions. Saliva coating on materials had no significant effect. ConclusionsThe fluoride-releasing restorative materials inhibited pH fall at the bacteria–material interface. The degree of inhibition of pH fall seemed to correspond to the amount of fluoride detected, suggesting that the inhibition was due to the fluoride released from these materials. Clinical significanceA little amount of fluoride actually released from the fluoride-releasing materials may have caries preventive potential for oral bacteria.

Effects of dietary combinations of organic acids and medium chain fatty acids on the gastrointestinal microbial ecology and bacterial metabolites in the digestive tract of weaning piglets1

Organic short and medium chain fatty acids are used in diets for piglets because they have an impact on the digestive processes and the intestinal microbiota. In this study, 48 pens (2 piglets/pen) were assigned randomly to 4 diets, without additive (control), with organic acids (OA; 0.416% fumaric and 0.328% lactic acid), with medium chain fatty acids (MCFA; 0.15% caprylic and capric acid), and a combination of OA and MCFA, to assess changes in the gastrointestinal microbiota with 12 pens per diet. Eight to nine piglets from each group were euthanized after 4 wk. Organic acids, MCFA, and pH in the digesta were determined and the intestinal microbiota was quantified by real-time PCR. The different diets had no effect on the growth performance. Concentration of added fumaric acid was below the detection limit in the upper small intestine whereas the concentration of lactic acid in the digesta was not affected by the treatments. The added MCFA was recovered in the MCFA treated groups in the stomach, but the concentrations declined in the upper small intestine. Concentration of short chain fatty acids was reduced in the colon digesta in piglets fed diets with OA compared with those fed unsupplemented diets (P = 0.029). The MCFA resulted in a pH reduction of the digesta, likely because of the effect on bacterial acid production. The addition of OA increased cell counts of Bacteroides-Porphyromonas-Prevotella group and clostridial clusters XIVa, I, and IV in the stomach, the clostridial cluster XIVa in the jejunum, and Bacteroides-Porphyromonas-Prevotella in the ileum and reduced counts of Streptococcus spp. in the colon (P < 0.05). The MCFA induced only minor changes in the gastrointestinal microbiota but increased cell counts for the Escherichia-Hafnia-Shigella group in the jejunum and the clostridial cluster XIVa in the colon digesta (P < 0.05). In the colon of piglets fed diets with organic OA, reduced mean cell counts of STb (est-II) positive Escherichia coli were found. In conclusion, OA and MCFA had effects on the intestinal microecology in piglets. The decrease of the intestinal pH and the reduction of E. coli virulence genes by OA could make the combination of short chain fatty acids and MCFA as interesting gut flora modifiers, which can eventually prevent postweaning diarrhea.

Effects of different fermentation methods on bacterial cellulose and acid production by Gluconacetobacter xylinus in Cantonese-style rice vinegar

A strain of acidogenic bacterium was isolated from the fermentation liquid of Cantonese-style rice vinegar produced by traditional surface fermentation. 16S rDNA identification confirmed the bacterium as Gluconacetobacter xylinus, which synthesizes bacterial cellulose, and the acid productivity of the strain was investigated. In the study, the effects of the membrane integrity and the comparison of the air-liquid interface membrane with immerged membrane on total acidity, cellulose production, alcohol dehydrogenase (ADH) activity and number of bacteria were investigated. The cellulose membrane and the bacteria were observed under SEM for discussing their relationship. The correlations between oxygen consumption and total acid production rate were compared in surface and shake flask fermentation. The results showed the average acid productivity of the strain was 0.02g/(100mL/h), and the integrity of cellulose membrane in surface fermentation had an important effect on total acidity and cellulose production. With a higher membrane integrity, the total acidity after 144 h of fermentation was 3.75 g/100 mL, and the cellulose production was 1.71 g/100 mL after 360 h of fermentation. However, when the membrane was crushed by mechanical force, the total acidity and the cellulose production were as low as 0.36 g/100 mL and 0.14 g/100 mL, respectively. When the cellulose membrane was forced under the surface of fermentation liquid, the total acid production rate was extremely low, but the activity of ADH in the cellulose membrane was basically the same with the one above the liquid surface. The bacteria were mainly distributed in the cellulose membrane during the fermentation. The bacterial counts in surface fermentation were more than in the shake flask fermentation and G. xylinus consumed the substrate faster, in surface fermentation than in shake flask fermentation. The oxygen consumption rate and total acid production rate of surface fermentation were respectively 26.13 times and 2.92 times that of shake flask fermentation.

Divalent Cations Enhance Fluoride Binding to Streptococcus mutans and Streptococcus sanguinis Cells and Subsequently Inhibit Bacterial Acid Production

One preventive effect of topical fluoride application is derived from the fact that fluoride can inhibit bacterial acid production. Furthermore, divalent cations such as Ca²⁺ and Mg²⁺ increase the binding of fluoride to bacterial cells. These findings suggest that exposure of oral bacteria to fluoride in the presence of divalent cations increases fluoride binding to bacterial cells and subsequently enhances fluoride-induced inhibition of bacterial acid production. This study investigated the effects of fluoride exposure (0–20,000 ppm F) in the presence of Ca²⁺ or Mg²⁺ prior to glucose challenge on pH fall ability by bacterial sugar fermentation, as well as fluoride binding to bacterial cells by exposure to fluoride, and fluoride release from bacterial cells during bacterial sugar fermentation, using caries-related bacteria, Streptococcus mutans and Streptococcus sanguinis. The pH fall by both streptococci was inhibited by exposure to over 250 ppm F in the presence of Ca²⁺ (p < 0.01), whereas in the presence of Mg²⁺, the pH fall by S. mutans and S. sanguinis was inhibited after exposure to over 250 and 950 ppm F, respectively (p < 0.05). The amounts of fluoride binding to and released from streptococcal cells increased with the concentration of fluoride the cells were exposed to in the presence of Mg²⁺, but were high enough even after 250 ppm F exposure in the presence of Ca²⁺. The enhanced inhibition of acid production in the presence of divalent cations is probably due to the improved efficiency of fluoride binding to bacterial cells being improved via these divalent cations.

Characterization of Lactobacillus brevis L62 strain, highly tolerant to copper ions

Lactic acid bacteria (LAB) as starter culture in food industry must be suitable for large-scale industrial production and possess the ability to survive in unfavorable processes and storage conditions. Approaches taken to address these problems include the selection of stress-resistant strains. In food industry, LAB are often exposed to metal ions induced stress. The interactions between LAB and metal ions are very poorly investigated. Because of that, the influence of non-toxic, toxic and antioxidant metal ions (Zn, Cu, and Mn) on growth, acid production, metal ions binding capacity of wild and adapted species of Leuconostoc mesenteroides L3, Lactobacillus brevis L62 and Lactobacillus plantarum L73 were investigated. The proteomic approach was applied to clarify how the LAB cells, especially the adapted ones, protect themselves and tolerate high concentrations of toxic metal ions. Results have shown that Zn and Mn addition into MRS medium in the investigated concentrations did not have effect on the bacterial growth and acid production, while copper ions were highly toxic, especially in static conditions. Leuc. mesenteroides L3 was the most efficient in Zn binding processes among the chosen LAB species, while L. plantarum L73 accumulated the highest concentration of Mn. L. brevis L62 was the most copper resistant species. Adaptation had a positive effect on growth and acid production of all species in the presence of copper. However, the adapted species incorporated less metal ions than the wild species. The exception was adapted L. brevis L62 that accumulated high concentration of copper ions in static conditions. The obtained results showed that L. brevis L62 is highly tolerant to copper ions, which allows its use as starter culture in fermentative processes in media with high concentration of copper ions.

Metabolomic Effects of Xylitol and Fluoride on Plaque Biofilm in Vivo

Dental caries is initiated by demineralization of the tooth surface through acid production from sugar by plaque biofilm. Fluoride and xylitol have been used worldwide as caries-preventive reagents, based on in vitro-proven inhibitory mechanisms on bacterial acid production. We attempted to confirm the inhibitory mechanisms of fluoride and xylitol in vivo by performing metabolome analysis on the central carbon metabolism in supragingival plaque using the combination of capillary electrophoresis and a time-of-flight mass spectrometer. Fluoride (225 and 900 ppm F−) inhibited lactate production from 10% glucose by 34% and 46%, respectively, along with the increase in 3-phosphoglycerate and the decrease in phosphoenolpyruvate in the EMP pathway in supragingival plaque. These results confirmed that fluoride inhibited bacterial enolase in the EMP pathway and subsequently repressed acid production in vivo. In contrast, 10% xylitol had no effect on acid production and the metabolome profile in supragingival plaque, although xylitol 5-phosphate was produced. These results suggest that xylitol is not an inhibitor of plaque acid production but rather a non-fermentative sugar alcohol. Metabolome analyses of plaque biofilm can be applied for monitoring the efficacy of dietary components and medicines for plaque biofilm, leading to the development of effective plaque control.

Oxidative versus reductive succinic acid production in the yeast saccharomyces cerevisiae

Bio-based succinic acid is receiving increasing attention, as it could provide a cost-effective, ecologicallysustainable alternative to the current petrochemical production process, thus promising a significantly higher market potential. The yeastSaccharomyces cerevisiae is a robust and well-established industrial production organism exhibiting an extraordinarily high acid- and osmotolerance. These features in conjunction with the sophisticated toolbox for genetic engineering make it particularly suitable for succinic acid production. The high tolerance towards acidity is a major advantage over previously established bacterial succinic acid production hosts, since it makes the use of neutralisation salts dispensable and thus enormously facilitates the downstream process. By constructing yeast strains capable of producing significant amounts of succinic acid, we have recently established S. cerevisiae as a promising host for succinic acid production. Our metabolic engineering strategy relied on the implementation of an oxidative production route using the glyoxylate cycle. We here discuss theoretical and practical aspects of oxidative and reductive succinic acid production routes in S. cerevisiae.

Phosphorus solubilization in the rhizosphere and its possible importance to determine phosphate plant availability in soil. A review with main emphasis on German results

Limited fertilization adapted to plant demand is of high economical and ecological relevance. This requires a reliable analysis of plant available P, based on knowledge of phosphorus dynamics in soils and P mobilization by plants. On chernozem-like soils, as well as under dry conditions, the double-lactate (DL) phosphate extraction methods apparently do not adequately reflect the P uptake ability of plants. This paper summarizes rhizosphere processes that affect P availability partly by reference of selected own experiments. Root exudates increased the double-lactate (DL) extractable P amount of soils in sterile and non sterile cultures. Microbial colonisation increased both the exudate amount and the specific ability of exudates to solubilize P. In spite of rapid exudate turnover, DL-P solubility was increased. Sugars released from P-deficient plants increased the P solubilizing ability of a bacterial strain (Enterobacter radicincitans), perhaps by changing bacterial acid production. Root exudates solubilized more P from soil than lactate extracts did. An investigation of physiological processes in the rhizosphere could contribute to a better understanding of nutrient availability and perhaps lead to the development of extraction methods that better reflect the availability of soil phosphorus to plants. Connecting field experiments with basic studies offers the opportunity to better understand plant nutritional processes to realize an effective and sustainable agriculture.

Fluoride released from glass-ionomer cement is responsible to inhibit the acid production of caries-related oral streptococci

Objectives Glass-ionomer cements (GICs) are known to have inhibitory effects on bacterial growth, but the biochemical mechanism of this property has not been fully understood. This study aimed to evaluate inhibitory effects of GIC on the acid production of caries-related oral streptococci, and to identify the components responsible for the inhibition. Methods An eluate was prepared by immersing set GIC in phosphate-buffered saline at 37 °C for 24 h. Fluoride and other elements in the eluate were quantified by fluoride ion electrode and atomic absorption photometry, respectively. Streptococcus mutans NCTC 10449 and Streptococcus sanguinis NCTC 10556 were used to evaluate the pH fall and the rate of acid production after the addition of glucose in the presence or absence of the eluate. Acidic end products from glucose were also assayed by carboxylic acid analyzer. Results The eluate contained silicon (1.24 ± 0.26 mM), fluoride (0.49 ± 0.02 mM) and aluminum (0.06 ± 0.00 mM), and inhibited the pH fall and the acid production rate of both streptococci at acidic pH, with a concomitant decrease in lactic acid production. These effects were comparable to those of a potassium fluoride solution containing the same concentration of fluoride as the eluate. Significance These results indicate that the GIC eluate inhibits the acid production of caries-related oral streptococci at acidic pH and that the effect is due to fluoride derived from the GIC. Thus, adjacent to GIC fillings, bacterial acid production and the subsequent bacterial growth may decrease, establishing a cariostatic environment.

Comparative Evaluation of in-Vitro Effects of Brazilian Green Propolis and Baccharis dracunculifolia Extracts on Cariogenic Factors of Streptococcus mutans

Streptococcus mutans triggers dental caries establishment by two major factors: synthesis of organic acids, which demineralize dental enamel, and synthesis of glucans, which mediate the attachment of bacteria to the tooth surface. Propolis is a natural product that may prevent dental caries. Baccharis dracunculifolia DC (Asteraceae), a native plant from Brazil, is the most important botanical origin for the production of green propolis (Brazilian propolis) by honeybees. However, whether B. dracunculifolia (Bd) has an anticariogenic effect, like green propolis, remains unknown. Herein, we have made a comparative evaluation of the effects of extracts from green propolis and Bd on the glucan synthesis and acidogenic potential of S. mutans. The inhibitory effects of the extracts on bacterial acid production were evaluated through the potentiometric measurement of pH from bacterial suspensions treated with serial concentrations of both extracts. Besides presenting close inhibitory values at the same concentration range, Bd leaf rinse and green propolis extracts had similar IC(50) values (0.41 and 0.34 mg/ml, respectively). Both extracts produced a bacteriostatic effect on S. mutans cultures at a concentration of 0.40 mg/ml. Estimated inhibitory values of green propolis and Bd leaf rinse extracts on the synthesis of insoluble glucans (IC(50)=12.9 and 25.0 microg/ml, respectively) and soluble glucans (IC(50)=50.4 and 49.1 microg/ml, respectively) were not significantly different from each other at p<0.05. The results demonstrate that Bd leaf rinse and green propolis extracts have similar inhibitory effects on the S. mutans cariogenic factors evaluated herein, and allowed us to suggest that Bd leaves may be a potential source for pharmaceutical products employed for this purpose.

Antibacterial activities and release kinetics of a newly developed recoverable controlled agent-release system.

We attempted to develop a resin with a recoverable antibacterial activity based on the desorption/adsorption of a cationic bactericide by the ion-exchange mechanism. The aims of this study were to investigate the release kinetics of the agent and the antibacterial activity of this newly designed resin system. An experimental resin was prepared by the addition of methacrylic acid as a cation-exchanger and a cationic antibacterial agent, cetylpyridinium chloride (CPC), to triethyleneglycol dimethacrylate. The amount of CPC desorbed from the experimental resin into buffer solutions at pH 4-8 was measured. The adsorption of CPC to control resin and re-adsorption of CPC to the experimental resin, which had once desorbed the agent, were also determined. The antibacterial activity of experimental resin against Streptococcus mutans was evaluated, and the relationship between bacterial acid production and antibacterial effect was assessed. The experimental resin desorbed CPC at pH < or = 6, and the amount of agent desorbed increased with increasing acidity. The control resin adsorbed CPC when immersed in CPC aqueous solution at a rate determined by the concentration of the agent and immersion time. The experimental resin, once desorbed CPC, could re-adsorb the bactericide by being exposed to a solution of the agent. Less plaque formed on the experimental resin, and the growth and survival of S. mutans was inhibited in the condition in which acid was produced. These results demonstrate that the resin system proposed was able to desorb and re-adsorb the cationic bactericide by an ion-exchange mechanism and could show an inhibitory effect on S. mutans growth and plaque formation.

MICROBIOLOGIC ASPECTS OF DENTAL PLAQUE AND DENTAL CARIES

Dental plaque is an example of a microbial biofilm with a diverse microbial composition; it is found naturally on teeth and confers advantages to the host, for example, by preventing colonization by exogenous, and often pathogenic, micro-organisms. In individuals with a high frequency sugar diet, or with a severely compromised saliva flow, the levels of potentially cariogenic bacteria (acid-producing and acid-tolerating species) can increase beyond those compatible with enamel health. This article discusses antimicrobial strategies to control dental caries, including; reducing plaque levels, in general or specific cariogenic bacteria in particular, by antiplaque or antimicrobial agents; reducing bacterial acid production by replacing fermentable carbohydrates in the diet with sugar substitutes, or by interfering with bacterial metabolism with fluoride or antimicrobial agents.

Release of mineral ions in dental plaque following acid production

The release of appreciable amounts of calcium, phosphate and fluoride found in whole plaque into the plaque-fluid phase, following bacterial acid production, can potentially reduce the driving force for tooth demineralization. However, limited information is available on this topic, particularly on the release of fluoride. This study sought to determine the change in calcium, phosphate and fluoride concentrations in plaque fluid after sucrose exposure. 48 h overnight-fasted supragingival plaque samples were collected from all tooth surfaces (with the exception of the lower lingual anterior teeth) of one half of an individual mouth, following a 1 min water rinse. Plaque samples were then collected from the other half of the same mouth, following a 292 mM sucrose rinse. Plaque fluid was isolated by centrifugation and analysed for total calcium and phosphate (ion chromatography) and for free fluoride (ion-specific electrode). Samples were collected from seven individuals. Following sucrose exposure, plaque-fluid pH decreased significantly from 6.5±0.3 to 5.4±0.2; calcium concentrations (mmol/l) also increased significantly ( p<0.01) from 1.9±0.5 to 5.0±2.1. Fluoride and phosphate concentrations in plaque fluid, however, did not increase significantly after sucrose exposure: mean concentrations (mmol/l) of fluoride after the water and sucrose rinses were 0.006±0.003 and 0.005±0.002, respectively, and mean phosphate concentrations (mmol/l) were 11.0±2.0 and 12.0±3.0, respectively. When results were expressed per wet plaque weight, phosphate concentrations were also found to increase significantly. The same trends were observed when additional plaque samples were treated in vitro with sucrose: fluoride-ion activity did not increase in plaque under in vivo-like conditions.

Ensiling salt-preserved shrimp waste with grass straw and molasses

Two experiments were conducted to evaluate the nutritional and fermentation characteristics of ensiled salt-preserved shrimp waste. In Experiment 1, 7.5% salt-preserved shrimp waste was stored for 6 days then mixed with molasses and perennial ryegrass straw. Treatments included three levels of shrimp waste (83, 65 and 47%, as mixed) within which there were three levels of dry molasses (0, 9, or 17%, as mixed). Mixtures were stored in 15-l mini-silos for 40 days. The resulting nine shrimp silages, which ranged from 33 to 52% DM and 11 to 21% CP, never reached a pH of <4.5, which is necessary for a stable grass silage. The silages that incorporated a high amount of shrimp waste (83%) and molasses (9 or 17%) with low levels of straw had high levels of lactic acid production but only a slight drop in pH. The high buffering capacity from the calcium carbonate in the shrimp waste may have diminished the effectiveness of the acids to change the pH of the final silage. When a lower level of shrimp waste (47%) and molasses were ensiled with high levels of grass straw, the pH remained high because of low lactic acid production. The use of low quality forages or, either the amount or source of carbohydrate substrates, may have limited bacterial growth and acid production. Without a pH of <4.5, the stability and the possible presence of bacterial pathogens in our shrimp waste silages make this a questionable feed to offer ruminants. In Experiment 2, two fistulated steers were used to evaluate the ruminal undegradable protein of fresh shrimp waste and shrimp silage (47% shrimp waste, 17% molasses). The samples were deposited into nylon bags and placed in the rumen at nine time points ranging from 0 to 96 h. Prior to insertion of the samples, steers were stuffed via the fistula with crustacean waste for a 2-week adjustment period. The ruminal undegradable protein was 57% for the fresh shrimp waste and 32% for the shrimp silage.

Mechanism of inhibition of acid production in Streptococcus mutans by sodium ions under strictly anaerobic conditions

Acids excreted and intracellular levels of glycolytic intermediates during glucose metabolism in streptococcus mutans NCTC 10449 under strictly anaerobic conditions were quantified in an attempt to understand the effect of sodium ions on bacterial acid production. In the presence of NaCl (0.15-0.30 M), the total amount of individual carboxylic acids excreted was inhibited by up to 31%. The intracellular level of fructose 1,6-bisphosphate increased by 58% and levels of 3-phosphoglycerate and pyruvate decreased by 46% and 12%, respectively. Sodium ions directly inhibited the activities of fructose 1,6-phosphate aldolase and triose phosphate isomerase. This indicated that the glycolytic enzymes responsible for the catalysis of fructose 1,6-bisphosphate to 3-phosphoglycerate were inhibited. However, in spite of the expected reduction in acid production intracellularly, the intracellular pH actually decreased in the presence of sodium ions. It is possible that the low intracellular pH inhibits the activity of the glycolytic enzymes involved in the breakdown of fructose 1,6-bisphosphate to 3-phosphoglycerate.

Rumen fermentation, bacterial and total volatile fatty acid (TVFA) production rates in cattle fed on urea-molasses-mineral block licks supplement

Of twenty rumen fistulated calves, four were allocated randomly to each of five tests. Animals were offered only wheat straw ad lib (T1), or in addition were supplemented with concentrate mixture (T2), or any one of three different types of UMMB-lick type A (T3), type B (T4) and type C (T5). A metabolism trial was conducted. Bacterial production rate (BPR) and total volatile fatty acid (VFA) production rate were estimated with an isotope dilution technique using 35S sodium sulfate and 14C sodium acetate, respectively.Nitrogen (N) balance in T1 animals were negative but, positive in other treatment groups. Total-N, ammonia-N, non-protein-N, TCA-precipitable-N in rumen on treatment groups T3 to T5 were comparable with T2 but higher than T1 (P < 0.01). Ammonia levels in rumen improved significantly on UMMB supplementation. Bacterial pool size and BPR were improved three-fold (P < 0.01) in T2 to T5 in comparison with T1. BPR per unit of digestible organic matter intake was higher on UMMB-lick type B and C than type A. Total VFA concentration, production rate and molar proportion of individual VFAs were higher (P < 0.01) and acetate to propionate ratio as well as non-glucogenic ratio were lower (P < 0.01) on treatment groups T3, T4 and T5 than on T1, while comparable with T2. YATP (gram of bacteria per mole of ATP) was 5.7, 8.9, 7.4, 9.7 and 9.6 in T1 to T5, respectively. Nitrogen incorporation efficiency by rumen bacteria was lower (P < 0.01) on supplementation of UMMB-licks compared to concentrate supplement. In conclusion, it would be possible for synergy in N and energy availability from UMMB-lick supplementation to wheat straw to stimulate microbial yield and fermentation in rumen like that from conventional concentrate supplement. The composition of UMMB-lick type B and type C was found to be better than type A in improving the microbial yield.

Salivary factors in caries models.

Consideration of salivary factors in caries models rarely extends beyond viewing saliva as a sink or diluent for plaque metabolic products, or as a source of buffering, for neutralizing plaque acids. In reality, saliva has a complex chemistry and a wide range of biochemical activities that may significantly affect plaque chemistry and microbiology. Thus, saliva is a major source of microbial nutrients, without which bacterial acid production is diminished. Buffering by salivary bicarbonate, and base production from urea and basic amino acids and peptides, significantly affect Stephan curves. Saliva is supersaturated with respect to basic calcium phosphate salts and contains novel inhibitors of calcium phosphate precipitation, while specific salivary proteins bind calcium. It seems important to consider if this system is reflected in plaque. Saliva, with contributions from serum and bacterial constituents, provides most of the precursors for the acquired enamel pellicle, which acts to slow demineralization during caries attack. Pellicle constituents appear to influence initial bacterial colonization of tooth surfaces and, therefore, may affect the microbial composition of plaque, but their detailed effects on plaque are poorly understood. Microbial adaptations to the anti-bacterial systems also seem important but are poorly investigated. Thus, saliva possesses an array of activities that have substantial actual or potential impact on plaque and, therefore, merit consideration for inclusion in systems intended to model dental caries.

Effect of pH on metal solubilization from sewage sludge: a neural-net-based approach

Microbial leaching for heavy metals removal from municipal sewage sludge is a complex biological process. The ultimate metal solubilization achieved depends on the type of sludge involved (nondigested, aerobically digested or anaerobically digested sludge), the decrease in sludge pH as a result of the leaching operation, and the concentration of metals initially present in the sludge. In addition, the system temperature exerts an indirect but strong influence by its effect on the bacterial growth and acid production process. A neural-net-based model was developed to predict the solubilization of six heavy metals, Cd, Cr, Cu, Ni, Pb, and Zn, from sewage sludge using the bioleaching process. The only input parameters required are the type of sludge, initial metal concentrations in the sludge, and the sludge pH. The model yielded satisfactory predictions of metal solubilization when tested with a number of actual experimental data. Key words: heavy metals, microbial leaching, modelling, neural-net, sewage sludge.

Modeling growth and off‐flavours production of spoiled beer bacteria, Pectinatus frisingensis

The genus Pectinatus includes strictly anaerobic Gram‐negative non‐spore‐forming mesophilic bacteria often referred to as beer‐spoilage bacteria. Pectinatus frisingensis was chosen as the reference strain. The organisms were grown in batch cultures under stringent anaerobic conditions in a synthetic medium and with pH regulation. Various glucose and lactate concentrations were used, and a low inoculum reproduced spoilage conditions in bottled beer. Propionate and acetate are the major compounds responsible for the off‐flavour of beer. Gompertz curves were fitted to acid‐biomass production and glucose consumption; thereby the lag‐phase, production rate and final concentrations were derived. Volatile fatty acids production began 19 h after biomass growth. There was no lineareffect of substrate on final concentration of propionate, acetate and biomass. An additive model is proposed for the prediction of bacterial growth and acid production on both glucose and lactate.

Effects of low fluoride concentrations on formation of caries-like lesions in human enamel in a sequential-transfer bacterial system

Human enamel surfaces were exposed to sequential batch cultures of Streptococcus mutans NCTC 10832 in a sucrose-rich medium containing 0–5 mg/l added fluoride (F). In 10-day experiments, subsurface lesion formation was partly inhibited by 1 mg/l F and completely by 2 and 5 mg/l F, but small lesions formed in 2 mg/l F in 21-day experiments. Analysis of the spent media, together with analogous, bacteria-free experiments, suggested that lesion inhibition involved two main effects. First, inhibition of bacterial acid production reduced the pH fall, resulting in reduced undersaturation with respect to hydroxyapatite and consequently reduced rate of demineralization. Secondly, interaction of F with enamel mineral resulted in a small increase in reprecipitation during periods of supersaturation and a much larger reduction in demineralization during periods of undersaturation. It is concluded that, at low F concentrations, inhibition of bacterial acid production is a major factor in lesion inhibition, which may contribute significantly to caries prevention in vivo where plaque fluid F levels are raised by frequent topical applications.