Feeding behavior of Ceratitis capitata (Diptera,Tephritidae): influence of carbohydrate ingestion

Experiments were conducted on Ceratitis capitata larvae and adults to determine the influence of sugar (glucose and sucrose) ingestion on some aspects of the feeding behavior of this species. The results indicate that larval choice of a diet containing glucose or sucrose is not influenced by the rearing diet, by carbohydrate deprivation or by previous experience (induction). Carbohydrate deprivation did not alter the discrimination threshold of the larvae for sucrose. In adult females, the discrimination threshold for sucrose was unchanged when the flies were submitted to carbohydrate deprivation during the adult phase although ingestion by deprived females was 35% higher than ingestion by the control group. However, adults submitted to carbohydrate deprivation during the larval phase suffered a profound reduction in discrimination threshold for sucrose.     

Carbohydrate digestion in Lutzomyia longipalpis' larvae (Diptera - Psychodidae)

Lutzomyia longipalpis is the principal species of phlebotomine incriminated as vector of Leishmania infantum, the etiological agent of visceral leishmaniasis in the Americas. Despite its importance as vector, almost nothing related to the larval biology, especially about its digestive system has been published. The objective of the present study was to obtain an overview of carbohydrate digestion by the larvae. Taking in account that phlebotomine larvae live in the soil rich in decaying materials and microorganisms we searched principally for enzymes capable to hydrolyze carbohydrates present in this kind of substrate. The principal carbohydrases encountered in the midgut were partially characterized. One of them is a α-amylase present in the anterior midgut. It is probably involved with the digestion of glycogen, the reserve carbohydrate of fungi. Two other especially active enzymes were present in the posterior midgut, a membrane bound α-glucosidase and a membrane bound trehalase. The first, complete the digestion of glycogen and the other probably acts in the digestion of trehalose, a carbohydrate usually encountered in microorganisms undergoing hydric stress. In a screening done with the use of p-nitrophenyl-derived substrates other less active enzymes were also observed in the midgut. A general view of carbohydrate digestion in L. longipalpis was presented. Our results indicate that soil microorganisms appear to be the main source of nutrients for the larvae.     

Pollen feeding proteomics: Salivary proteins of the passion flower butterfly,Heliconius melpomene

While most adult Lepidoptera use flower nectar as their primary food source, butterflies in the genus Heliconius have evolved the novel ability to acquire amino acids from consuming pollen. Heliconius butterflies collect pollen on their proboscis, moisten the pollen with saliva, and use a combination of mechanical disruption and chemical degradation to release free amino acids that are subsequently re-ingested in the saliva. Little is known about the molecular mechanisms of this complex pollen feeding adaptation. Here we report an initial shotgun proteomic analysis of saliva from Heliconius melpomene. Results from liquid-chromatography tandem mass-spectrometry confidently identified 31 salivary proteins, most of which contained predicted signal peptides, consistent with extracellular secretion. Further bioinformatic annotation of these salivary proteins indicated the presence of four distinct functional classes: proteolysis (10 proteins), carbohydrate hydrolysis (5), immunity (6), and “housekeeping” (4). Additionally, six proteins could not be functionally annotated beyond containing a predicted signal sequence. The presence of several salivary proteases is consistent with previous demonstrations that Heliconius saliva has proteolytic capacity. It is likely that these proteins play a key role in generating free amino acids during pollen digestion. The identification of proteins functioning in carbohydrate hydrolysis is consistent with Heliconius butterflies consuming nectar, like other lepidopterans, as well as pollen. Immune-related proteins in saliva are also expected, given that ingestion of pathogens is a likely route to infection. The few “housekeeping” proteins are likely not true salivary proteins and reflect a modest level of contamination that occurred during saliva collection. Among the unannotated proteins were two sets of paralogs, each seemingly the result of a relatively recent tandem duplication. These results offer a first glimpse into the molecular foundation of Heliconius pollen feeding and provide a substantial advance towards comprehensively understanding this striking evolutionary novelty.     

Muscle glycogen resynthesis during recovery from cycle exercise: no effect of additional protein ingestion.

In the present study, we have investigated the effect of carbohydrate and protein hydrolysate ingestion on muscle glycogen resynthesis during 4 h of recovery from intense cycle exercise. Five volunteers were studied during recovery while they ingested, immediately after exercise, a 600-ml bolus and then every 15 min a 150-ml bolus containing 1) 1.67 g. kg body wt(-1). l(-1) of sucrose and 0.5 g. kg body wt(-1). l(-1) of a whey protein hydrolysate (CHO/protein), 2) 1.67 g. kg body wt(-1). l(-1) of sucrose (CHO), and 3) water. CHO/protein and CHO ingestion caused an increased arterial glucose concentration compared with water ingestion during 4 h of recovery. With CHO ingestion, glucose concentration was 1-1.5 mmol/l higher during the first hour of recovery compared with CHO/protein ingestion. Leg glucose uptake was initially 0.7 mmol/min with water ingestion and decreased gradually with no measurable glucose uptake observed at 3 h of recovery. Leg glucose uptake was rather constant at 0.9 mmol/min with CHO/protein and CHO ingestion, and insulin levels were stable at 70, 45, and 5 mU/l for CHO/protein, CHO, and water ingestion, respectively. Glycogen resynthesis rates were 52 +/- 7, 48 +/- 5, and 18 +/- 6 for the first 1.5 h of recovery and decreased to 30 +/- 6, 36 +/- 3, and 8 +/- 6 mmol. kg dry muscle(-1). h(-1) between 1.5 and 4 h for CHO/protein, CHO, and water ingestion, respectively. No differences could be observed between CHO/protein and CHO ingestion ingestion. It is concluded that coingestion of carbohydrate and protein, compared with ingestion of carbohydrate alone, did not increase leg glucose uptake or glycogen resynthesis rate further when carbohydrate was ingested in sufficient amounts every 15 min to induce an optimal rate of glycogen resynthesis.     

Effect of naloxone on pancreatic and gastric endocrine function in response to carbohydrate and fat-rich test meals

The present study was designed to determine the effect of naloxone, a specific opiate receptor antagonist, on postprandial levels of insulin, glucagon, pancreatic polypeptide (PP), somatostatin-like immunoreactivity (SLI) and gastrin in response to carbohydrate and fat-rich test meals in a group of 6 healthy volunteers. The addition of naloxone to a meal consisting of 50 g sucrose dissolved in 200 ml water augmented the rise of plasma insulin levels significantly during the first 30 min after its ingestion and reduced the decrease of plasma glucagon. During the ingestion of a fat-rich meal in form of 200 ml cream naloxone reduced the rise in plasma insulin and pancreatic polypeptide and elevated glucagon levels during the last 30 min of the experimental period. When sucrose was dissolved in 200 ml cream the addition of naloxone augmented the postprandial rise of insulin levels between 15 and 60 min after ingestion of the meal and elicited an increase of plasma SLI and PP levels throughout the entire experimental period which indicates that post-prandial levels of insulin, glucagon, PP and SLI are modulated via endogenous opiate receptors during the ingestion of carbohydrate and fat test meals and that this effect depends on the composition of the ingested nutrients. These data raise the possibility that endogenous opiates participate in the regulation of postprandial insulin, glucagon, somatostatin and pancreatic polypeptide release not only in certain disease states as demonstrated recently for insulin secretion in type II diabetes mellitus but endogenous opiates may also be of importance under physiological conditions.     

Diet-induced adaptation of intestinal fructose absorption in the rat.

The effect of dietary sucrose, fructose and glucose on the intestinal absorption of fructose and glucose was investigated in adult rats in vivo: Glucose absorption was not affected by the type of dietary carbohydrate, while the absorption of fructose was increased by the ingestion of the sucrose or fructose diet, as compared with the glucose diet. An almost maximal increase of fructose absorption was already observed when the quarter of the total dietary carbohydrates was replaced by fructose. Faecal fructose elimination declined during the feeding experiment. The enhanced intestinal absorption of the fructose load in rats fed the fructose diet was manifested by higher concentrations of fructose, but also of glucose and lactate in the hepatic portal blood.     

Aphid activities during sieve element punctures

Aphid salivation in sieve elements and phloem sap ingestion were linked to waveforms in the Electrical Penetration Graph (EPG). Non-viruliferousRhopalosiphum padi (L.) (Hemiptera, Aphididae) on barley yellow dwarf virus (BYDV) infected wheat could acquire the virus, which was used as an indication for phloem sap ingestion, whereas virus inoculation by viruliferous aphids on healthy plants was associated with salivation in sieve elements or other phloem cells. Probing was monitored and the waveforms recorded were related to ELISA results of test plants. The EPG patterns A, B, and C are indicative of the stylet pathway phase, whereas patterns E1 and E2 reflect the phloem (sieve element) phase with an unknown activity (E1) or with ingestion and concurrent salivation (E2). Aphids showing pathway and E1 rarely acquired virus, suggesting that little or no phloem sap ingestion can occur during these patterns, whereas those showing additionally pattern E2 did so substantially, indicating phloem sap ingestion. The main pattern related to virus inoculation was E1, although some aphids were able to inoculate plants during pathway. Pattern E1 clearly reflects the most important salivation into sieve elements. Pattern E2 had no clear contribution to virus inoculation, supporting the present hypothesis that during this pattern the saliva is mixed with the phloem sap in the single canal at the stylet tips and ingested immediately, without reaching the plant tissue. Sustained sap ingestion did not affect virus inoculation. So, BYDV inoculation mainly occurs during the first period of a sieve element puncture which is always formed by E1. Implications on persistent virus transmission are discussed.     

Gene silencing in phlebotomine sand flies: Xanthine dehydrogenase knock down by dsRNA microinjections

Lutzomyia longipalpis are vectors of medically important visceral leishmaniasis in South America. Blood-fed adult females digest large amounts of protein, and xanthine dehydrogenase is thought to be a key enzyme involved in protein catabolism through the production of urate. Large amounts of heme are also released during digestion with potentially damaging consequences, as heme can generate oxygen radicals that damage lipids, proteins and nucleic acids. However, urate is an antioxidant that may prevent such oxidative damage produced by heme. We investigated xanthine dehydrogenase by developing the RNAi technique for sand flies and used this technique to knock down the Lu. longipalpis xanthine dehydrogenase gene to evaluate its role in survival of adult females after blood feeding. The gene sequence of Lu. longipalpis xanthine dehydrogenase is described together with expression in different life cycle stages and RNAi knock down. Semi-quantitative RT-PCR of xanthine dehydrogenase expression showed a significant increase in expression after bloodmeal ingestion. Microinjection of dsRNA via the thorax of 1-day-old adult female sand flies resulted in approximately 40% reduction of xanthine dehydrogenase gene expression in comparison to flies injected with a control dsRNA. A significant reduction of urate in the whole body and excretions of Lu. longipalpis was observed after dsRNA xanthine dehydrogenase microinjection and feeding 96h later on rabbit blood. Sand flies injected with XDH dsRNA also exhibit significantly reduced life span in comparison with the mock-injected group when fed on sucrose or when rabbit blood fed, showing that urate could be indeed an important free radical scavenger in Lu. longipalpis. The demonstration of xanthine dehydrogenase knock down by dsRNA microinjection, low mortality of microinjected insects and the successful bloodfeeding of injected insects demonstrated the utility of RNAi as a tool for functional analysis of genes in phlebotomine sand flies.     

Sucrose metabolism during cotyledon development of Vicia faba L. is controlled by the concerted action of both sucrose-phosphate synthase and sucrose synthase: expression patterns, metabolic regulation and implications for seed development

The roles of sucrose-phosphate synthase (Sps) and sucrose synthase (Sus) in developing embryos of Vicia faba have been characterized. In the cotyledons the expression of both Sps and Sus is initiated in cells differentiating into storage tissue. This stage is characterized by a switch in the carbohydrate state from a high to a low hexoses to sucrose ratio. The carbohydrate state was found earlier to be controlled by seed coat-associated invertase. During cotyledon development the Sps-enzyme undergoes a cycle of deactivation and reactivation: the activated state is associated with the prestorage phase, desiccation and germination and the deactivated state with the storage phase. Sus activity is associated with the storage phase. Sps and Sus are differentially influenced by free sugars. Feeding hexoses to storage phase cotyledons increases levels of Sps-mRNA but not Sus-mRNA, Sps activity and Sps activation state and impairs storage functions evidenced by an increased sucrose to starch ratio and a downregulation of storage protein legumin B-mRNA. Sus enzyme activity is inhibited by free hexoses in vitro. It is proposed that the changing carbohydrate state during cotyledon development controls the ratio of Sps to Sus. Sps may have some significance for the initiation of the storage process possibly decreasing hexoses and/ or increasing sucrose. The relevance of the changing carbohydrate state with respect to development and storage processes is discussed.     

Effect of carbohydrate ingestion on adipose tissue lipolysis during long-lasting exercise in trained men

To study whethersucrose administration acts on lipid mobilization during prolongedexercise, we used subcutaneous abdominal adipose tissue microdialysisin eight well-trained subjects submitted at random to two 100-minexercises (50% maximal aerobic power) on separate days. After 50 minof exercise, the subjects ingested either a sucrose solution (0.75 g/kgbody wt) or water. By using a microdialysis probe, dialysate wasobtained every 10 min from the subjects at rest, during exercise, andduring a 30-min recovery period. During exercise without sucrose,plasma and dialysate glycerol increased significantly. With sucrose,the response was significantly lower for dialysate glycerol(P < 0.05). Plasma free fatty acidlevel was lower after sucrose than after water ingestion(P < 0.05). With water ingestion,plasma catecholamines increased significantly, whereas insulin fell(P < 0.05). With sucrose ingestion,the epinephrine response was blunted, whereas the insulin level wassignificantly increased. In conclusion, the use of adipose tissuemicrodialysis directly supports a lower lipid mobilization duringexercise when sucrose is supplied, which confirms that the availabilityof carbohydrate influences lipid mobilization.

     

Obesity by choice: the powerful influence of nutrient availability on nutrient intake

The consumption of nutrients by rodents is markedly influenced by the number of containers of each nutrient provided. Most rats given a choice from separate sources of protein, carbohydrate, and fat thrived if given one cup of each but half failed to thrive if given one cup of each and three extra cups of carbohydrate or fat. Rats given five bottles of sucrose solution and one bottle of water became fatter than rats given five bottles of water and one of sucrose. These studies in rats may point to a model for human obesity, in which the availability of food can override physiological controls of ingestion.     

Effect of soluble carbohydrates on digestion of cellulose by pure cultures of rumen bacteria

The rate of cellulose digestion in the presence of either glucose or cellobiose was studied for the three predominant species of cellulolytic rumen bacteria: Ruminococcus albus, Ruminococcus flavefaciens, and Bacteroides succinogenes. When a soluble carbohydrate was added to cellulose broth, the lag phase of cellulose digestion was shortened. Presumably, this was due to greater numbers of bacteria, because increasing the size of the inoculum had a similar effect. Cellulose digestion occurred simultaneously with utilization of the soluble carbohydrate. The rate of cellulose digestion slowed markedly for B. succinogenes and R. flavefaciens and slowed less for R. albus after the cellobiose or glucose had been utilized, and was accompanied by a decrease in pH. Both the rate and the extent of cellulose digestion were partially inhibited when the initial pH of the medium was 6.3 or below. R. albus appeared to be less affected by a low-pH medium than were B. succinogenes and R. flavefaciens. When a soluble carbohydrate was added to the fermentation during the maximum-rate phase of cellulose digestion, the rate of cellulose digestion was not affected until after the soluble carbohydrate had been depleted and the pH had decreased markedly. Prolonged exposure of the bacteria to a low pH had little if any effect on their subsequent ability to digest cellulose. Cellulase activity of intact bacterial cells appeared to be constitutive in nature for these three species of rumen bacteria.     

Effect of soluble carbohydrates on digestion of cellulose by pure cultures of rumen bacteria.

The rate of cellulose digestion in the presence of either glucose or cellobiose was studied for the three predominant species of cellulolytic rumen bacteria: Ruminococcus albus, Ruminococcus flavefaciens, and Bacteroides succinogenes. When a soluble carbohydrate was added to cellulose broth, the lag phase of cellulose digestion was shortened. Presumably, this was due to greater numbers of bacteria, because increasing the size of the inoculum had a similar effect. Cellulose digestion occurred simultaneously with utilization of the soluble carbohydrate. The rate of cellulose digestion slowed markedly for B. succinogenes and R. flavefaciens and slowed less for R. albus after the cellobiose or glucose had been utilized, and was accompanied by a decrease in pH. Both the rate and the extent of cellulose digestion were partially inhibited when the initial pH of the medium was 6.3 or below. R. albus appeared to be less affected by a low-pH medium than were B. succinogenes and R. flavefaciens. When a soluble carbohydrate was added to the fermentation during the maximum-rate phase of cellulose digestion, the rate of cellulose digestion was not affected until after the soluble carbohydrate had been depleted and the pH had decreased markedly. Prolonged exposure of the bacteria to a low pH had little if any effect on their subsequent ability to digest cellulose. Cellulase activity of intact bacterial cells appeared to be constitutive in nature for these three species of rumen bacteria.     

Genetically determined response to different ingested carbohydrates in the production of diabetes

It has been shown that the metabolic responses to the ingestion of carbohydrates depend upon a) the type of the ingested carbohydrate and b) the genetic build-up of the recipient. In the non-susceptible animal, the ingestion of high sucrose, fructose or glucose diets will ensue in "normal" metabolic responses. In the susceptible animal the ingestion of these same carbohydrates will result in impairment of the carbohydrate and lipid metabolism, which will ultimately lead to the development of diabetes and diabetic angiopathy while their siblings with the same genetic build-up consuming starch, will not.     

Control of postprandial hyperglycaemia by galactosyl maltobionolactone and its novel anti-amylase effect in mice

The ability to control carbohydrate digestion is useful in the treatment of diabetes mellitus and obesity. In the present study, we examined whether recently developed 4(2)-O-beta-D-galactosyl maltobionolactone (LG2O) having anti-amylase activity is able to control postprandial blood glucose concentration in mice. In addition, we tried to determine how LG2O regulates carbohydrate delivery in the gut lumen by conducting in vivo and in vitro studies. Male non-diabetic ddY mice and KK-A(y) mice, a spontaneously diabetic strain, had free access to a carbohydrate rich diet supplemented with LG2O (3 or 10 g/kg) for 0.5 hr, and blood glucose concentration was measured. LG2O suppressed any steep increase in postprandial blood glucose concentration in both ddY and KK-A(y) mice. Corresponding to the blood glucose response, LG2O also markedly suppressed any increase in postprandial plasma insulin concentration. After ingestion of the diet, LG2O produced a 1.5-3.5 fold increase in the gut contents and reducible sugar content in the small intestine but not in the stomach. Although alpha-amylase activity in the stomach was much lower compared with the activity in the small intestine, LG2O still strongly inhibited alpha-amylase activity in the stomach. In contrast, LG2O had little or no influence on alpha-amylase activity in the proximal intestine. From the in vitro carbohydrate digestion stimulation, LG2O at 7.5 mM decreased glucose production by 75% for dextrin, 25% for alpha-starch and 60% for raw starch. In conclusion, administration of LG2O inhibits carbohydrate digestion in the gut, and produces significant improvements in both blood glucose and insulin response following ingestion as part of the diet, and this evidence provides support for its therapeutic potential in treating diabetes mellitus and obesity.     

Extracellular Glycoside Hydrolase Activities in the Human Oral Cavity

Carbohydrate availability shifts when bacteria attach to a surface and form biofilm. When salivary planktonic bacteria form an oral biofilm, a variety of polysaccharides and glycoproteins are the primary carbon sources; however, simple sugar availabilities are limited due to low diffusion from saliva to biofilm. We hypothesized that bacterial glycoside hydrolase (GH) activities would be higher in a biofilm than in saliva in order to maintain metabolism in a low-sugar, high-glycoprotein environment. Salivary bacteria from 13 healthy individuals were used to grow in vitro biofilm using two separate media, one with sucrose and the other limiting carbon sources to a complex carbohydrate. All six GHs measured were higher in vitro when grown in the medium with complex carbohydrate as the sole carbon source. We then collected saliva and overnight dental plaque samples from the same individuals and measured ex vivo activities for the same six enzymes to determine how oral microbial utilization of glycoconjugates shifts between the planktonic phase in saliva and the biofilm phase in overnight dental plaque. Overall higher GH activities were observed in plaque samples, in agreement with in vitro observation. A similar pattern was observed in GH activity profiles between in vitro and ex vivo data. 16S rRNA gene analysis showed that plaque samples had a higher abundance of microorganisms with larger number of GH gene sequences. These results suggest differences in sugar catabolism between the oral bacteria located in the biofilm and those in saliva.     

EFFECT OF REPEATED INGESTION OF WATER AND SOLID SUCROSE ON THE DIFFERENTIAL SENSITIVITY TO SUCROSE AND ASPARTAME

The differential sensitivity to sucrose or Aspartame in water was studied over the course of 90 min sessions, using a sequential method. The assessors had not eaten since 9 a.m. and the sessions began at 12 noon. Two factors were studied: water ingestion and solid sucrose ingestion every 5 min. The ingestion of water had no effect on discriminative sensitivity. However, the ingestion of solid sucrose had an effect; it maintained discriminant ability in contrast to the protocol without ingestion. The reason for this effect of sucrose is not known; it might be metabolic, hedonic or perhaps something else.     

In situ activities of hexokinase and fructokinase in relation to phosphorylation status of root meristem cells of Vicia faba during reactivation from sugar starvation

The plant cell cycle is equipped with two principal control points: PCP1 in G1 and PCP2 in G2 phase. These checkpoints can arrest the cell cycle in response to carbohydrate starvation, while sugar presence can revive the replication and mitotic activity. The process of cell cycle revival is strongly repressed by okadaic acid (OA) or 6-dimethylaminopurine (6-DMAP), inhibitors of specific protein phosphatases 1 or 2A or kinases (cyclin-dependent kinases), respectively. In the present study, it was investigated whether inhibition of cell cycle revival is performed through interference of the above-mentioned inhibitors with the metabolic pathway of sucrose applied to the cells. Changes of hexokinase (HK) and fructokinase (FK) activities, key enzymes of hexose metabolism, were analyzed in Vicia faba root meristem cells arrested in G1 and G2 phase by carbohydrate starvation as well as in those recovered with glucose or sucrose in the presence of OA or 6-DMAP. It was shown that in the sugar-starved cells, the activity of both enzymes decreased significantly. During cell regeneration with carbohydrates, the activity of HK was induced more by sucrose than by glucose, while FK remained inactive after glucose addition. Moreover, in situ investigation of the activities of HK and FK showed that OA-induced and 6-DMAP-induced repression of the cell cycle revival is connected with the interference of these drugs in the metabolic pathway of sucrose. It was also indicated that stronger OA-induced and 6-DMAP-induced inhibition of the replication and mitosis revival, at the early stages of sucrose regeneration, was correlated with the stronger influence of these inhibitors on HK and FK activities.     

Biochemical changes during sucrose deprivation in higher plant cells. Phosphorus-31 nuclear magnetic resonance studies

An experimental arrangement was described that enables nuclear magnetic resonance spectra of compressed plant cells to be recorded while circulating a medium through the sample. The system provided a convenient arrangement for monitoring by 31P NMR the behavior of plant cells over a long period of time under different conditions such as sucrose starvation. Perfusion of compressed sycamore cells with sucrose-free culture medium triggered a progressive decrease in the glucose 6-P and uridine-5'-diphosphate-alpha-D-glucose resonances over 30 h. When almost all the intracellular carbohydrate pool had disappeared the nucleotide triphosphate resonances decline progressively. These changes were accompanied by a Pi accumulation in the vacuole and a phosphorylcholine (P-choline) accumulation in the cytoplasm. The very long lag phase observed for ATP and P-choline evolution was comparable with that observed for the progressive intracellular digestion of cytoplasmic constituents (Journet, E., Bligny, R. and Douce, R. (1986) J. Biol. Chem. 261, 3193-3199). Addition of sucrose in the circulating system after a long period of sucrose starvation led to a disappearance of the cytoplasmic Pi resonance and a marked increase in that of glucose 6-P. Under these conditions the vacuolar Pi pool did not fluctuate to buffer the Pi in the cytoplasm. The results suggest that Pi which has been sequestered in the vacuole during the course of sucrose starvation is not restored to the cytoplasm for rapid metabolic processes. Furthermore, the presence of P-choline in plant cells in large excess should be considered as a good marker of membrane utilization after a long period of sucrose starvation and is very likely related to stress.     

Effect of Nutrients upon Streptococcus mutans BHT and Streptococcus mitior LPA-1 Growing in Pure or Mixed Culture on Human Teeth in an Artificial Mouth

Streptococcus mutans and Streptococcus mitior were both capable of colonizing the surfaces of human teeth in an artificial mouth. Although fermentable carbohydrate was not essential for colonization, highest numbers were recovered after 5 d if 5–0°_***0 (w/v) sucrose had been available intermittently. When grown together in mixed culture the interaction of the two species was affected profoundly by the available nutrients. In the presence of synthetic saliva alone, Strep. mitior was strongly antagonistic to Strep. mutans. When a nutrient broth containing sucrose was also provided intermittently there was slight inhibition of Strep. mutans accompanied by an increase in the proportion of Strep. mitior in the plaque, although the former was the dominant organism under these conditions. When 0–5% (w/v) glucose replaced the sucrose, mutual antagonism occurred and fewer organisms were recovered than if only synthetic saliva had been available. One reason why a high-sucrose diet encourages colonization by Strep. mutans may be that insoluble extracellular polysaccharide confers a competitive advantage upon it in the face of antagonistic agents such as the hydrogen peroxide produced by Strep. mitior.