Composition and Metabolic Activities of Bacterial Biofilms Colonizing Food Residues in the Human Gut

Bacteria growing in the human large intestine live in intimate association with the host and play an important role in host digestive processes, gut physiology, and metabolism. Fecal bacteria have been investigated extensively, but few studies have been done on biofilms that form on digestive wastes in the large bowel. The aims of this investigation were to investigate the composition and metabolic activities of bacterial communities that colonize the surfaces of food residues in fecal material, with respect to their role in the fermentation of complex carbohydrates. Fresh stools were obtained from 15 healthy donors, and food residues were separated by filtration. Adherent bacteria were removed by surfactant treatment for microbiological analysis and fermentation studies. Scanning electron microscopy and fluorescent in situ hybridization in conjunction with confocal laser scanning microscopy (CLSM) were used to visualize intact biofilms. Results showed that bacterial populations strongly adhering to particulate matter were phenotypically similar in composition to unattached communities, with bacteroides and bifidobacteria predominating. Biofilms comprised a mixture of living and dead bacteria, and CLSM showed that bifidobacteria in the biofilms occurred as isolated dispersed cells and in microcolonies near the interface with the substratum. Fermentation experiments with a variety of complex carbohydrates demonstrated that biofilm populations were more efficient in digesting polysaccharides, while nonadhering communities fermented oligosaccharides most rapidly. Acetate was the principal fermentation product formed by biofilm bacteria, whereas higher levels of butyrate were produced by nonadherent populations, showing that the two communities were metabolically distinct.     

Patterns of Gut Bacterial Colonization in Three Primate Species

Host fitness is impacted by trillions of bacteria in the gastrointestinal tract that facilitate development and are inextricably tied to life history. During development, microbial colonization primes the gut metabolism and physiology, thereby setting the stage for adult nutrition and health. However, the ecological rules governing microbial succession are poorly understood. In this study, we examined the relationship between host lineage, captive diet, and life stage and gut microbiota characteristics in three primate species (infraorder, Lemuriformes). Fecal samples were collected from captive lemur mothers and their infants, from birth to weaning. Microbial DNA was extracted and the v4 region of 16S rDNA was sequenced on the Illumina platform using protocols from the Earth Microbiome Project. Here, we show that colonization proceeds along different successional trajectories in developing infants from species with differing dietary regimes and ecological profiles: frugivorous (fruit-eating) Varecia variegata, generalist Lemur catta, and folivorous (leaf-eating) Propithecus coquereli. Our analyses reveal community membership and succession patterns consistent with previous studies of human infants, suggesting that lemurs may serve as a useful model of microbial ecology in the primate gut. Each lemur species exhibits distinct species-specific bacterial diversity signatures correlating to life stages and life history traits, implying that gut microbial community assembly primes developing infants at species-specific rates for their respective adult feeding strategies.     

Context-Dependent Competition in a Model Gut Bacterial Community

Understanding the ecological processes that generate complex community structures may provide insight into the establishment and maintenance of a normal microbial community in the human gastrointestinal tract, yet very little is known about how biotic interactions influence community dynamics in this system. Here, we use natural strains of Escherichia coli and a simplified model microbiota to demonstrate that the colonization process on the strain level can be context dependent, in the sense that the outcome of intra-specific competition may be determined by the composition of the background community. These results are consistent with previous models for competition between organisms where one competitor has adapted to low resource environments whereas the other is optimized for rapid reproduction when resources are abundant. The genomic profiles of E. coli strains representing these differing ecological strategies provide clues for deciphering the genetic underpinnings of niche adaptation within a single species. Our findings extend the role of ecological theory in understanding microbial systems and the conceptual toolbox for describing microbial community dynamics. There are few, if any, concrete examples of context-dependent competition on a single trophic level. However, this phenomenon can have potentially dramatic effects on which bacteria will successfully establish and persist in the gastrointestinal system, and the principle should be equally applicable to other microbial ecosystems.     

Metal Toxicity Affects Fungal and Bacterial Activities in Soil Differently

Although the toxic effect of heavy metals on soil microorganism activity is well known, little is known about the effects on different organism groups. The influence of heavy metal addition on total, bacterial, and fungal activities was therefore studied for up to 60 days in a laboratory experiment using forest soil contaminated with different concentrations of Zn or Cu. The effects of the metals differed between the different activity measurements. During the first week after metal addition, the total activity (respiration rate) decreased by 30% at the highest level of contamination and then remained stable during the 60 days of incubation. The bacterial activity (thymidine incorporation rate) decreased during the first days with the level of metal contamination, resulting in a 90% decrease at the highest level of contamination. Bacterial activity then slowly recovered to values similar to those of the control soil. The recovery was faster when soil pH, which had decreased due to metal addition, was restored to control values by liming. Fungal activity (acetate-in-ergosterol incorporation rate) initially increased with the level of metal contamination, being up to 3 and 7 times higher than that in the control samples during the first week at the highest levels of Zn and Cu addition, respectively. The positive effect of metal addition on fungal activity then decreased, but fungal activity was still higher in contaminated than in control soil after 35 days. This is the first direct evidence that fungal and bacterial activities in soil are differently affected by heavy metals. The different responses of bacteria and fungi to heavy metals were reflected in an increase in the relative fungal/bacterial ratio (estimated using phospholipid fatty acid analysis) with increased metal load.     

Estimation of Nitrifying Bacterial Activities by Measuring Oxygen Uptake in the Presence of the Metabolic Inhibitors Allylthiourea and Azide

The effects of two metabolic inhibitors on an enriched nitrifying biomass during incubation for short periods of time were investigated by determining respirometric measurements. Allylthiourea (86 μM) and azide (24 μM) were shown to be strong, selective inhibitors of ammonia and nitrite oxidation, respectively. Consequently, a differential respirometry method for estimating nitrifying and heterotrophic bacterial activities within a mixed biomass is proposed.     

Low-Dose Aspartame Consumption Differentially Affects Gut Microbiota-Host Metabolic Interactions in the Diet-Induced Obese Rat

Aspartame consumption is implicated in the development of obesity and metabolic disease despite the intention of limiting caloric intake. The mechanisms responsible for this association remain unclear, but may involve circulating metabolites and the gut microbiota. Aims were to examine the impact of chronic low-dose aspartame consumption on anthropometric, metabolic and microbial parameters in a diet-induced obese model. Male Sprague-Dawley rats were randomized into a standard chow diet (CH, 12% kcal fat) or high fat (HF, 60% kcal fat) and further into ad libitum water control (W) or low-dose aspartame (A, 5–7 mg/kg/d in drinking water) treatments for 8 week (n = 10–12 animals/treatment). Animals on aspartame consumed fewer calories, gained less weight and had a more favorable body composition when challenged with HF compared to animals consuming water. Despite this, aspartame elevated fasting glucose levels and an insulin tolerance test showed aspartame to impair insulin-stimulated glucose disposal in both CH and HF, independently of body composition. Fecal analysis of gut bacterial composition showed aspartame to increase total bacteria, the abundance of Enterobacteriaceae and Clostridium leptum. An interaction between HF and aspartame was also observed for Roseburia ssp wherein HF-A was higher than HF-W (P<0.05). Within HF, aspartame attenuated the typical HF-induced increase in the Firmicutes:Bacteroidetes ratio. Serum metabolomics analysis revealed aspartame to be rapidly metabolized and to be associated with elevations in the short chain fatty acid propionate, a bacterial end product and highly gluconeogenic substrate, potentially explaining its negative affects on insulin tolerance. How aspartame influences gut microbial composition and the implications of these changes on the development of metabolic disease require further investigation.     

A Model for Bacterial Colonization of Sinking Aggregates

Sinking aggregates provide important nutrient-rich environments for marine bacteria. Quantifying the rate at which motile bacteria colonize such aggregations is important in understanding the microbial loop in the pelagic food web. In this paper, a simple analytical model is presented to predict the rate at which bacteria undergoing a random walk encounter a sinking aggregate. The model incorporates the flow field generated by the sinking aggregate, the swimming behavior of the bacteria, and the interaction of the flow with the swimming behavior. An expression for the encounter rate is computed in the limit of large Péclet number when the random walk can be approximated by a diffusion process. Comparison with an individual-based numerical simulation is also given.     

The Cockroach Origin of the Termite Gut Microbiota: Patterns in Bacterial Community Structure Reflect Major Evolutionary Events

Termites digest wood and other lignocellulosic substrates with the help of their intestinal microbiota. While the functions of the symbionts in the digestive process are slowly emerging, the origin of the bacteria colonizing the hindgut bioreactor is entirely unknown. Recently, our group discovered numerous representatives of bacterial lineages specific to termite guts in a closely related omnivorous cockroach, but it remains unclear whether they derive from the microbiota of a common ancestor or were independently selected by the gut environment. Here, we studied the bacterial gut microbiota in 34 species of termites and cockroaches using pyrotag analysis of the 16S rRNA genes. Although the community structures differed greatly between the major host groups, with dramatic changes in the relative abundances of particular bacterial taxa, we found that the majority of sequence reads belonged to bacterial lineages that were shared among most host species. When mapped onto the host tree, the changes in community structure coincided with major events in termite evolution, such as acquisition and loss of cellulolytic protists and the ensuing dietary diversification. UniFrac analysis of the core microbiota of termites and cockroaches and construction of phylogenetic tree of individual genus level lineages revealed a general host signal, whereas the branching order often did not match the detailed phylogeny of the host. It remains unclear whether the lineages in question have been associated with the ancestral cockroach since the early Cretaceous (cospeciation) or are diet-specific lineages that were independently acquired from the environment (host selection).     

Expression of Human Paraoxonase 1 Decreases Superoxide Levels and Alters Bacterial Colonization in the Gut of Drosophila melanogaster

Paraoxonases (PON) are a family of proteins (PON1, 2 and 3) with multiple enzymatic activities. PON1 interferes with homoserine lactone-mediated quorum sensing in bacteria and with reactive oxygen species (ROS) in humans and mice. PON1 gene mutations have been linked to multiple traits, including aging, and diseases of the cardiovascular, nervous and gastrointestinal system. The overlapping enzymatic activities in the PON family members and high linkage disequilibrium rates within their polymorphisms confound animal and human studies of PON1 function. In contrast, arthropods such as Drosophila melanogaster have no PON homologs, resulting in an ideal model to study interactions between PON genotype and host phenotypes. We hypothesized that expression of PON1 in D. melanogaster would alter ROS. We found that PON1 alters expression of multiple oxidative stress genes and decreases superoxide anion levels in normal and germ-free D. melanogaster. We also found differences in the composition of the gut microbiota, with a remarkable increase in levels of Lactobacillus plantarum and associated changes in expression of antimicrobial and cuticle-related genes. PON1 expression directly decreased superoxide anion levels and altered bacterial colonization of the gut and its gene expression profile, highlighting the complex nature of the interaction between host genotype and gut microbiota. We speculate that the interaction between some genotypes and human diseases may be mediated by the presence of certain gut bacteria that can induce specific immune responses in the gut and other host tissues.     

Inherited Fungal and Bacterial Endosymbionts of a Parasitic Wasp and Its Cockroach Host

Bacterial endosymbionts of insects are increasingly being recognized as common, diverse, and integral to the biology of their hosts. Inherited fungal symbionts have been largely overlooked, however, even though insect guts appear to be a key habitat for an incredible array of fungal diversity. Like bacteria, fungal symbionts also likely play important roles in the ecology and evolution of their insect associates. The objective of this study was to lay the foundations for understanding the roles of the vertically transmitted fungal and bacterial associates of both the brownbanded cockroach, Supella longipalpa, and its parasitic wasp, Comperia merceti. We used culture-dependent and culture-independent molecular methods and phylogenetic analyses in order to identify the symbionts. Two fungal associates of brownbanded cockroaches were found. To our knowledge, this is the first record of vertically transmitted fungal symbionts in the order Blattaria. The wasp was found to house a close relative of one of the cockroach fungi but no bacterial symbionts. Finally, the brownbanded cockroaches also harbored three lineages of bacterial symbionts: Blattabacterium and two lineages of Wolbachia, indicating the number of vertically transmitted symbionts in this insect may be as many as five.     

基于肠道微生态探讨黄连素在代谢性疾病中的抗炎作用

肠道菌群与能量代谢密切相关,其组成和代谢紊乱可通过多种途径导致胰岛素抵抗,肥胖和2型糖尿病。黄连素因具有减重、降糖、调脂等作用被广泛用于肥胖、2型糖尿病及非酒精性脂肪性肝病等代谢性疾病的辅助治疗;研究表明,黄连素可调节肠道菌群的组成和代谢,改善肠道微生态环境,从而改善胰岛素抵抗和代谢。本文综述了黄连素通过肠道菌群-炎症轴在干预代谢性疾病的研究进展,以期为代谢性疾病的治疗寻找新的策略,并为今后该领域的深入研究提供指导意义。     

A Thick-Walled Organism Isolated from the Cockroach Gut by Using a Spent Medium Technique

By using a conditioning technique whereby complex media are inoculated several times with bacteria from the hindgut of the cockroach Eublaberus posticus, a succession of bacterial types occurred. An obligately anaerobic, pleomorphic, thick-walled, gram-positive organism is described which was isolated by this culturing technique.     

Fungal and Bacterial Colonization of Submerged Leaf Litter in a Mediterranean Stream

Microbial colonization dynamics of fungi and bacteria were analyzed in an intermittent Mediterranean forested stream using two different leaf substrata (Platanus acerifolia and Populus nigra). Results showed that fungal and bacterial biomass accumulation was stimulated on both leaves due to a flooding episode that increased dissolved inorganic nitrogen (DIN) and dissolved oxygen (DO) availability in the stream water. Leaf mass loss coincided with the parallel increase in microbial biomass and extracellular enzymatic activities after the flood event. Differences in litter quality favoured bacterial biomass accumulation and β‐glucosidase and cellobiohydrolase enzymatic activities in the soft Populus species. Microbial heterotrophs colonization of submerged leaf litter and organic matter use in Mediterranean‐type streams are modulated by environmental conditions, especially the hydrological variability. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Metabolic Proximity in the Order of Colonization of a Microbial Community

Microbial biofilms are often composed of multiple bacterial species that accumulate by adhering to a surface and to each other. Biofilms can be resistant to antibiotics and physical stresses, posing unresolved challenges in the fight against infectious diseases. It has been suggested that early colonizers of certain biofilms could cause local environmental changes, favoring the aggregation of subsequent organisms. Here we ask whether the enzyme content of different microbes in a well-characterized dental biofilm can be used to predict their order of colonization. We define a metabolic distance between different species, based on the overlap in their enzyme content. We next use this metric to quantify the average metabolic distance between neighboring organisms in the biofilm. We find that this distance is significantly smaller than the one observed for a random choice of prokaryotes, probably reflecting the environmental constraints on metabolic function of the community. More surprisingly, this metabolic metric is able to discriminate between observed and randomized orders of colonization of the biofilm, with the observed orders displaying smaller metabolic distance than randomized ones. By complementing these results with the analysis of individual vs. joint metabolic networks, we find that the tendency towards minimal metabolic distance may be counter-balanced by a propensity to pair organisms with maximal joint potential for synergistic interactions. The trade-off between these two tendencies may create a “sweet spot” of optimal inter-organism distance, with possible broad implications for our understanding of microbial community organization.     

The Colonization Dynamics of the Gut Microbiota in Tilapia Larvae

The gut microbiota of fish larvae evolves fast towards a complex community. Both host and environment affect the development of the gut microbiota; however, the relative importance of both is poorly understood. Determining specific changes in gut microbial populations in response to a change in an environmental factor is very complicated. Interactions between factors are difficult to separate and any response could be masked due to high inter-individual variation even for individuals that share a common environment. In this study we characterized and quantified the spatio-temporal variation in the gut microbiota of tilapia larvae, reared in recirculating aquaculture systems (RAS) or active suspension tanks (AS). Our results showed that variation in gut microbiota between replicate tanks was not significantly higher than within tank variation, suggesting that there is no tank effect on water and gut microbiota. However, when individuals were reared in replicate RAS, gut microbiota differed significantly. The highest variation was observed between individuals reared in different types of system (RAS vs. AS). Our data suggest that under experimental conditions in which the roles of deterministic and stochastic factors have not been precisely determined, compositional replication of the microbial communities of an ecosystem is not predictable.     

Probiotic Lactobacillus rhamnosus GG Enhanced Th1 Cellular Immunity but Did Not Affect Antibody Responses in a Human Gut Microbiota Transplanted Neonatal Gnotobiotic Pig Model

This study aims to establish a human gut microbiota (HGM) transplanted gnotobiotic (Gn) pig model of human rotavirus (HRV) infection and diarrhea, and to verify the dose-effects of probiotics on HRV vaccine-induced immune responses. Our previous studies using the Gn pig model found that probiotics dose-dependently regulated both T cell and B cell immune responses induced by rotavirus vaccines. We generated the HGM transplanted neonatal Gn pigs through daily feeding of neonatal human fecal suspension to germ-free pigs for 3 days starting at 12 hours after birth. We found that attenuated HRV (AttHRV) vaccination conferred similar overall protection against rotavirus diarrhea and virus shedding in Gn pigs and HGM transplanted Gn pigs. HGM promoted the development of the neonatal immune system, as evidenced by the significantly enhanced IFN-γ producing T cell responses and reduction of regulatory T cells and their cytokine production in the AttHRV-vaccinated pigs. The higher dose Lactobacillus rhamnosus GG (LGG) feeding (14 doses, up to 10⁹ colony-forming-unit [CFU]/dose) effectively increased the LGG counts in the HGM Gn pig intestinal contents and significantly enhanced HRV-specific IFN-γ producing T cell responses to the AttHRV vaccine. Lower dose LGG (9 doses, up to 10⁶ CFU/dose) was ineffective. Neither doses of LGG significantly improved the protection rate, HRV-specific IgA and IgG antibody titers in serum, or IgA antibody titers in intestinal contents compared to the AttHRV vaccine alone, suggesting that an even higher dose of LGG is needed to overcome the influence of the microbiota to achieve the immunostimulatory effect in the HGM pigs. This study demonstrated that HGM Gn pig is an applicable animal model for studying immune responses to rotavirus vaccines and can be used for studying interventions (i.e., probiotics and prebiotics) that may enhance the immunogenicity and protective efficacy of vaccines through improving the gut microbiota.     

Diet-Induced Metabolic Improvements in a Hamster Model of Hypercholesterolemia Are Strongly Linked to Alterations of the Gut Microbiota

The mammalian gastrointestinal microbiota exerts a strong influence on host lipid and cholesterol metabolism. In this study, we have characterized the interplay among diet, gut microbial ecology, and cholesterol metabolism in a hamster model of hypercholesterolemia. Previous work in this model had shown that grain sorghum lipid extract (GSL) included in the diet significantly improved the high-density lipoprotein (HDL)/non-HDL cholesterol equilibrium (T. P. Carr, C. L. Weller, V. L. Schlegel, S. L. Cuppett, D. M. Guderian, Jr., and K. R. Johnson, J. Nutr. 135:2236-2240, 2005). Molecular analysis of the hamsters'' fecal bacterial populations by pyrosequencing of 16S rRNA tags, PCR-denaturing gradient gel electrophoresis, and Bifidobacterium-specific quantitative real-time PCR revealed that the improvements in cholesterol homeostasis induced through feeding the hamsters GSL were strongly associated with alterations of the gut microbiota. Bifidobacteria, which significantly increased in abundance in hamsters fed GSL, showed a strong positive association with HDL plasma cholesterol levels (r = 0.75; P = 0.001). The proportion of members of the family Coriobacteriaceae decreased when the hamsters were fed GSL and showed a high positive association with non-HDL plasma cholesterol levels (r = 0.84; P = 0.0002). These correlations were more significant than those between daily GSL intake and animal metabolic markers, implying that the dietary effects on host cholesterol metabolism are conferred, at least in part, through an effect on the gut microbiota. This study provides evidence that modulation of the gut microbiota-host metabolic interrelationship by dietary intervention has the potential to improve mammalian cholesterol homeostasis, which has relevance for cardiovascular health.The mammalian gut microbiota interacts intimately with its host, affecting both host metabolic and immunological phenotypes with important consequences for health (18, 22, 32). Recent studies have revealed complex linkages between the gut microbiome and host metabolism, with the microbes exerting effects on the energy balance by influencing glucose and lipid metabolism (2, 7, 28). This intimate metabolic relationship is most likely the consequence of a long coevolutionary process that resulted in a mutualistic relationship between the host and its microbial partners (25). However, life in industrialized societies has introduced profound changes into the human environment (e.g., diet, antibiotics, hospital deliveries, hygiene, etc.) that are markedly different from the conditions to which humans have evolved and that are likely to have occurred too abruptly for the human microbiome to adjust. Consequently, aberrations of the gut microbiota induced through lifestyle factors could be relevant to the etiology of several complex human diseases whose occurrence has markedly increased in developed countries. Interestingly, imbalances in the gut microbiota have been reported for obesity, type 1 and 2 diabetes, some allergies, and inflammatory bowel diseases in humans and animal models (7, 24, 43, 45, 48). The connection between gut bacteria and disease suggests an intriguing paradigm on how to view and potentially treat complex diseases. Specific bacterial populations in the intestine could be pharmaceutical targets to maintain or restore metabolic functions (6, 17).Coronary heart disease (CHD) continues to be a major cause of death in developed countries and is another example of a “western disease” that is less common in underdeveloped countries but increases in frequency with adoption of western customs (4). Most risk factors for CHD (obesity, high blood pressure, type 2 diabetes, heredity, high cholesterol, and diet) have been linked to the gut microbiota (7, 17, 20, 30, 45), and gut bacteria have been suggested to play a role in the etiology of cardiovascular disease (16, 33). Cholesterol metabolism is a key factor in susceptibility to CHD, and as early as 1959, it has been shown that germfree rats have higher serum cholesterol concentrations than their conventional counterparts do (12). Several mechanisms have been proposed by which gut bacteria could influence host cholesterol metabolism (13). Bacterial conversions of bile acids (such as the formation of secondary bile acids) are likely to play a role, as they affect enterohepatic circulation, de novo synthesis of bile acids, emulsification, and cholesterol absorption (10, 28, 30). A further mechanism by which gut bacteria might influence cholesterol metabolism is through Fiaf (fasting-induced adipocyte factor), which is selectively suppressed in the intestinal epithelium by the gut microbiota (1, 2). Fiaf is an important regulator of lipid metabolism (e.g., through its inhibition of lipoprotein lipase) and has been shown to increase total cholesterol and high-density lipoprotein (HDL) cholesterol levels when overexpressed in transgenic mice (26).There are several reasons why hamsters are an excellent model for studying the metabolic relationships among diet, cholesterol metabolism, and gut microbiota in relation to health. First, hamsters are omnivorous, and their blood lipid profiles respond to diets in a predictive manner similar to humans (5). Second, unlike mice and rats which lack cholesterol ester transfer protein, hamsters exhibit all of the enzymatic pathways in lipoprotein and bile metabolism that are also present in humans. They exhibit limited hepatic synthesis of cholesterol and bile acids, resulting in more relevant data when extrapolating to humans (23). Third, hamsters develop atherosclerosis in a predictive manner in response to dietary manipulation (31).Using the Golden Syrian hamster model, Carr and coworkers have shown that the hexane-extractable lipid fraction of grain sorghum whole kernels (GSL), when included in the hamsters'' diet, leads to a significant reduction of plasma non-HDL and liver cholesterol levels while increasing HDL cholesterol levels (8). We extended this research and performed a comprehensive molecular characterization of the fecal microbiota of the hamsters by pyrosequencing of 16S rRNA tags, denaturing gradient gel electrophoresis (DGGE), and Bifidobacterium specific quantitative real-time PCR (qRT-PCR) in order to test whether metabolic effects of GSL were associated with specific modifications of the gut microbiota.     

Bacterial Colonization of Particles: Growth and Interactions

Marine particles in the ocean are exposed to diverse bacterial communities, and colonization and growth of attached bacteria are important processes in the degradation and transformation of the particles. In an earlier study, we showed that the initial colonization of model particles by individual bacterial strains isolated from marine aggregates was a function of attachment and detachment. In the present study, we have investigated how this colonization process was further affected by growth and interspecific interactions among the bacteria. Long-term incubation experiments showed that growth dominated over attachment and detachment after a few hours in controlling the bacterial population density on agar particles. In the absence of grazing mortality, this growth led to an equilibrium population density consistent with the theoretical limit due to oxygen diffusion. Interspecific interaction experiments showed that the presence of some bacterial strains (“residents”) on the agar particles either increased or decreased the colonization rate of other strains (“newcomers”). Comparison between an antibiotic-producing strain and its antibiotic-free mutant showed no inhibitory effect on the newcomers due to antibiotic production. On the contrary, hydrolytic activity of the antibiotic-producing strain appeared to benefit the newcomers and enhance their colonization rate. These results show that growth- and species-specific interactions have to be taken into account to adequately describe bacterial colonization of marine particles. Changes in colonization pattern due to such small-scale processes may have profound effects on the transformation and fluxes of particulate matter in the ocean.     

Oxalobacter formigenes Colonization and Oxalate Dynamics in a Mouse Model

Animal and human studies have provided compelling evidence that colonization of the intestine with Oxalobacter formigenes reduces urinary oxalate excretion and lowers the risk of forming calcium oxalate kidney stones. The mechanism providing protection appears to be related to the unique ability of O. formigenes to rely on oxalate as a major source of carbon and energy for growth. However, much is not known about the factors that influence colonization and host-bacterium interactions. We have colonized mice with O. formigenes OxCC13 and systematically investigated the impacts of diets with different levels of calcium and oxalate on O. formigenes intestinal densities and urinary and intestinal oxalate levels. Measurement of intestinal oxalate levels in mice colonized or not colonized with O. formigenes demonstrated the highly efficient degradation of soluble oxalate by O. formigenes relative to other microbiota. The ratio of calcium to oxalate in diets was important in determining colonization densities and conditions where urinary oxalate and fecal oxalate excretion were modified, and the results were consistent with those from studies we have performed with colonized and noncolonized humans. The use of low-oxalate purified diets showed that 80% of animals retained O. formigenes colonization after a 1-week dietary oxalate deprivation. Animals not colonized with O. formigenes excreted two times more oxalate in feces than they had ingested. This nondietary source of oxalate may play an important role in the survival of O. formigenes during periods of dietary oxalate deprivation. These studies suggest that the mouse will be a useful model to further characterize interactions between O. formigenes and the host and factors that impact colonization.