Transient response of Enterobacter aerogenes under a dual nutrient limitation in a chemostat.

Utilizing a chemostat with a dual nutrient limitation of nitrogen and phosphate, we examined the transient response of the culture following a pulse of one of the limiting nutrients (ammonia). This method provided quantitative evidence that cells can be grown under dual nutrient limitation. Furthermore, the pattern of response was consistent with the hypothesis that phosphate limitation restricts nucleic acid synthesis in the cell and that nitrogen limitation restricts protein synthesis. The net result is that under a phosphate limitation there is a restricted biosynthetic capacity which we feel is closely associated with the RNA content of the cell.     

Competition between oral Streptococcus species in the chemostat under alternating conditions of glucose limitation and excess

Abstract Oral Streptococcus species experience carbohydrate limitation interrupted by periods of substrate excess following food intake by the host. To investigate the competitiveness of various streptococcal species under fluctuating carbohydrate supply, 2-membered chemostat cultures were run.
Under continuous limitation of glucose or sucrose, all 6 Streptococcus mutans test strains were outcompeted by Streptococcus sanguis P4A7 or Streptococcus milleri B448. This indicated that S. mutans had a lower affinity for glucose and sucrose than S. sanguis and S. milleri .
Mixed cultures were then subjected to hourly pulses with glucose. Under these conditions S. mutans Ny344 competed successfully with S. milleri B448, but still lost the competition against S. sanguis P4A7. The streptococci responded to pulses by taking up glucose at the maximum rate almost instantaneously. S. sanguis P4A7 had the highest rate of glucose uptake while the q _max value of S. mutans Ny344 was higher than that of S. milleri B448. This suggested a causal relationship between q _max and competitiveness.     

Species coexistence under resource competition with intraspecific and interspecific direct competition in a chemostat

Competition theory has developed separately for direct competition and for exploitative competition. However, the combined effects of the two types of competition on species coexistence remain unclear. To examine how intraspecific and interspecific direct competition contributes to the coexistence of species competing for a single resource, we constructed a chemostat-type resource competition model. With general functions for intraspecific and interspecific direct competition, we derived necessary and sufficient conditions (except for a critical case that rarely occurs in a biological sense) that determine the number of stably coexisting species. From these conditions, we found that the number of coexisting species is determined just by the invasibility of each species into subcommunities with a smaller number of species. In addition, using a combination of rigorous mathematical theory and a simple graphical method, we can demonstrate how the stronger intraspecific direct competition facilitates species invasion, leading to a larger number of coexisting species.     

Association between competition and obligate mutualism in a chemostat

In this paper, we consider a simple chemostat model involving two obligate mutualistic species feeding on a limiting substrate. Systems of differential equations are proposed as models of this association. A detailed qualitative analysis is carried out. We show the existence of a domain of coexistence, which is a set of initial conditions in which both species survive. We demonstrate, under certain supplementary assumptions, the uniqueness of the stable equilibrium point which corresponds to the coexistence of the two species.     

Association between competition and obligate mutualism in a chemostat

In this paper, we consider a simple chemostat model involving two obligate mutualistic species feeding on a limiting substrate. Systems of differential equations are proposed as models of this association. A detailed qualitative analysis is carried out. We show the existence of a domain of coexistence, which is a set of initial conditions in which both species survive. We demonstrate, under certain supplementary assumptions, the uniqueness of the stable equilibrium point which corresponds to the coexistence of the two species.     

A mathematical description of the behaviour of mixed chemostat cultures of an autotrophic nitrifier and a heterotrophic nitrifier/aerobic denitrifier; a comparison with experimental data

Abstract A general, unstructed mathematical model has been used to describe the behaviour of nutrient-limited growth of two bacteria in a continuous co-culture. The experimental system consisted of a two-membered mixed culture of the heterotrophic nitrifier/aerobic denitrifier, Thiosphaera pantotropha , and the autotrophic nitrifier, Nitrosomonas europaea , competing for ammonia in chemostat culture. The outcome of competition was only dependent on the Monod constants and the growth yields of the two bacteria. The model shows that both bacteria will oxidize equal amounts of ammonia when the cell ratio of T. pantotropha/N. europaea is 260.     

Maximization of steady-state bacterial production in a chemostat with pH and substrate control.

This analytical study deals with the steady-state behavior and control of microbial growth in continuous cultures. A second order Haldane-Monod model of continuous cultures is used as a basis for study of the effects of the adjustment of pH by the addition of acidic (or basic) materials. The treatment of a hydrogen ion concentration, in addition to substrate and microbial concentrations as state variables, results in a third order system of equations describing the process. The analysis of the system in equilibrium yields several admissible steady states, that is, steady states which satisfy all constraints. An optimal control problem is formulated and subsequently solved to maximize steady-state microbial production.     

Regulation of glucose catabolism in Thiobacillus A2 grown in the chemostat under dual limitation by succinate and glucose

In contrast to its diauxic behaviour in batch culture, Thiobacillus A2 grew in chemostat culture using glucose and succinate as dual limiting substrates. Biomass production under dual limitations was the sum of that on single substrates with each substrate being oxidized and assimilated to similar extents in single and dual substrate-limited cultures. In glucose and glucose + succinate-limited cultures glucose was oxidized largely by the Entner-Doudoroff and pentose phosphate pathways, but other mechanisms also contributed and the ratios of pathways depended on substrate ratios and the previous substrate-history of the culture. Variations in specific activities of enzymes of carbohydrate metabolism following switches from single to mixed substrates were considerable, ranging from fourfold for fructose diphosphate aldolase to more than 200-fold for hexokinase, fructose diphosphatase, glucose 6-phosphate and 6-phosphogluconate dehydrogenases. Changes in specific activities occurred only over prolonged time periods in the chemostat, probably reflecting low concentrations of free substrates in carbon-limited cultures and consequent low levels of catabolite repression.     

Coexistence of S. cerevisiae and E. coli in chemostat under substrate competition and product inhibition

A mixed culture of Saccharomyces cerevisiae and Escherichia coli was established in a stable coexistence steady state in a chemostat under constant operating conditions. The species competed for glucose, the growth-limiting resource, and produced acetate and ethanol. The acetic acid was shown to be very inhibitory to E. coli in pure culture at pH 5 while ethanol inhibition was only marginal. No significant inhibition of S. cerevisiae growth was observed by either acetate or ethanol. Pure culture parameters were measured and used in the analysis. Linearized stability analysis for the case when both organisms produce the inhibitor showed that a transition through three stable outcomes was possible as the feed concentration is lowered. Experimental studies verified these predictions, and successive transitions from a yeast growth steady state, to a coexistence steady state, and to an E. coli growth steady state were obtained by lowering the glucose concentration in the feed from 10 to 5 to 2.5 g/L, respectively. This dynamic behavior is distinct from the outcomes of other competition-inhibition combinations and experimentally demonstrates for the first time that coexistence is possible due to substrate competition and product inhibition.     

Periodic coexistence in the chemostat with three species competing for three essential resources

A chemostat model of three species of microorganisms competing for three essential, growth-limiting nutrients is considered. J. Husiman and F.J. Weissing [Nature 402 (1999) 407] show numerically that this model can generate periodic oscillations. The present contribution is concerned with rigorous analysis regarding the existence of periodic oscillations in this model. Our analysis is based on the following observation made by Huisman and Weissing: there is a cyclic replacement of species, if each species becomes limited by the resource for which it is the intermediate competitor. Using a permanence theory, an index theory, and a Poincaré-Bendixson theory for three-dimensional competitive systems, we analytically succeed to give sufficient conditions for the existence of periodic orbits in the limit sets in this model. The results in this paper suggest that with a wide range of parameter values, sustained periodic oscillations of species abundances for the model are possible, without involving external disturbances. Our results also suggest that competition is not necessarily destructive, i.e., in the case of existence of sustained periodic oscillations, if one of three competitors is absent, one of the other two rivals cannot survive.     

Asymmetric competition between two tit species: a reciprocal removal experiment

1. In a 5-year field experiment, competition for food was tested between great tit ( Parus major L.) and blue tit ( P. caeruleus L.), two common hole-nesters in Central Europe. Experimental ('allopatric') populations of both species were created during the breeding seasons by preventing the nesting or egg laying of one of the competing species.
2. An asymmetric relationship was found between the two tits; blue tits were more successful in the competitive interaction. Detectable effects were found only in nestling condition. Great tits raised lighter nestlings when breeding sympatrically with blue tits.
3. A possible mechanism is suggested that could be responsible for the different competitive abilities of the two species; blue tits are more effective in utilizing the most abundant size categories of caterpillar food supply than great tits.     

Costs of reproduction in a lizard species: a comparison of observational and experimental data

Life history theory predicts that increasing investments into reproduction compromises survival and future reproduction. However, demonstrating such costs is confounded by positive correlations between life history traits. For example, individuals in good condition may be good at both surviving and reproducing. We studied such processes in a viviparous snow skink lizard ( Niveoscincus microlepidotus ) from high elevation sites in Tasmania, Australia. Our results show a stark difference in costs of reproduction between unmanipulated females from the natural population versus experimentally manipulated females (using follicle stimulating hormones). In the unmanipulated females, females with relatively larger reproductive investments survived better than females with smaller reproductive investments. In the experimental group, however, females forced to 'over-invest' into a larger clutch survived less well than controls. Thus, our study confirms the potential dangers of non-experimental estimation of costs of reproduction.     

Interspecific competition between two species of the bean weevil

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Contributions from the Entomological Laboratory, Kyoto University, No. 110.     

Mycoplasma pneumoniae and Mycoplasma genitalium: a comparison of two closely related bacterial species

The rapid progress in sequencing large quantities of DNA will provide an increasing number of complete genome sequences of closely related bacterial species as well as of pairs of isolates from the same species with different features, such as a pathogenic and an apathogenic representative. This opens the way to apply subtractive comparative analysis as a tool to select from the large pool of all bacterial genes a relatively small set of genes that can be correlated with the expression of a certain phenotype. These selected genes can then be the target for further functional analyses.     

Dynamics of a chemostat in which two organisms compete for a common substrate

When two organisms compete for a given substrate without preying on one another, the possible steady states depend on the relative disposition of the two growth curves and the position of the point (Z,θ), whose coordinates are the nutrient feed concentration and dilution rate. It is shown how the stability of each steady state can be understood and qualitative phase portraits can be drawn for each of the 31 distinct types of situations.     

Nitrogen competition between three dominant plant species and microbes in a temperate grassland

Background and aims

To test the hypothesis that dominant plant species could acquire different nitrogen (N) forms over a spatial scale and they also have the ability to compete for available N with microbes.

Methods

A short-term ¹⁵N labeling experiment was conducted in the temperate grassland ecosystem of North China in July of 2013. Three N forms (NO₃ ^? _, NH₄ ⁺ and glycine) labeled with ¹⁵N were injected into the two soil depths (0–5 and 5–15 cm) surrounding each plant to explore N acquisition by plants and microbes. Three dominant plant species (Artemisia frigida, Cleistogenes squarrosa and Artemisia capillaris) were investigated.

Results

Two hours after ¹⁵N labeling, all three dominant plant species absorbed both organic and inorganic N, but different patterns were observed at two soil depths. Uptake of NO₃ ^? was significantly higher at 0–5 cm than at 5–15 cm soil depth among all the dominant plant species. ¹⁵N recovery by microbes was significantly higher than plants. However, ¹⁵N recovery by plants showed different patterns over soil depths.

Conclusions

Dominant plant species in the temperate grassland have different patterns in acquisition of N added to soil in organic form and absorption of inorganic N, and microbes were more effectively than plants at competing for N in a short-term period.

     

Mathematical modeling for mixed culture growth of two bacterial populations with opposite substrate preferences

An unstructured mathematical model is proposed for mixed culture growth of two different bacterial species that exhibit "opposite" substrate preferences in response to the "same" environmental conditions. The model incorporates enzymatic control mechanisms such as induction, repression, and inhibition in the microorganisms as manifested in their preferential utilization of substrates and microbial interactions such as amensalism and competition. The model predicts cell mass, substrate concentrations, dissolved oxygen tension, as well as key enzyme levels. The predictions of the model are compared with experimental data for pure culture growth and for mixed culture growth on two substrates, glucose and citrate, in a batch reactor.     

Mathematical models of microbial growth and competition in the chemostat regulated by cell-bound extracellular enzymes

A mathematical model of growth and competitive interaction of microorganisms in the chemostat is analyzed. The growth-limiting nutrient is not in a form that can be directly assimilated by the microorganisms, and must first be transformed into an intermediate product by cell-bound extracellular enzymes. General monotone functions, including Michaelis-Menten and sigmoidal response functions, are used to describe nutrient conversion and growth due to consumption of the intermediate product. It is shown that the initial concentration of the species is an important determining factor for survival or washout. When there are two species whose growth is limited by the same nutrient, three different modes of competition are described. Competitive coexistence steady states are shown to be possible in two of them, but they are always unstable. In all of our numerical simulations, the system approaches a steady state corresponding to the washout of one or both of the species from the chemostat.Research supported by NSF grant DMS-90-96279Research supported by NSERC grant A-9358     

The CANON system (Completely Autotrophic Nitrogen-removal Over Nitrite) under ammonium limitation: interaction and competition between three groups of bacteria

The CANON system (Completely Autotrophic Nitrogen Removal Over Nitrite) can potentially remove ammonium from wastewater in a single, oxygen-limited treatment step. The usefulness of CANON as an industrial process will be determined by the ability of the system to recover from major disturbances in feed composition. The CANON process relies on the stable interaction between only two bacterial populations: Nitrosomonas-like aerobic and Planctomycete-like anaerobic ammonium oxidising bacteria. The effect of extended periods of ammonium limitation was investigated at the laboratory scale in two different reactor types (sequencing batch reactor and chemostat). The lower limit of effective and stable nitrogen removal to dinitrogen gas in the CANON system was 0.1 kg N m(-3) day(-1). At this loading rate, 92% of the total nitrogen was removed. After prolonged exposure (> 1 month) to influxes lower than this critical NH4+-influx, a third population of bacteria developed in the system and affected the CANON reaction stoichiometry, resulting in a temporary decrease in nitrogen removal from 92% to 57%. The third group of bacteria were identified by activity tests and qualititative FISH (Fluorescence In Situ Hybridisation) analysis to be nitrite-oxidising Nitrobacter and Nitrospira species. The changes caused by the NH4+-limitation were completely reversible, and the system re-established itself as soon as the ammonium limitation was removed. This study showed that CANON is a robust system for ammonium removal, enduring periods of up to one month of ammonium limitation without irreversible damage.