The impact of flow rate (simulated leaching) on plasmid transfer frequency between bacteria in a model rhizosphere system.

AIMS: To investigate the effect of flow rate and inoculation order on plasmid transfer frequency between bacteria in a model rhizosphere system. METHODS AND RESULTS: A physical model system was constructed and used to demonstrate that although flow rate did affect plasmid transfer frequency for an introduced strain, the flow rates necessary for a significant effect on an established population were much higher than typical water flow rates found through soil. Plasmid transfer frequency was highly sensitive to strain inoculation order. CONCLUSION: Flow rate may not have a significant effect on plasmid transfer frequency between established bacterial populations in the rhizosphere. SIGNIFICANCE AND IMPACT OF THE STUDY: This study contributes to the current debate over the release and spread of genetically modified organisms into the environment. It also demonstrates that model controlled systems may be used to rapidly obtain initial data about the potential behaviour of microorganisms, prior to more costly and lengthy glasshouse and field trials.

Substrate Concentration and Plasmid Transfer Frequency between Bacteria in a Model Rhizosphere.

The influence of substrate concentration on plasmid transfer frequency in the rhizosphere was investigated using a physical model employing a hollow fiber membrane instead of a real root. The absolute number of transconjugants produced increased with increasing substrate (glucose) concentration, but the plasmid transfer frequency decreased exponentially with increasing substrate concentration from 4.4 × 10(-3) at 90 µg ml(-1) glucose to 1.35 × 10(-5) at 3600 µg ml(-1) glucose. These results were found to be heavily dependant on donor to recipient ratio and distribution of strains, but independent of initial donor and recipient inoculum density and regime. These observations also show that plasmid transfer frequency is reduced at high substrate concentrations, which suggests that plasmid transfer is either stimulated when growth limiting nutrient is scarce or inhibited when it is abundant.

Pathway kinetics and metabolic control analysis of a high-yielding strain of Penicillium chrysogenum during fed batch cultivations.

A kinetic model representing the pathway for the biosynthesis of penicillin by P. chrysogenum has been developed. The model is capable of describing the flux through the biosynthetic pathway, and model simulations correspond well with measurements of intermediates and end products. One feature of the present model structure is that it assumes the kinetics of the enzyme isopenicillin N synthetase (IPNS) to be first order with respect to the dissolved oxygen concentration in the range of 0.070 to 0.18 mM (25% to 70% saturation with air). Thus, it indicates the importance that molecular oxygen has on the rate of the reaction catalyzed by this enzyme, and consequently as an enhancer of the specific rate of penicillin production. Using the kinetic model, metabolic control analysis (MCA) of the pathway was performed. The determined flux control coefficients suggested that, during the production phase, the flux is controlled by IPNS as this enzyme becomes saturated with tripeptide delta-(L-alpha-amino-adipyl)-L-cysteinyl-D-valine (LLD-ACV). In the simulations, oxygen was shown to be a bottleneck alleviator by stimulating the rate of IPNS which prevents the accumulation of LLD-ACV. As a consequence of this stimulation, the rate-controlling step was moved to another place in the pathway.

The relationship between cell and population growth.

A steady-state model of cell volume frequency distribution using the method of Williams (1971) is derived. Results are compared to a Monte Carlo simulation of cell growth and division. It is suggested that the Monte Carlo method might be of value for investigating cell and population properties for which analytic methods are not currently available.

Genetic transformation and cell morphology of Bacillus subtilis grown in Mg+(+)-limited chemostat culture.

The rate and frequency of genetic transformation of Bacillus subtilis grown in Mg+(+)-limited chemostat culture are dependent on the dilution rate (D) of the system and achieved maximum values at D = 0.23 h-1. Mg+(+)-limitation induced a morphological change in the cells from their normal rod shape to extended helices. Although this change in shape was a transient phenomenon, under some conditions it persisted for several days and resulted in an apparent increase in the transformation frequency.

Effect of pentoses and pentitols on fermentation of hay by mixed populations of ruminal microorganisms.

Consecutive batch culture, a technique which involves sequential transfer of cultures to fresh medium at regular intervals, was used to establish mixed ruminal-microbial populations in an anaerobic medium containing highly digestible hay. Once volatile fatty acid production was stable, perturbations were imposed in consecutive cultures by the addition of one of each of the following pentoses or analogous pentitols: l-arabinose, d-lyxose, d-ribose, d-xylose, l-arabitol, d-arabitol (lyxitol), ribitol, and xylitol. With the exception of d-lyxose, the addition of pentoses caused marked increases in propionate and valerate production, and except for d-arabitol, pentitol addition caused increases in butyrate and valerate production. On transfer to and continued incubation in the control medium, volatile fatty acid production reverted to preperturbed levels. The presence of pentitols and pentoses significantly reduced the endpoint pH of cultures and the proportion of hay that was fermented. With all added substrates, the response to the perturbation was at its maximum within one incubation (i.e., within 48 h). Similarly, the variables being monitored all returned to control levels within one incubation. On the basis of these results, it is suggested that changes were related to the need to maintain a redox balance within anaerobic cultures rather than any significant changes in the microbial population that was present.

Computer control of carbon-nitrogen ratio in Spirulina platensis.

An on-line computer was used to control the ratio of carbon to nitrogen in algal biomass. An indirect method of growth and biomass estimation was utilized. This was based on balancing the amount of CO(2) carbon in and out of the algal bioreactor. It was shown that growth conditions govern the morphology and composition of Spirulina platensis. Cells grown under light limitation were narrower, had high levels of phycocyanin pigments, and were packed full of small lipid granules. Whereas cells grown under nitrogen limitation lost their characteristic blue-green color, had reduced levels of phycocyanin, were fatter, and were packed full of larger lipid granules.

Estimating the mode of growth of individual microbial cells from cell volume distributions.

Two new methods are derived for inferring the mode of growth of individual microbial cells from measurements made of the volume distributions of populations. One is based on statistics of the observed distribution and has the particular advantage that it is very easy to use. The second, which requires gradient centrifugation, yields the mode of growth directly, rather than by comparison with theoretically derived distributions. Both methods have been found to be more sensitive than those previously suggested.

Behaviour of Dictyostelium discoideum amoebae and Escherichia coli grown together in chemostat culture.

When Dictyostelium discoideum amoebae and Escherichia coli were grown together in chemostat culture damped oscillations in the population densities of the organisms occurred followed by a sudden increase in bacterial numbers and a concommitant decrease in the number of amoebae. After the system had come to equilibrium altering the dilution rate resulted in a monotonic change in the experimental variables to new steady state levels. A square wave increase in the concentration of limiting nutrient in the feed medium during the oscillatory phase of culture produced a sinusoidal response indistinguishable from that prior to the perturbation. The results are more complicated than those predicted by simple models of microbial predator-prey dynamics although they correspond most nearly to models which incorporate saturation kinetics.