The dynamics of the macromolecular composition of biomass.

The biomass composition of microorganisms depends on the growth conditions. This study explores whether a two-component model can explain how the elemental and macromolecular composition of the biomass of bacteria varies with the specific growth rate. The model describes the rates at which microorganisms assimilate substrates into reserves and utilize reserves for maintenance and growth. Crucial model assumptions are that biomass consists of reserves and structure and that each of these components has an invariant composition. The composition of biomass can vary when the ratio between reserves and structure varies. Literature data on the macromolecular composition of Escherichia coli, cultivated on various substrates, show that the protein, RNA and DNA content of biomass follow a distinctive trend when plotted as a function of the dry-weight-specific growth rate. This observation leads to the proposition that the macromolecular composition of E. coli depends directly on the growth rate, and only indirectly on the carbon- and energy-source used as substrate. We show that the variation of the macromolecular composition of E. coli over its entire range of growth rates can be described with invariant macromolecular compositions of the reserve and structural components of biomass. The model is also applied to our data on a succinate-limited continuous culture of Paracoccus denitrificans.

Survival and production in variable resource environments.

A dynamic energy budget (DEB) model describes the rates at which organisms assimilate and utilize energy from food for maintenance, growth, reproduction and development. We study the dynamic behavior of one particular DEB model, Kooijman's kappa-rule model, whose key assumption is that somatic and reproductive tissues are competing for energy. We assume an environment in which the food density fluctuates either periodically or stochastically (pink noise). Both types of fluctuations stimulate growth; the magnitude of the (average) increase in size depends on both the strength and duration of the fluctuations. In a stochastic environment, the risk of mortality due to starvation increases with increasing fluctuation intensity. The mean lifespan is also a function of the model parameter kappa characterizing the partitioning of energy between somatic and reproductive tissues. Organisms committing a large fraction of resources to reproduction endure periods of food shortage relatively well. The effects of food fluctuations on reproduction are complex. With stochastic food, reproduction in survivors increases with increasing fluctuation intensities, but lifetime reproduction decreases. Periodic fluctuations may enhance reproduction, depending on the value of kappa. Thus, a variable food supply stimulates growth, increases mortality and may enhance reproduction, depending on life history.

Simultaneous NH3 oxidation and N2 production at reduced O2 tensions by sewage sludge subcultured with chemolithotrophic medium.

The ammonia oxidation rate by sewage sludge was determined as a function of the dissolved oxygen tension. Samples of sludge were taken from a domestic waste water treatment pilot plant in which sludge was completely retained by membrane filtration. The samples were subcultured chemolithotrophically in recycling reactors. The gas supplied was a mixture of pure argon and oxygen. The KO2 for ammonia oxidation was estimated to be 0.97 (+/- 0.16) kPa dissolved oxygen. Together with ammonia oxidation and oxygen consumption, dinitrogen gas was produced. So, aerobic denitrification occurred. At dissolved oxygen tensions of 1.25 kPa and higher, the dinitrogen production rate (per N-mole) equalled 20% of the ammonia oxidation rate. This proportion was even 58% at 0.3 kPa dissolved oxygen. At 0.15 kPa dissolved oxygen, however, nitrification hardly proceeded, while dinitrogen production soon stopped. Most likely, a nitrifier concomitantly oxidized ammonia and reduced nitrite to dinitrogen.

Progression of scoliosis in children with myelomeningocele.

In a retrospective follow-up of 64 patients with myelomeningocele and scoliosis of more than 10 degrees, the development of the scoliosis curve was watched. The mean follow-up was 4.3 years. Most progression of scoliosis can be expected before 15 years of age. Fifty-four percent of the patients with scoliosis of 40 degrees or more progressed more than 5 degrees per year. Progression was dependent on the scoliosis angle. A multivariate model for the prediction of the scoliosis progress in a 1-year perspective was applied. The model included the current scoliosis angle, the age of the patient, the skeletal level of the dysraphism, and the patient's ambulation capacity.

Brace treatment of scoliosis in children with myelomeningocele.

In twenty-one children with myelomeningocele and progressive scoliosis, treatment of scoliosis was attempted with a Boston type underarm brace. Thirteen children finished full brace treatment (average treatment time 2.5 years) and the patients were included in a follow-up more than 2 years after the end of treatment with no further progression of the scoliosis. Two patients are still undergoing brace treatment. Six children were operated due to continued progression of the scoliosis. Among 14 patients with scoliosis 45 degrees or less at the start of brace treatment, only 1 patient progressed and underwent operation. The brace had a temporary effect on severe scoliosis, decreasing the rate of progression. Complications were few. The brace caused decubitus ulcer in one patient, and two patients developed increased pressure of the urinary tract.

Prevalence of scoliosis in children with myelomeningocele in western Sweden.

In a cross-sectional study of the prevalence of scoliosis in patients with myelomeningocele, 131 patients 5-20 years of age were investigated with clinical examination and radiograms. Sixty-nine percent of the myelomeningocele patients had scoliosis, this prevalence being present already at a young age (6 years). The occurrence of scoliosis increased drastically at high levels of dysraphism, being 20% in patients with sacral myelomeningocele but 94% in patients with thoracic MMC level. Between ages 5 and 10, the mean scoliosis increased successively from 15 to 33 degrees. After this age there was no significant further increase. The ambulatory status of the patients was strongly correlated to the scoliosis incidence. Forty-nine percent of the patients had a uni- or bilateral hip dislocation. There was no correlation between the side of the scoliosis convexity and the side of hip dislocation.

Influence of surgical treatment of scoliosis in children with spina bifida on ambulation and motoric skills.

The influence on motoric skills of surgical treatment of scoliosis in 14 children with myelomeningocele was studied. Fifty percent of the children had increased flexion contractures of the hips at follow-up, and all but one patient had impaired motor ability. There was no influence on activities of daily life, but 57% of children had lost some of their ambulation capacity. Postoperative physiotherapy is highly advisable. Intensive attempts to treat these children conservatively to prevent scoliosis progression is suggested.

Microbial growth dynamics on the basis of individual budgets.

The popular theories for microbial dynamics by Monod, Pirt and Droop are shown to be special cases of a model for individual budgets, in which growth and maintenance are on the expense of reserve materials. The dynamics of reserve materials is a first order process with a relaxation time proportional to cell length; maintenance is proportional to cell volume, and uptake, which depends hyperbolically on substrate density, is proportional to cell volume as well. Because of the latter, population dynamics depends on the behaviour of the individuals in a simple way, such that the cell volume distribution has no quantitative effect. When uptake is proportional to the surface area of the cell, which is realistic from a physical point of view, the relation between the individual level and the population one becomes more complicated and the cell size and shape distribution affects population dynamics. It is shown how the changing shape of rods modifies uptake and, consequently, growth. The concept of energy conductance, defined as the ratio of the maximum surface area specific uptake and the volume specific energy reserve has been introduced in the analysis of microbial dynamics. The first tentative results indicate that the value for E. coli is close to the mean value for a wide variety of animals. Properties of the model for cell suspension at constant substrate densities are analyzed and tested against a variety of experimental data from the literature on both the individual and the population level.