Small and medium sized mammals as predators of ticks (Ixodoidea) in South Africa

A total of 1,640 stomach contents from 56 South African species of small and medium sized mammals were examined for ticks. Only six ticks were found in three specimens of the caracal Felis caracal. Although identification of the tick species was not possible owing to fragmentation and partial digestion, the genus Haemaphysalis was represented in four of these cases. We conclude that predation by small and medium sized mammals on ticks is uncommon in South Africa.     

Nonlinear effect of dynamic self-organization in macromolecular systems caused by photocontrolled electron flux

We use the electron-conformational interaction approach to develop a physical model which self-consistently describes the photomobilized electron transfer kinetics and structure conformational transitions in reaction centers (RCs) of purple bacteria. We consider the kinetics of electron transition from pigment onto primary acceptor and the subsequent charge recombination accounting for the change of distance between the above-mentioned cofactors. It is shown that, given natural values of RC parameters, the kinetic constant's dependence on the acting light intensity is monotone. As opposed to the previous case, similar dependencies for the chain of electron transfer between primary and secondary quinone acceptors revealed anS-like relationship. This can lead to bistability of the RC optical transmission coefficient and a fundamental dependence of charge recombination kinetics upon the prehistory of the RC's interaction with exciting radiation.     

Free and immobilized cultures of Spirulina maxima for swine waste treatment

We have analyzed the behavior of spirulina maxima at increasing concentration of ammonium nitrogen present in swine waste when it is either growing in suspension or immobilized in polymeric supports. We compared the response of spirulina maxima growth to different concentrations of aeration stabilized swine waste (total phosphorus, ammonium nitrogen) as a way to determine the treatment efficiency of both systems. At a dilution of 50 % of swine waste, the suspended system reached the best results for biomass concentration and nutrient removal. In the immobilized system, at dilutions of 25 and 50 % of swine waste, more than 90 % ammonium nitrogen removal was obtained, and the optimal cell concentration for immobilization was 2 g/l (wet basis).     

Displacement and Return Movement of Chloroplasts in the Marine Dinophyte Pyrocystis noctiluca. Experiments with Optical Tweezers

Infrared laser traps (optical tweezers) were used to study laser-induced organelle movements in the marine alga Pyrocystis noctiluca (Dinophyta). These cells are highly suitable for optical micromanipulation due to their large size and extensive vacuole. Experiments were done with plastids held by optical tweezers and moved from the nuclear area into the vacuole. The subsequent retraction movement was analysed for speed. The displaced organelles remained connected to their original position by a thin cytoplasmic strand, often less than 1 μm in diameter. When the organelles were released they rapidly returned at an initial rate of 81.7 ± 7.8 μm . s^?1 (overall displacement 50 μm, measured distance 20 μm, 25 °C ± 1 °C, number of cells 22), slowing down with progressive retraction of the connecting strand. The return movement was reduced to 4.2 ± 0.2 μ .s^?1 (n = 10) when the organelles were displaced and held for 1 min. Displacement to a longer distance increased the rate of return movement. A change from a high to a low environmental temperature significantly reduced movement from 94.5 ± 9.0 . s^?1 (30 °C ± 1 °C, n = 22) to 34.5 ± 2.7 μm .s^?1 (5°C ± 1 °C, n = 22). Nocodazole and N-ethylmaleimide (NEM), inhibitors of microtubules and acto-myosin, respectively, did not affect the retraction of the connecting strand, but at high concentrations of NEM it became increasingly difficult to move organelles away from the nuclear area. We suggest that the return movement of organelles within laser-induced artificial strands mainly depends on the viscoelastic properties of the tonoplast. The quantification of these properties by optical tweezers allows determination of reactions of plant cells to temperature changes.     

A robust fed-batch feeding strategy for optimal parameter estimation for baker's yeast production

Parameter identification of structured models is often a problem in biotechnology, because the poor data situation and the number of unknown parameters only allow for inaccurate estimates. But often only a subset of all kinetic parameters of the model are of interest for production purposes, e.g. for fed-batch cultivation. These parameters should be estimated with a given accuracy. In addition, the experiments for information acquisition with respect to these parameters should be as simple as possible and should consider some practical restrictions. In this contribution a fed-batch feeding strategy is proposed to allow for an accurate estimation of yield and of critical growth rate of baker's yeast. The feeding also allows for economic and stereotyped use of staff and equipment and is therefore suitable for routine use in screening of strains and media. The overall pattern is similar to that one, usually used in production scale to minimize errors by limited model validity. After an initial phase for achieving a reproducible state three different growth rates are adjusted to cover the range of possible critical growth rates. From biomass and ethanol measurements yield and critical growth rate can be estimated with an accuracy of about 2.1%. The fermentation pattern ends up with a constant feeding rate to simulate a limited oxygen transfer rate and to allow for an uptake of residual sugar and ethanol before a dough test can be carried out. Beside experimental results simulations and sensitivity analyses are shown.List of Symbols P ethanol concentration - S substrate concentration - S _f substrate concentration in feed - T fermentation time - V fermenter volume - X biomass concentration - C measurement error covariance matrix - F Fisher information matrix - X state variables - Y output variables - X _p state sensitivity functions with respect to parameters - Y _p output sensitivity functions - e eigenvectors - k vector of limitation and inhibition parameters - n number of observations - q _in feeding stream - q _b stream for samples and ammonia feed - r vector of specific turnover rates - y vector of yields - specific weight - eigenvalues - specific growth rate - _set exponent in exponential feeding - standard deviation Dedicated to the 65th birthday of Professor Fritz Wagner.A. O. Ejiofor and B. O. Solomon are grateful to the Alexander von Humboldt Stiftung for granting them fellowships and to GBF for providing all the materials necessary for their successful research stay in Germany.     

Loss of antibody productivity is highly reproducible in multiple hybridoma subclones

An immunoglobulin G (IgG(2b)) producing hybridoma cell line (S3H5/gamma2bA2) was cloned and subcloned. Twenty subclones were grown in parallel while being adapted in a stepwise fashion to serum-free medium. Following adaptation to serum-free medium, it was found that 16 of the 20 subclones remained at a relatively constant proportion of nonproducing cells. Three of the remaining subclones transiently deviated from this balance but eventually returned toward this population composition. One subclone continued to lose productivity. A population balance was reached at approximately 8% of the population being nonproducers. The loss of antibody productivity was thus highly reproducible. (c) 1993 John Wiley & Sons, Inc.