The mechanism of flocculation of a Saccharomyces cerevisiae cell homogenate using polyethyleneimine

The effects of polyethyleneimine, electrolyte concentration and pH on the electrophoretic mobility values for whole yeast, yeast homogenate and borax clarified homogenate were observed. The polyethyleneimine concentration at which there is optimum clarification of the borax-clarified homogenate corresponded to a mobility value of zero. Increased floc size up to the point of zero mobility was obtained by use of polyethyleneimine of larger molecular weight and also higher concentration. The effect of ageing under shear on the size of the flocs was examined and indicated reversibility of break-up at low shear rates while at higher shear rates the flocs were irreversibly broken. Such growth and ageing observations suggest floc formation comprises a two-stage process; the initial formation of primary particles by polymer bridging, and the subsequent formation of larger flocs as these primary particles are brought together by charge neutralisation.UCL is the Biotechnology and Biological Science Research Council sponsored Advanced Centre for Biochemical Engineering and we are grateful to the Council for financial support.     

The influence of pyrithione on the growth of micro-organisms

Pyrithione was active against a range of micro-organisms, the most resistant being Gram-negative bacteria. The growth curves for Klebsiella pneumoniae and Bacillus licheniformis showed a drug-dependent lag phase. Candida albicans grew with a drug-dependent growth rate. EDTA antagonized the effects of pyrithione.     

24-Methyl-23-dehydrocholesterol: a new sterol intermediate in C-24 demethylation from the nematodes Panagrellus redivivus and Caenorhabditis elegans?

Panagrellus redivivus produced 24-methyl-23-dehydrocholesterol as 4.0% of the 4-desmethylsterols when propagated in a medium containing campesterol as the dietary sterol. The re-examination of previous data revealed that Caenorhabditis elegans produced 1.8% 24-methyl-23-dehydrocholesterol when propagated in medium containing campesterol. 24-Methyl-23-dehydrocholesterol was not detected when the nematodes were propagated in medium containing 22-dihydrobrassicasterol or 24-methylenecholesterol. This may be a result of the greater efficiency of dealkylation of the latter two sterols. This is the first report of the natural occurrence of this sterol in a non-photosynthetic organism, and the first report in organisms that dealkylate 24-alkylsterols.     

THE EVOLUTION OF REPRODUCTIVE ISOLATION AS A CORRELATED CHARACTER UNDER SYMPATRIC CONDITIONS: EXPERIMENTAL EVIDENCE

A set of experiments is described that tests the general hypothesis that sympatric speciation is genetically feasible whenever reproductive isolation evolves indirectly as a correlated character. We specifically test the hypothesis that disruptive selection on habitat preference can lead to sympatric speciation when individuals mate locally within their selected habitat. Drosophila melanogaster was used as a model system. A 35-generation experiment using a complex habitat maze led to complete reproductive isolation between subpopulations using different spatiotemporal habitats. The reproductive isolation that developed over the course of the experiment was a result of offspring returning to mate in the habitat type selected by their parents, i.e., a gradual breakdown in migration between habitats.