Summary: Synchronous cultures of Alcaligenes eutrophus (Hydrogenomonas eutrophah16) were prepared by a size selection method which gave high synchrony indices (0·70 to 0·85). Unlike the smooth exponential increase which was observed in exponentially growing cultures, the respiration rate [μg-atom O min−1 (ml culture)−1] of synchronous cultures exhibited a periodic increase which was composed of two ‘steps’ with rises centred at approximately 0·4 and 0·9 of the cell-cycle. The respiration of exponentially growing and synchronous cultures was stimulated by the uncoupling agent carbonyl cyanide m-chlorophenylhydrazone; the degree of stimulation varied during the cell-cycle and was inversely related to the respiration rate. The observed discontinuous patterns of respiration are discussed with reference to the reported respiratory properties of other micro-organisms in synchronous cultures.
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