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A simple rapid method for preparing synchronous cultures of Bacillus subtilis has been used to investigate changes in density during the cell cycle. Asynchronous cells separated on a stepped Percoll density gradient had a mean cell density of 1.117 g ml-1 +/- 0.004. Samples from a synchronous culture exhibited variation (ca. 1.5%) in mean cell density which was greatest at the onset of cell division. An asynchronous control culture showed little variation in density. These results are discussed in relation to previous work on Escherichia coli.

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Paracoccus denitrificans was synchronised by a selection method, and factors affecting respiratory activity during the cell cycle were investigated. Oxygen uptake rates increased discontinuously in 2 steps, with mid‐points of rises centred at around 0.3 and 0.8 of a cycle. Addition of KCN (0.1 mM) resulted in a constant level of inhibition of respiration (approx. 70%) irrespective of cell age. However, the addition of a respiratory uncoupler stimulated oxygen uptake rates in a cell‐cycle dependent manner. Maxima of stimulation occurred at points intermediate to the rises in respiratory activity and were centred at around 0.18 and 0.55 of the cell cycle.