The influence of poly-β-hydroxybutyrate accumulation on cell volume and buoyant density in Alcaligenes eutrophus

TLDR: Percoll is a more suitable medium than sucrose to measure the density of live bacterial cells because of the high osmolarity of sucrose which gives artificial results.

Abstract: Accumulation of poly-β-hydroxybutyrate (PHB) was studied in Alcaligenes eutrophus strain N9A. Under nitrogen limitation and heterotrophic conditions, the cells accumulated PHB at a rate of 50 fg cell-1 h-1. Volume increased from 1.208 to 3.808 μm3 and buoyant density from 1.110 to 1.145 pg μm-3 with an increase in PHB from 0 up to 1.699 pg cell-1. Volume was found to change linearly with PHB content. The changes were due to increases in cell width and not in cell length. PHB explained 93% of the changes in cellular volume. The relationship between density and PHB was hyperbolic. PHB explained 96% of the changes in density. When a mutant strain unable to accumulate PHB was analyzed together with the wild type, the PHB-less mutant and the wild type showed densities of 1.100 pg μm-3 and 1.120 pg μm-3, respectively, in gradients of 65% Percoll. In sucrose gradients, nevertheless, the results were reversed. This discrepancy was explained by the high osmolarity of sucrose which gives artificial results. Thus, we conclude that Percoll is a more suitable medium than sucrose to measure the density of live bacterial cells.