Polyhydroxyalkanoates in Bacterial Cells - More Than just Storage Materials

Abstract

Since polyhydroxyalkanoates (PHAs) belong among the most widespread storage materials within bacteria genus, it can be assumed that the ability of PHAs accumulation represent significant advantage in the natural environments. Of course, probably the most obvious is the possibility to utilize PHAs when external carbon sources are depleted. Nevertheless, it is likely that PHAs play much more complex role in the stress response of bacteria. Therefore, the aim of this work was to investigate possible influence of PHA accumulation in bacteria on physical properties of the cells and their cytoplasm with respect to possible stress survival. Cells of bacteria Cupriavidus necator H16 with various poly(3-hydroxybutyrate) (PHB) content (25 – 91 % of cell dry weight) were subjected to analytical centrifugation employing LUMiSizer. We observed that slope of index instability of bacterial culture increased with rise of PHB content in cells. It indicates that accumulation of PHB granules in bacterial cells affects overall physico-mechanical properties of the cells in particular sedimentation potential and density which might influence their behavior in natural environment such as sedimentation, surface absorption and subsequent biofilm formation. Furthermore, when investigated by Cryo-SEM, the PHB containing cells showed needle-type plastic deformations while these structures were absent in the cells without polymer. This suggests that native intracellular PHB granules reveal completely different mechanical and physico-chemical properties than any other component of bacterial cytoplasm and their flexibility even in deeply-frozen state is significantly higher than that of PHB isolated from bacterial cells. Based on these observations, it can be expected that, aside from their involvement in metabolism, presence of PHB granules dramatically changes physico-mechanical properties of cytoplasm and overall properties of cells which might represent important advantage when cells are exposed to stress conditions.