| The effects of different humectants on growth, compatible solute uptake, membrane-associated protein synthesis, and pyruvate dehydrogenase activity in the osmotolerant foodborne pathogen Staphylococcus aureus were investigated. While growth in the presence of the impermeant humectants, sodium chloride and sucrose, induced accumulation of proline and glycine betaine by cells, growth in the presence of the permeant humectant glycerol did not. When compatible solutes were omitted from low aw media, growth was very poor in the presence of impermeant humectants. In contrast, the addition of compatible solutes had essentially no effect on growth when cells were grown in low aw media containing glycerol. The pyruvate dehydrogenase multienzyme complex (PDHC) was found to be upregulated by osmotic stress in S. aureus. The upregulation was detectable both at the levels of activity and relative abundance, and was judged to be about 4-fold when sodium chloride was used to adjust water activity (aw) of the growth medium to 0.94. It was found that S. aureus consumes greater amounts of carbon source per unit of biomass produced when growing under conditions of osmotic stress, indicating a role for energy metabolism in osmoadaptation. The upregulation of the PDHC was found to be humectant-dependent and was greatest when impermeant and non-metabolizable humectants were used to adjust a w, while permeant humectants and humectants Imported into the cell via phosphoenolpyruvate-dependent sugar transport systems did not give rise to increased levels or activity of the PDHC, indicating that osmotic upregulation of the PDHC is subject to catabolite repression in this organism. The presence or absence of PDHC upregulation in response to osmotic stress was found to correlate with the observed growth inhibitory action of the humectant A model for staphylococcal osmoadaptation and the involvement of primary metabolism therein is proposed and discussed. |
