A mathematical model for growth and osmoregulation in halophilic bacteria

Many molecular details of the ecophysiology of halophilic bacteria that use compatible solutes to maintain osmotic equilibrium have been examined. We ask whether the details are consistent and complete enough to predict growth and osmoregulation in these bacteria by integrating this information in a mathematical model. Parameterized for the halophilic organism Halomonas elongata, the model predicts the substrate and salt dependence of growth, the uptake of potassium and ectoine and the synthesis of ectoine. It is shown that salt (NaCl) dependence of growth can be modelled by substrate inhibition kinetics. Osmoregulation is known to involve accumulation of both ectoine and potassium glutamate in H. elongata. Using published and newly determined parameters, osmoregulatory models using either direct turgor or two-step (turgor and potassium) signalling are compared. The results are consistent with a role for potassium as a second messenger for hyperosmotic stress. Simulations of osmotic upshifts show a transient overregulation of the intracellular solute levels, as has been previously observed in experiments. A possible adaptive value of this overregulation as 'pre-emptive' behaviour in an environment with frequent dry periods leading to steadily increasing osmolarity is proposed. As a result of growth parameter estimation, a maximum P : O value of 2 for H. elongata can be inferred. In conclusion, the model developed here reproduces essential aspects of growth and osmoregulation in halophilic bacteria with a minimal set of assumptions.