Briefly, (600 words maximum) discuss your data with respect to the mechanism of transport of isoleucine into Staphylococcus aureus, explaining the mode of action of each of the uncouplers used in your discussion.
Isoleucine uptake into Staphylococcus aureus occurs by the action of a H+ coupled symport \cite{Niven_1972}. The process is active and relies on Δp to function efficiently. As Δp = ΔΨ + ΔpH, any reduction in the electrical or chemical potential over the cytoplasmic membrane may result in a reduction of isoleucine uptake. The first two experiments performed (No Glucose and +Glucose) show the effect a fully functioning electron transport chain has on isoleucine uptake. Under the presence of glucose, concentrations of isoleucine within the cell after 15 mins were 47-48x greater than outside the cell - with glucose to catabolise, the cell was able to maintain Δp by pumping hydrogen ions out of the cell at the same rate as symporting isoleucine into the cell, allowing for a large increase of intracellular isoleucine concentrations. In comparison to the experiment performed under no glucose conditions, where concentrations of [14C]-isoleucine were only 13-15x the concentrations outside of the cell, showing while the cell is still able to actively transport into the cell, it is doing so at a much reduced rate, due to starvation. Analysis of the action of the ionophores used in experiments 3-6 are best viewed in comparison to a fully functioning ETC, and one without glucose.
Carbonyl Cyanide m-chlorophenyl hydrazone (CCCP) is a weak acid, and protonophore that selectively increases the cell membranes permeability to H+ ions, it does this by diffusing into the cytoplasmic membrane in its HA form and releasing a H+ ion into the cytoplasm, A- is then free to diffuse through to the positively charged side of the membrane (at a voltage dependant rate) and adsorb another H+ ion \cite{Benz_1983}. As the protons now have an alternative route down their concentration gradient out of the cell, this vastly reduces the effectiveness of the electron transport chain to produce Δp. As both ΔpH and ΔΨ are lower than they should be, this explains why the concentration of isoleucine with the cell struggles to increase within the cell. The results are however, significantly different to the concentration of isoleucine within a cell starved of glucose ( ρ = 0.00567 ), showing how even a damaged ETC is more effective than the inability to power one.
Valinomycin is an ionophore for K+ ions, the expected hypothetical result of addition of valinomycin to the ability to uptake isoleucine is that it will have a similar, but muted effect compared to CCCP. As an ionophore for K+ ions, it allows a route of equilibration for ΔΨ over the cells membrane, which will reduce the ability of S. aureus to uptake isoleucine.
Nigericin is also an ionophore with an overall structure similar to valinomycin, and like valinomycin nigericin is an ionophore for K+, however when accepting the K+ ion, nigericin releases a H+ ion (allowing it to remain uncharged and move through the cytoplasmic membrane) \cite{ferguson2013}. The net result of nigericin's action is an equalisation of K+ and H+ concentrations between the cell and the outside.The net result is a reduction in ΔpH, as K+ is transported out of the cell, [H+] increases intracellularly to remove