Valinomycin is an neutrally charged depsipeptide ionophore ,with high selectivity for K+ ions (over 10,000x favourability of K+ over Na+) \cite{Hensley_1975,Varma2008}. The expected hypothetical result of addition of valinomycin will be a reduction in ΔΨ over the cytoplasmic membrane.  As an ionophore for K+ ions, it allows a route of equilibration for ΔΨ as K+ can now move freely into the cytoplasm to counteract the ΔΨ produced by the electron transport chain. This, in turn, will reduce Δp,  which will reduce the ability of S. aureus to uptake isoleucine. Compared to CCCP, which directly affects both ΔpH and ΔΨ, valinomycin's effect on Δp is significantly smaller (ρ = 0.0009373) - with concentrations of [14C]-isoleucine within cells treated with valinomycin being nearly twice as high as in cells treated with CCCP.
Nigericin is an ionophore with an overall structure similar to valinomycin \cite{Ahmed_1983}, 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 in an electroneutral exchange) \cite{ferguson2013}. The biochemical result of nigericin's action is an equalisation of both K+ and H+ concentrations between the cell and the periplasm. The net result is a reduction in ΔpH, as K+ is transported out of the cytoplasm, [H+] increases within the cytoplasm to reducing ΔpH over the cytoplasmic membrane.
When used in combination with valinomycin, the cell has a reduction of ΔpH AND ΔΨ. Nigericin equilibrates ΔpH, while valinomycin allows equilibration of ΔΨ by providing an another route for K+ ions in and out of the cell. The joint effect vastly reduces Δp, in an overall effect similar to CCCP, which also reduced ΔΨ and ΔpH. This can be seen by the t-test performed between +Glucose, +CCCP and +Glucose, +Nigericin, +Valinomycin, which showed no statistically significant difference between the effects of the two experiments on [14C]-isoleucine uptake ( ρ = 0.2146).