The supernatant was removed, and radioactivity in cells and supernatant was counted by liquid scintillation spectrometry. Internal pH was calculated from the distribution of 14C and 3H between the pellet and the supernatant. The accumulation of benzoic acid in E. ruminantium was abolished Adriamycin concentration by pretreatment of the

cells with 10 μM tetrachlorosalicylanilide, a protonophore (Hamilton, 1968), suggesting there was little or no active uptake or binding of benzoic acid by the cells. The chemical potential gradient (ZΔpH) generated by the pH gradient across the cell membrane was calculated from the Nernst relationship: Z = 2.3 RT/F or 62 mV at 39 °C. Intracellular volume was calculated using a separate aliquot of culture. One millilitre of culture was incubated with 3H2O (7 μCi, 125 μCi mL−1 and hydroxy [14C] methyl inulin (0.7 μCi, 11.1 mCi mmol−1) for 10 min, before centrifuging as before. The distribution of 14C-inulin and 3H2O in the pellet and the supernatant

allowed the exclusion volume of inulin compared to H2O to be calculated and hence the intracellular volume (Rottenberg, 1979). The electrical potential (Δψ) was calculated from the uptake of the lipophilic cation [phenyl-14C]tetraphenylphosphonium bromide (TPP+). One millilitre of culture was incubated under CO2 with TPP+ check details (0.05 μCi, 31 mCi mmol−1) and 3H3O (0.5 μCi, 16 μCi mL−1), then centrifuged and counted as before. Δψ was calculated from the distribution of TPP+ between the intra- and extracellular space (Rottenberg, 1979). The total transmembrane potential (Δp) was calculated as Δp = Δψ − ZΔpH. Nonspecific uptake/binding

of TPP+ was corrected by subtracting the apparent uptake in cells that had been treated with toluene (1% v/v, 1 h). Intracellular K+, Na+ and Ca2+ concentrations were measured in cells that had been centrifuged and resuspended in 25% TCA, then diluted in deionized water. Hydroxyl [14C] methyl inulin (0.7 μCi mL−1, 11.1 mCi mmol−1) SPTLC1 was added to the cultures before centrifugation to allow corrections to be made for extracellular medium trapped in the cell pellet. Na+ and K+ were analysed by atomic emission spectrometry on a Pye Unicam SP9 atomic absorbance spectrometer, while Ca2+ was determined by atomic absorbance on the same instrument. ATP pools were measured by a luciferase method (Wallace & West, 1982) in cells 2 h after the addition of the ionophores. Protein was determined using Folin reagent (Herbert et al., 1971). Tetronasin was a gift from Coopers Animal Health Limited, Berkhamsted, Herts. Monensin was from Sigma. TCS and TPP were gifts from I.R. Booth, University of Aberdeen. [Carboxy-14C]-benzoate was from New England Nuclear, Stevenage, Herefordshire. All other radiochemicals were from Amersham. The potency of monensin and tetronasin against E. ruminantium, S. bovis, P. albensis and L. casei was determined by inoculating bacteria into media in which the concentration of ionophore was serially doubled.