Potentiation by metal ions of the efficacy of the ionophores, monensin and tetronasin, towards four species of ruminal bacteria.

Concentrations of Na(+), K(+) and Ca(2+) in the growth medium were varied within limits normally found in vivo to determine how cation concentrations affect the sensitivity of ruminal bacteria to the ionophores, monensin (a Na(+)/H(+) and K(+)/H(+) exchanger) and tetronasin (Ca(2+)/H(+)). High [Na(+)] (172 mM cf. 137 mM in control medium) enhanced the efficacy of monensin towards Eubacterium ruminantium 2388, Streptococcus bovis C277, Lactobacillus casei LB17 and Prevotella albensis M384. High [K(+)] (35 mM cf. 19 mM) alone caused a decreased potency of both ionophores, except with L. casei. Added Ca(2+) (7.4 cf. 2.8 mM) increased the potency of tetronasin when [Na(+)] was low. High [Na(+)] alone also potentiated the efficacy of tetronasin. Monensin caused intracellular [Na(+)] and [K(+)] to be decreased in the most sensitive of these organisms, E. ruminantium, whereas only intracellular [Ca(2+)] fell with tetronasin. The changes were small; however, Δp fell by only 20 mV after 2 h when ionophores caused immediate cessation of growth. ATP concentrations fell by 77% and 75% with monensin and tetronasin, respectively. Thus, altering cation concentrations might be used to potentiate the efficacy of ionophores, by increasing the rate of energy expenditure to maintain ionic homoeostasis in sensitive bacteria.