However, the measured values of TPP+ distribution also indicated that ionophores had only a minor influence on membrane potential. Some acidification of the cytoplasm occurred, but the total protonmotive force was

only decreased by about 20%. In contrast, the ATP pool fell by 75%. It should be noted that the experiments were performed with late-exponential or stationary-phase cells to reflect the conditions that pertain predominantly in the rumen (Hobson & Wallace, 1982). It seems improbable that the mechanisms differ in more active bacteria, as in mid-exponential phase, although the magnitude of the gradients and pools may be different. Russell and his colleagues ABT-199 in vitro have made similar observations with other species of ruminal bacteria. The high apparent intracellular concentrations of Na+ and K+ were similar to those measured in S. bovis (Russell, 1987). Ruminal bacteria have been described as mildly halophilic, based on their requirements of Na+ for growth (Caldwell et al., 1973; Caldwell & Hudson, 1974). The membrane potential fell by < 10% when monensin

was added to S. bovis, although intracellular pH was affected to a greater extent (Russell, 1987). The protonmotive force of a ruminal Peptostreptococcus was unaffected by monensin, yet the ATP pool fell by two-thirds (Chen & Russell, 1989). It therefore appears that it is not the collapse of transmembrane ion gradients that causes the toxic effect of ionophores on intact bacteria, but the energy expenditure required to support the increased energy demand of homoeostatic selleck screening library mechanisms maintaining the gradients. Any extra demands induced by adding different cations may therefore have an influence on the efficacy of an ionophore, even if the ion is not translocated by the ionophore. In conclusion,

it may be possible to enhance the efficacy of ionophores by adding salts of mineral cations to the diet. However, the spectrum of antibacterial activity against different species, upon which ionophore depends for its nutritional effects, may well be different P-type ATPase when the added cations are present, depending on the ion gradients present in different species. Thus, the nutritional effects of the ionophores (Chen & Russell, 1989) may not be the same at different cation concentrations. The present results also have implications for mechanisms by which ruminal bacteria may become resistant to ionophores. Adaptive resistance to ionophores involves changes in the permeability of the cell envelope (Newbold et al., 1992; Callaway & Russell, 1999), which may well affect changes in transmembrane ion gradients. One of the fears concerning the use of antimicrobials in livestock production is that transmissible resistance factors will arise and by transfer to human pathogens will render antibiotic therapy ineffective (Goodrich et al., 1984). However, there is no evidence that such resistance arises by exposure to ionophores such as monensin (Russell & Houlihan, 2003; Phillips, 2007).