Strategies of nutrient transport by ruminal bacteria.

The survival of bacteria in natural environments like the rumen depends on the ability of the bacteria to scavenge nutrients. It is now evident that ruminal bacteria use a variety of transport mechanisms. Hydrophobic substances, such as ammonia and acetate, are permeable to the lipid bilayers of cell membranes and can be taken up by passive diffusion. Hydrophilic compounds (e.g., sugars, amino acids, peptides) do not easily pass through lipid bilayers and must be transported across cell membranes on carrier proteins. Facilitated diffusion can display saturable kinetics but does not result in accumulation of solute. Active transport can establish extremely high concentration gradients, and this work may be driven by the hydrolysis of chemical bonds (e.g., ATP) or ion gradients, which are coupled to solute symport. Many solute symports involve protons, but sodium systems also are common in ruminal bacteria. The phosphotransferase system chemically modifies sugars as they pass across the cell membrane, and several ruminal bacteria have this method of group translocation. Many feed additives have either a direct or indirect effect on rumen bacterial transport. For instance, ionophores can inhibit transport by destroying (sometimes even reversing) ion gradients, lowering intracellular pH, or causing excessive ATP hydrolysis.