Microbial transport: adaptations to natural environments.

The cytoplasmic membrane of bacteria is the matrix for metabolic energy transducing processes such as proton motive force generation and solute transport. Passive permeation of protons across the cytoplasmic membrane is a crucial determinant in the proton motive generating capacity of the organisms. Adaptations of the membrane composition are needed to restrict the proton permeation rates especially at higher temperatures. Thermophilic bacteria cannot sufficiently restrict this proton permeation at their growth temperature and have to rely on the much lower permeation of Na + to generate a sodium motive force for driving metabolic energy-dependent membrane processes. Specific transport systems mediate passage across the membrane at physiological rates of all compounds needed for growth and metabolism and of all end products of metabolism. Some of transport systems, the secondary transporters, transduce one form of electrochemical energy into another form. These transporters can play crucial roles in the generation of metabolic energy. This is especially so in anaerobes such as Lactic Acid Bacteria which live under energy-limited conditions. Several transport systems are specifically aimed at the generation of metabolic energy during periods of energy-limitation. In their natural environment bacteria are also often exposed to cytotoxic compounds, including antibiotics. Many bacteria can respond to this live-threatening condition by overexpressing powerful drug-extruding multidrug resistance systems.