Biophysical characterization of in- and efflux in Gram-negative bacteria

Gram-negative bacteria developed a number of tools to avoid accumulation of cell-toxic compounds. The outer membrane as a first defense system is tightly packed reducing permeation through the lipid membrane. Water-soluble compounds may penetrate through membrane channels called porins. Once inside the periplasmic space special enzymes may welcome the foreign molecule for inactivation. The molecules entering the inner membrane will be harvested by efflux pumps and ejected back to the extra-cellular space. Bacteria modulate all these barriers through the level of protein expression or mutations. In order to understand the function of the involved proteins a quantification of the individual transport elements is necessary. Here we describe recent biophysical methods to characterize molecular transport across membranes.