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NPN is a small hydrophobic fluorescent probe which has been used to define the hydrophobic and self-promoted uptake pathways across the outer membranes of gram-negative bacteria (17). NPN fluoresces weakly in aqueous environments but strongly in hydrophobic environments, such as the membrane interior. It is usually excluded from the outer membrane by the outer monolayer of the outer membrane, which comprises the polyanion lipopolysaccharide, stabilized by divalent cation cross bridging. In most bacteria, a destabilizing polycation, such as polymyxin B, is required to permeabilize the outer membrane to NPN, a result consistent with and diagnostic of self-promoted uptake of the polycation. H. pylori (Fig. 1) demonstrated relatively high intrinsic uptake (50 arbitrary units) of NPN compared to E. coli (10 to 20 arbitrary units) or P. aeruginosa (10 arbitrary units), a result consistent with its relatively high susceptibility to hydrophobic agents. The higher background level of NPN fluorescence indicated that either the outer membrane of in vitro-grown H. pylori was permeable to hydrophobic compounds or there was a lack of active efflux in in vitro-grown H. pylori.

from http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=89666