Excretion of endogenous cadaverine leads to a decrease in porin-mediated outer membrane permeability.

The permeability of the outer membrane of Escherichia coli to hydrophilic compounds is controlled by porin channels. Electrophysiological experiments showed that polyamines inhibit ionic flux through cationic porins when applied to either side of the membrane. Externally added polyamines, such as cadaverine, decrease porin-mediated fluxes of beta-lactam antibiotics in live cells. Here we tested the effects of endogenously expressed cadaverine on the rate of permeation of cephaloridine through porins, by manipulating in a pH-independent way the expression of the cadBA operon, which encodes proteins involved in the decarboxylation of lysine to cadaverine and in cadaverine excretion. We report that increased levels of excreted cadaverine correlate with a decreased outer membrane permeability to cephaloridine, without any change in porin expression. Cadaverine appears to promote a sustained inhibition of porins, since the effect remains even after removal of the exogenously added or excreted polyamine. The cadaverine-induced inhibition is sufficient to provide cells with some resistance to ampicillin but not to hydrophobic antibiotics. Finally, the mere expression of cadC, in the absence of cadaverine production, leads to a reduction in the amounts of OmpF and OmpC proteins, which suggests a novel mechanism for the environmental control of porin expression. The results presented here support the notion that polyamines can act as endogenous modulators of outer membrane permeability, possibly as part of an adaptive response to acidic conditions.