The influence of outer membrane (OM) permeability on carbapenem susceptibility was studied in strains of Enterobacter cloacae, a species in which carbapenem resistance depends upon the conjunction of overproduction of the chromosomal cephalosporinase and reduction of OM permeability. Relative trans-OM diffusion rates were measured using the liposome swelling assay. Proteoliposomes were reconstituted with OM from the members of an isogenic set of E. cloacae strains, selected in vivo or in vitro, which produced either porins F and D (wild-type), or F or D only, or neither. For all but one mutant, and compared with the wild-type strain, the respective increases in MICs and decreases in trans-OM diffusion of carbapenems were: nil and 13 to 18%; 4- to 32-fold and 33 to 50%; > or = 64-fold and > or = 90%. Our results suggest (i) that carbapenems (and other beta-lactam antibiotics) diffuse through porins F and D, but more rapidly through porin F, and (ii) that OM permeability is the critical factor in determining the level of MICs of carbapenems for cephalosporinase-overproducing strains of E. cloacae. The OM of one particular low-level carbapenem-resistant and porin F- and D-deficient mutant was at least five times more permeable to carbapenems than the similarly porin-deficient high-level resistant mutants. We infer from this observation the possible existence of an alternative carbapenem penetration pathway which could be associated with two as yet uncharacterized overproduced OM proteins of about 22 and 47 kDa.