Recently, j8-lactam agents containing iron-chelating moieties, such as E0702, which contains catechol, and pirazmonam and U-78,608, which contain 3-hydroxypyridone, have been developed. By determining the susceptibility to these agents of Escherichia coli mutants lacking various iron-repressible outer membrane proteins, we showed that two of these proteins with hitherto unknown functions, Fiu and Cir, were apparently involved in the transport of monomeric catechol and its analogs. These results confirm the conclusion of Curtis and co-workers, which was obtained by using a different set of catechol-containing antibiotics (N. A. C. Curtis, R. L. Eisenstadt, S. J. East, R. J. Cornford, L. A. Walker, and A. J. White, Antimicrob. Agents Chemother. 32:1879Chemother. 32: -1886Chemother. 32: , 1988. E0702 was shown to enhance the uptake of radioactive ferric iron into wild-type cells but not into cir fiu double mutants. By combining the influx of E0702 with its hydrolysis by a periplasmic P-lactamase, we showed that the wild-type cells transported unliganded E0702 at a rate comparable to or even higher than the rate of transport of the E0702-Fe3+ complex. We postulate that the main function of Cir and Fiu may be to recapture the hydrolytic products of enterobactin, such as 2,3-dihydroxybenzoic acid and 2,3-dihydroxybenzoylserine.Most nutrients used by gram-negative bacteria move across the outer membrane through the nonspecific channels of porin (16), and a similar mechanism of diffusion is thought to be used by most antibiotics that are effective against these bacteria, including most ,-lactams (13). Some nutrients, however, are too large for efficient diffusion through the porin channel, and these compounds, including iron-siderophore complexes, are often taken up by specific pathways present in the outer membrane. Thus, Escherichia coli, for example, is known to have at least seven outer membrane proteins whose production is regulated by the availability of ferric iron in the environment; four of them, the products of the fepA, fecA, fluA, and AhuE genes, have been identified as the receptors for ferric enterobactin, ferric citrate, fermchrome, and coprogen, respectively (12,22). Recently, several P-lactam compounds with attached iron-chelating moieties have been developed (9,18,24). Moreover, evidence suggestive of the utilization of a siderophore receptor for outer membrane penetration was obtained for at least one of these compounds, E0702, because E. coli became resistant to this compound if it had tonB mutations, which abolish the transport of all known siderophores as well as vitamin B12 across the outer membrane (26). However, the identities of the outer membrane receptors used by these 1-lactams were unknown, and this study was undertaken to identify these receptors. In the course of the present study, we learned that another laboratory studied the outer membrane permeation of a group of catechol-containing cephalosporins by using a similar approach (3). Our results, however, have been obtained by using an...