[Penetration of meropenem in gram-negative bacilli. Differences in activity with imipenem].

The outer membrane of gramnegative bacteria cell-wall contain channels formed by proteins known as porins, which facilitate the penetration of molecules into the cell. Imipenem and meropenem possess an important intrinsic activity against most gramnegative bacteria due to their high affinity for the penicillin-binding proteins PBP-2 and/or PBP-3. Meropenem is slightly more active against certain species of Enterobacteriaceae, Pseudomonas aeruginosa and nonfermenters bacilli. In this sense the differences in the activity between the two carbapenems may be attributable to differences in their affinity for PBPs, differences in their resistance to beta-lactamases hydrolysis, or to the differences in the capacity to employ certain porin channels. OprF is the main porin channel involved in the beta-lactam penetration of bacteria, though OprC and OprD2 may also contribute to the penetration of carbapenems into P. aeruginosa. However, evidences suggest that although impenem requires the presence of OprD2, meropenem may use other pathways for penetration. In Escherichia coli both carbapenems use ompF and ompC, and no specific porin channels have been detected. Only in the case of Enterobacter cloacae may exceptions exists among Enterobacteriaceae.