Effect of a combination of benzylpenicillin or ampicillin and dicloxacillin on peptidoglycan synthesis in a cell-free enzyme system from a beta-lactamase producing strain of Citrobacter freundii.

The membrane fraction prepared from beta-lactamase producing Citrobacter freundii GN346 catalyzed in vitro peptidoglycan synthesis from uridine-5'-diphosphate-N-acetylmuramyl-L-alanyl-D-glutamyl-meso-diaminopimelyl- D-alanyl-D-alanine and uridine-5'-diphosphate-N-acetylglucosamine, which was accompanied by the release of alanine from the carboxyl terminal end of the former substrate. Though this reaction was inhibited by benzylpenicillin (PCG) and ampicillin (ABPC), the reaction was relatively insensitive compared with that catalyzed by the membrane fraction from a derived beta-lactamaseless mutant strain GN346/16. In contrast, the enzyme activity of the parent strain was strongly inhibited by a combination of PCG or ABPC and dicloxacillin (MDIPC). The beta-lactamase present in the membrane fraction from the parent strain showed stronger activity than that from the mutant strain, and the activity was inhibited by MDIPC as in the case of the soluble enzyme localized in the periplasmic space.