Novel morphological changes in gram-negative bacteria caused by combination of bulgecin and cefmenoxime

The mode of action of bulgecin was investigated by examining its bactericidal and bacteriolytic activities, its effect on bacterial morphology, and its interaction with penicillin-binding proteins (PBPs). Bulgecin alone did not show any antibacterial activity against Escherichia coli and Serratia marcescens, but in concert with cefmenoxime, it induced potent growth-inhibitory and bactericidal activities. Electron microscopic examination of E. coli cells exposed to bulgecin combined with cefmenoxime revealed that a bulge developed in the middle of the cell, and additional smaller bulges were formed halfway between the central bulge and the polar ends. At the site of bulge development, vesicular mesosomelike structures appeared in the cytoplasm, the peptidoglycan layer facing them became faint, and the outer membrane protruded to form blebs. These morphological changes were quite different from those caused by the mecillinam-cefmenoxime combination that produces big bulges in E. coli. When S. marcescens was exposed to the combination of bulgecin and cefmenoxime, not only bulge formation, but also branching of the cells was observed. Bulgecin neither showed affinity for any PBPs of E. coli nor affected the binding of cefmenoxime or mecillinam to the PBPs.