Electron microscopy of Pseudomonas aeruginosa treated with sulbenicillin and dibekacin.

A possible mechanism responsible for the combined effects of sulbenicillin and dibekacin on Pseudomonas aeruginosa IAM 1007 was investigated. The bactericidal activity of the above two drugs in combination was very strong. The regrowth of test strains after removal of the drugs was suppressed markedly, even when they were exposed to sulbenicillin plus dibekacin at a subinhibitory concentration of individual drugs. Sulbenicillin caused elongation of the bacterial cells. At the early stage of elongation, no demonstrable changes of ultrastructure of the cell wall were observed. At the late stage, lysis of the peptidoglycan layer occurred and spheroplast was formed. However, most of the outer membrane of the cell wall remained intact. Sulbenicillin acts upon the peptidoglycan layer, but not on the outer membrane. Thus it is difficult for sulbenicillin alone to cause cell lysis. On the other hand, dibekacin caused destruction of ribosomes and lysis of the outer membrane of the cell wall. Both sulbenicillin and dibekacin act on the cell wall, the former on the peptidoglycan layer (the inner membrane) and the latter on the outer membrane. The combined use of sulbenicillin and dibekacin caused elongation of bacilli and severe destruction of the inner and outer membranes of the cell wall. These morphological changes occurred even when the concentration of the individual drug was lower than its minimum inhibitory concentration (MIC). Furthermore, the cells elongated by sulbenicillin were ruptured easily when treated with dibekacin subsequently. The bacilli treated with dibekacin at a concentration lower than MIC and then treated with sulbenicillin at a concentration lower than MIC showed a marked elongation of the cells, which indicated that the effects of sulbenicillin was enhanced by dibekacin. These findings suggested strongly that sulbenicillin and dibekacin act on cell wall constituents and that their effects were complementary and synergistic.