In vitro and in vivo complex formations of polymyxin B and lipopolysaccharides (LPS) from resistant and sensitive cells of Serratia marcescens were studied by polyacrylamide gel electrophoresis in sodium dodecyl sulfate and electron microscopy. In vitro treatment of LPS from resistant cells with polymyxin B gave two populations of spherical complexes of different molecular weights as determined electrophoretically. Similar treatment of LPS from sensitive cells resulted in dissociation of the LPS-protein and subsequent complexing with the LPS moiety into stable spheres.In vivo treatment of resistant cells with polymyxin B resulted in LPS-polymyxin B complexes which were comparatively smaller and existed in two morphological forms: spheres and linear ribbons.LPS from the sensitive cells were degraded extensively into small rods and an amorphous mass by the in vivo polymyxin B treatment.In both systems, the electrophoretic results consistently matched the electron microscopic evidences for complex formation of LPS with polymyxin B. It is suggested that the disruptive effects of polymyxin B on LPS in the outer membrane of S. marcescens may be the explanation for the change in permeability barrier in the resistant cells and disorganization of the outer membrane and subsequent death in the sensitive cells. Furthermore, the ability of the LPS to complex with the polymyxin B molecules in resistant cells may be the basis of their resistance to the antibiotic.Polymyxin B, a cationic polypeptide antibiotic, interacts with various anionic cellular components of gram-negative bacteria such as lipopolysaccharide (LPS),1,1) phospholipids (PL),3-6) deoxyribonucleic acid (DNA) 7-9) and ribosomes.8-10) Polymyxin B treatment of LPS isolated from susceptible bacteria results in a breakdown of the LPS molecule (in vitro effect).11) When whole cells are treated with the antibiotic, blebs are formed on the outer membrane of the cell envelope7.12-14) and subsequently, disorganization of the outer membrane occurs if the cells are susceptible to the antibiotic.Although the formation of the blebs in the outer membrane has been suggested to be caused by an aggregation of the antibiotic with outer membrane components such as LPS and PL9) (in vivo effect), direct evidences for the formation of such aggregates or complexes have not been shown, nor was the sequence of events from the moment of complex formation to the disorganization of the outer membrane described. In this communication, we report the electrophoretic and electron microscopic evidences of the in vitro and in vivo formation of complexes between polymyxin B and LPS from cells of Serratia marcescens, which were either resistant or sensitive to the antibiotic.

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