The effects of subinhibitory concentrations of different beta-lactam antibiotics and one quinolone on the sedimentation of outer membranes (OMs) of Escherichia coli and on the qualitative properties and immunogenicity of OM components were studied. Membranes were prepared by osmotic lysis of plasmolyzed bacteria. OM and cytoplasmic membrane vesicles were separated by sucrose density ultracentrifugation. Two peaks of OM vesicles with different buoyant densities could be isolated; the quantitative contribution of these to the total OM varied, depending upon the growth phase. In early log phase, the OM consisted mainly of lighter material; in late log and stationary phases, the OM consisted mainly of heavier material. Moxalactam, imipenem, and ciprofloxacin inhibited the formation of heavier material in all growth phases. The immunogenicity of OM vesicles was tested in mice by the hemolytic plaque test. The lighter OM material was markedly less immunogenic than the heavier OM material. The vesicles from antibiotic-treated bacteria and those from early-log-phase cells were less immunogenic than vesicles from untreated late-log-phase and stationary-phase bacteria. These changes were found for the immune response against lipopolysaccharides, as well as against OM proteins. Thus, the immunogenicity of OM components seems to be dependent upon the quantitative composition of lighter and heavier compounds, which is strongly influenced by growth phase and treatment with certain antibiotics
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