Antibiotics of the vancomycin group are shown to enhance their affinities for the bacterial cell wall by the devices of either dimerization (vancomycin and other glycopeptides which dimerize even more strongly) or use of a membrane anchor (teicoplanin); a chelate mechanism is suggested in both cases, as supported by antagonism experiments with the cell wall analog di-N-acetyl-L-Lys-D-Ala-D-Ala. These results may have implications for other binding processes which occur near membrane surfaces.Recent reviews of structure-activity relationships for antibiotics of the vancomycin group (10) have not considered dimerization of the antibiotics, which we now believe plays an important role in the mode of action (5,8,9,16). Here, we present results consistent with a mechanism by which dimeric (e.g., vancomycin [Fig. 1A]) and lipoylated (e.g., teicoplanin [ Fig. 1C]) glycopeptides enhance the binding affinities to their cellular targets by preferential location of the antibiotic near the site of cell wall biosynthesis.The antibacterial activities of vancomycin antibiotics result from their affinities for the Opeptidyl-D-Ala-D-Ala sequence present in the growing cell wall of gram-positive bacteria (11,14,18); bound antibiotic inhibits the transglycosylase and transpeptidase enzymes responsible for construction of the cell wall (13). While the antibiotic activity can generally be correlated with the association constant for the cell wall, as measured with the cell wall analog di-N-acetyl-L-Lys-D-Ala-D-Ala (DALAA), antibiotics which dimerize strongly show anomalously high in vitro activities (4, 6, 9).Back-to-back homodimers (Fig. 2) are formed by most antibiotics of the vancomycin group (with the exception of teicoplanin [see below]), as shown by nuclear magnetic resonance studies (4,8,16). Dimerization is promoted by structural epitopes which appear as additions to the basic heptapeptide motif, including chlorine atoms and sugars at two sites (8). Furthermore, dimerization increases the affinity for cell wall analogs by a factor of 1 to 10, as shown in nuclear magnetic resonance experiments (9); conversely, cell wall fragments enhance dimerization by factors of 2 to 100 (9).This evidence suggests that dimerization may have physiological significance in the action of antibiotics of the vancomycin group. This was investigated by determining the effects of the cell wall analog DALAA on the potencies of glycopeptide antibiotics exhibiting a range of dimerization constants by an agar diffusion assay by the methods of Rake et al. (12). Glycopeptides (1 g) and DALAA (1 to 200 g) were added as aqueous solutions to paper disks (6-mm diameter; Whatman AA), which were dried before being placed on the surfaces of agar plates (1-mm agar thickness); antibiotic medium 1 (Difco Laboratories, Detroit,) was inoculated with Bacillus subtilis ATCC 6633 (30-l spore suspension/10 ml of agar Difco Laboratories, Detroit, Mich.), which was used as the indicator organism. After incubation at 37ЊC for 18 h, inhibition zone diameters were measured. D...

The mode of action of a semisynthetic glycopeptide active against vancomycin-resistant bacteria has been investigated. It is shown that the antibiotic, biphenylchloroeremomycin or LY307599, dimerizes strongly and anchors to membranes. It is hypothesized that these two locating devices, previously identified by us when acting separately, might combine to give enhanced binding at a cell surface. This hypothesis is tested experimentally by showing that glycopeptides can bind cell-wall precursor analogues from resistant bacteria (terminating in -D-lactate) in a similar manner to those from susceptible bacteria (terminating in -D-alanine) and by using model cell surfaces where the benefits of dimerization can be expressed and studied. These model systems use vesicles to represent the cell membrane, to which cell wall analogues are anchored Via a docosanoyl chain, so mimicking the arrangement encountered at the cell surface. Using 1 H NMR spectroscopy, we demonstrate enhanced binding due to dimerization and propose that this enhancement will act cooperatively with membrane anchoring in biphenylchloroeremomycin.

The association of vancomycin group antibiotics with the growing bacterial cell wall was investigated by using the cell wall precursor analog di-N-acetyl-Lys-D-Ala-D-Ala in competition binding experiments. The affinities of the antibiotics for the -D-Ala-D-Ala-containing cell wall precursors of Bacillus subtilis ATCC 6633 (a model for vancomycin-susceptible gram-positive bacteria) and for the -D-Ala-D-Lac-containing cell wall precursors of Leuconostoc mesenteroides (a model for vancomycin-resistant strains of Enterococcus faecium and Enterococcus faecalis) were determined by a whole-cell assay. The binding of strongly dimerizing antibiotics such as eremomycin to the bacterial surface was thus shown to be enhanced by up to 2 orders of magnitude (relative to the binding in free solution) by the chelate effect, whereas weakly dimerizing antibiotics like vancomycin and antibiotics carrying lipid tails (teicoplanin) benefited less (ca. 1 order of magnitude). The affinity measured in this way correlates well with the MIC of the antibiotic, and a consequence of this is that future design of semisynthetic vancomycin-group antibiotics should attempt to incorporate chelate effect-enhancing structural features.

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