Small Molecule Antibiotics Inhibit Distinct Stages of Bacterial Outer Membrane Protein Assembly

Several antibacterial compounds have recently been discovered that potentially inhibit the activity of BamA, an essential subunit of a heterooligomer (the barrel assembly machinery or BAM) that assembles outer membrane proteins (OMPs) in Gram-negative bacteria, but their mode of action is unclear. To address this issue, we examined the effect of three inhibitors on the biogenesis of a model E. coli OMP (EspP) in vivo. We found that darobactin potently inhibited the interaction of a conserved C-terminal sequence motif (the "β signal") with BamA, but had no effect on assembly if added at a postbinding stage. In contrast, Polyphor peptide 7 and MRL-494 inhibited both binding and at least one later step of assembly. Taken together with previous studies that analyzed the binding of darobactin and Polyphor peptide 7 to BamA in vitro, our results strongly suggest that the two compounds inhibit BAM function by distinct competitive and allosteric mechanisms. In addition to providing insights into the properties of the antibacterial compounds, our results also provide direct experimental evidence that supports a model in which the binding of the β signal to BamA initiates the membrane insertion of OMPs. IMPORTANCE There is a clear need to develop novel broad-spectrum antibiotics to address the global problem of antimicrobial resistance. Multiple compounds that have strong antibacterial activity have recently been described that appear to inhibit the activity of the barrel assembly machinery (BAM), an essential complex that catalyzes the assembly (i.e., folding and membrane insertion) of outer membrane proteins (OMPs) in all Gram-negative bacteria. We analyzed the effects of three of these compounds on OMP biogenesis in vivo and found that they inhibited different stages of the assembly process. Because these compounds have distinct modes of action, they can be used in combination to reduce the emergence of resistant strains. As a corollary, we obtained evidence that these compounds will be valuable tools in future studies on BAM function.