Gram-negative bacteria shield themselves from antibiotics by producing an outer membrane (OM) that forms a formidable permeability barrier. Multidrug resistance among these organisms is a particularly acute problem that is exacerbated by the OM. The poor penetrance of many available antibiotics prevents their clinical use, and efforts to discover novel classes of antibiotics against Gram-negative bacteria have been unsuccessful for almost 50 years. Recent insights into how the OM is built offer new hope. Several essential multiprotein molecular machines (Bam, Lpt, and Lol) work in concert to assemble the barrier and offer a swathe of new targets for novel therapeutic development. Murepavadin has been at the vanguard of these efforts, but its recently reported phase III clinical trial toxicity has tempered the anticipation of imminent new clinical options. Nonetheless, the many concerted efforts aimed at breaking down the OM barrier provide a source of ongoing optimism for what may soon come through the development pipeline. We will review the current state of drug development against the OM assembly targets, highlighting insightful new discovery approaches and strategies.

Gram-negative bacteria produce lipid-anchored lipoproteins that are trafficked to their outer membrane (OM). These lipoproteins are essential components in each of the molecular machines that build the OM, including the Bam machine that assembles β-barrel proteins and the Lpt pathway that transports lipopolysaccharide. Stress responses are known to monitor Bam and Lpt function, yet no stress system has been found that oversees the fundamental process of lipoprotein trafficking. We used genetic and chemical biology approaches to induce several different lipoprotein trafficking stresses in Escherichia coli. Our results identified the Cpx two-component system as a stress response for monitoring trafficking. Cpx is activated by trafficking defects and is required to protect the cell against the consequence of the resulting stress. The OM-targeted lipoprotein NlpE acts as a sensor that allows Cpx to gauge trafficking efficiency. We reveal that NlpE signals to Cpx while it is transiting the inner membrane (IM) en route to the OM and that only a small highly conserved N-terminal domain is required for signaling. We propose that defective trafficking causes NlpE to accumulate in the IM, activating Cpx to mount a transcriptional response that protects cells. Furthermore, we reconcile this new role of NlpE in signaling trafficking defects with its previously proposed role in sensing copper (Cu) stress by demonstrating that Cu impairs acylation of lipoproteins and, consequently, their trafficking to the OM. IMPORTANCE The outer membrane built by Gram-negative bacteria such as Escherichia coli forms a barrier that prevents antibiotics from entering the cell, limiting clinical options at a time of prevalent antibiotic resistance. Stress responses ensure that barrier integrity is continuously maintained. We have identified the Cpx signal transduction system as a stress response that monitors the trafficking of lipid-anchored lipoproteins to the outer membrane. These lipoproteins are needed by every machine that builds the outer membrane. Cpx monitors just one lipoprotein, NlpE, to detect the efficiency of lipoprotein trafficking in the cell. NlpE and Cpx were previously shown to play a role in resistance to copper. We show that copper blocks lipoprotein trafficking, reconciling old and new observations. Copper is an important element in innate immunity against pathogens, and our findings suggest that NlpE and Cpx help E. coli survive the assault of copper on a key outer membrane assembly pathway.

Gram-negative bacteria have an outer membrane, which acts as a protective barrier and excludes many antibiotics. The limited number of antibiotics active against Gram-negative bacteria, along with rising rates of antibiotic resistance, highlights the need for efficient antibiotic discovery efforts.

No abstract

No abstract

The outer membrane (OM) of Gram-negative bacteria is an essential organelle that acts as a formidable barrier to antibiotics. Increasingly prevalent resistance to existing drugs has exacerbated the need for antibiotic discovery efforts targeting the OM. Acylated proteins, known as lipoproteins, are essential in every pathway needed to build the OM. The central role of OM lipoproteins makes their biogenesis a uniquely attractive therapeutic target, but it also complicates in vivo identification of on-pathway inhibitors, as inhibition of OM lipoprotein biogenesis broadly disrupts OM assembly. Here, we use genetics to probe the eight essential proteins involved in OM lipoprotein biogenesis. We define a biological signature consisting of three simple assays that can characteristically identify OM lipoprotein biogenesis defects in vivo. The few known chemical inhibitors of OM lipoprotein biogenesis conform to the biological signature. We also examine MAC13243, a proposed inhibitor of OM lipoprotein biogenesis, and find that it fails to conform to the biological signature. Indeed, we demonstrate that MAC13243 activity relies entirely on a target outside of the OM lipoprotein biogenesis pathway. Hence, our signature offers simple tools to easily assess whether antibiotic lead compounds target an essential pathway that is the hub of OM assembly.

Autosomal dominant Hyper-IgE (HIES) syndrome is a primary immunodeficiency disorder characterized by elevated serum IgE levels, severe eczema and recurrent Staphylococcus aureus and Candida albicans skin infections.Dominant-negative mutations in STAT3 are found in majority of the HIES patients. However, the role of impaired STAT3 signaling in contributing to the increased IgE levels in HIES is not well understood. Herein, we used a mouse model of atopic dermatitis-like skin inflammation induced by S. aureus epicutaneous exposure in cre/lox mice with cell-specific deletion of STAT3 in keratinocytes (K5-cre ERT2 Â STAT3 fl/fl mice) and CD4 + T cells (CD4-cre ERT2 Â STAT3 fl/fl mice). We found that S. aureus epicutaneous exposure induced B cell expansion and markedly elevated serum IgE levels along with concomitant increases in serum IgG and IgA levels. Unexpectedly, mice with STAT3 deletion in keratinocytes but not in CD4 + T cells had significantly higher serum IgE levels compared with wildtype (WT) mice. CD4 + T cells were required for the IgE production, since CD4 + T cell depletion resulted in significantly reduced levels of serum IgE. Interestingly, we found S. aureus epicutaneous exposure induced significantly higher mRNA levels of IL-17A, IL-17F, IL-12p40, and IL-23p19 in the skin of mice with STAT3 deletion in keratinocytes compared with WT mice. Finally, S. aureus epicutaneous exposure of mice deficient in both IL-17A and IL-17F failed to increase serum IgE levels. Taken together, these results indicate that S. aureus epicutaneous exposure in the setting of impaired STAT3-signaling in keratinocytes enhanced IL-17A/F responses that resulted in elevated serum IgE production. This provides insights into a novel role of S. aureus epicutaneous exposure and STAT3-signaling in keratinocytes in regulating IgE production and provides a mechanistic explanation for the increased serum IgE levels in HIES.