Multidrug-resistant (MDR) bacterial infections are a serious threat to public health. Among the most alarming resistance trends is the rapid rise in the number and diversity of β-lactamases, enzymes that inactivate β-lactams, a class of antibiotics that has been a therapeutic mainstay for decades. Although several new β-lactamase inhibitors have been approved or are in clinical trials, their spectra of activity do not address MDR pathogens such as Acinetobacter baumannii. This report describes the rational design and characterization of expanded-spectrum serine β-lactamase inhibitors that potently inhibit clinically relevant class A, C and D β-lactamases and penicillin-binding proteins, resulting in intrinsic antibacterial activity against Enterobacteriaceae and restoration of β-lactam activity in a broad range of MDR Gram-negative pathogens. One of the most promising combinations is sulbactam-ETX2514, whose potent antibacterial activity, in vivo efficacy against MDR A. baumannii infections and promising preclinical safety demonstrate its potential to address this significant unmet medical need.

Fis, the most abundant DNA-binding protein in Escherichia coli during rapid growth, has been suspected to play an important role in defining nucleoid structure. Using bulk-phase and single-DNA molecule experiments, we analyze the structural consequences of non-specific binding by Fis to DNA. Fis binds DNA in a largely sequence-neutral fashion at nanomolar concentrations, resulting in mild compaction under applied force due to DNA bending. With increasing concentration, Fis first coats DNA to form an ordered array with one Fis dimer bound per 21 bp and then abruptly shifts to forming a higher-order Fis-DNA filament, referred to as a low-mobility complex (LMC). The LMC initially contains two Fis dimers per 21 bp of DNA, but additional Fis dimers assemble into the LMC as the concentration is increased further. These complexes, formed at or above 1 microM Fis, are able to collapse large DNA molecules via stabilization of DNA loops. The opening and closing of loops on single DNA molecules can be followed in real time as abrupt jumps in DNA extension. Formation of loop-stabilizing complexes is sensitive to high ionic strength, even under conditions where DNA bending-compaction is unaltered. Analyses of mutants indicate that Fis-mediated DNA looping does not involve tertiary or quaternary changes in the Fis dimer structure but that a number of surface-exposed residues located both within and outside the helix-turn-helix DNA-binding region are critical. These results suggest that Fis may play a role in vivo as a domain barrier element by organizing DNA loops within the E. coli chromosome.

A high-throughput phenotypic screen based on a Citrobacter freundii AmpC reporter expressed in Escherichia coli was executed to discover novel inhibitors of bacterial cell wall synthesis, an attractive, well-validated target for antibiotic intervention. Here we describe the discovery and characterization of sulfonyl piperazine and pyrazole compounds, each with novel mechanisms of action. E. coli mutants resistant to these compounds display no cross-resistance to antibiotics of other classes. Resistance to the sulfonyl piperazine maps to LpxH, which catalyzes the fourth step in the synthesis of lipid A, the outer membrane anchor of lipopolysaccharide (LPS). To our knowledge, this compound is the first reported inhibitor of LpxH. Resistance to the pyrazole compound mapped to mutations in either LolC or LolE, components of the essential LolCDE transporter complex, which is required for trafficking of lipoproteins to the outer membrane. Biochemical experiments with E. coli spheroplasts showed that the pyrazole compound is capable of inhibiting the release of lipoproteins from the inner membrane. Both of these compounds have significant promise as chemical probes to further interrogate the potential of these novel cell wall components for antimicrobial therapy. IMPORTANCEThe prevalence of antibacterial resistance, particularly among Gram-negative organisms, signals a need for novel antibacterial agents. A phenotypic screen using AmpC as a sensor for compounds that inhibit processes involved in Gram-negative envelope biogenesis led to the identification of two novel inhibitors with unique mechanisms of action targeting Escherichia coli outer membrane biogenesis. One compound inhibits the transport system for lipoprotein transport to the outer membrane, while the other compound inhibits synthesis of lipopolysaccharide. These results indicate that it is still possible to uncover new compounds with intrinsic antibacterial activity that inhibit novel targets related to the cell envelope, suggesting that the Gram-negative cell envelope still has untapped potential for therapeutic intervention.

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Vibrio cholerae, the cause of cholera, has two circular chromosomes. The parAB genes on each V. cholerae chromosome act to control chromosome segregation in a replicon-specific fashion. The chromosome I (ChrI) parAB genes (parAB1) govern the localization of the origin region of ChrI, while the chromosome II (ChrII) parAB genes (parAB2) control the segregation of ChrII. In addition to ParA and ParB proteins, Par systems require ParB binding sites (parS). Here we identified the parS sites on both V. cholerae chromosomes. We found three clustered origin-proximal ParB1 binding parS1 sites on ChrI. Deletion of these three parS1 sites abrogated yellow fluorescent protein (YFP)-ParB1 focus formation in vivo and resulted in mislocalization of the ChrI origin region. However, as observed in a parA1 mutant, mislocalization of the ChrI origin region in the parS1 mutant did not compromise V. cholerae growth, suggesting that additional (non-Par-related) mechanisms may mediate the partitioning of ChrI. We also identified 10 ParB2 binding parS2 sites, which differed in sequence from parS1. Fluorescent derivatives of ParB1 and ParB2 formed foci only with the cognate parS sequence. parABS2 appears to form a functional partitioning system, as we found that parABS2 was sufficient to stabilize an ordinarily unstable plasmid in Escherichia coli. Most parS2 sites were located within 70 kb of the ChrII origin of replication, but one parS2 site was found in the terminus region of ChrI. In contrast, in other sequenced vibrio species, the distribution of parS1 and parS2 sites was entirely chromosome specific.

cIn Gram-negative bacteria, lipoproteins are transported to the outer membrane by the Lol system. In this process, lipoproteins are released from the inner membrane by the ABC transporter LolCDE and passed to LolA, a diffusible periplasmic molecular chaperone. Lipoproteins are then transferred to the outer membrane receptor protein, LolB, for insertion in the outer membrane. Here we describe the discovery and characterization of novel pyridineimidazole compounds that inhibit this process. Escherichia coli mutants resistant to the pyridineimidazoles show no cross-resistance to other classes of antibiotics and map to either the LolC or LolE protein of the LolCDE transporter complex. The pyridineimidazoles were shown to inhibit the LolA-dependent release of the lipoprotein Lpp from E. coli spheroplasts. These results combined with bacterial cytological profiling are consistent with LolCDE-mediated disruption of lipoprotein targeting to the outer membrane as the mode of action of these pyridineimidazoles. The pyridineimidazoles are the first reported inhibitors of the LolCDE complex, a target which has never been exploited for therapeutic intervention. These compounds open the door to further interrogation of the outer membrane lipoprotein transport pathway as a target for antimicrobial therapy.T he most distinguishing feature of Gram-negative bacteria is their cell envelope, which is comprised of both an inner and an outer membrane bilayer. The outer membrane has a unique composition of lipoproteins, ␤-barrel proteins, lipopolysaccharides, and phospholipids. Lipoproteins, membrane proteins that are covalently modified with lipids, are involved in a variety of integral cellular functions, such as the synthesis and maintenance of the cell surface and the transport of substrates (reviewed in reference 1). Lipoproteins are synthesized as precursors in the cytoplasm. Upon transit across the inner membrane by either the Sec or Tat machinery, the export signal peptide is cleaved, and attached to its amino terminus is a lipid moiety. This lipid serves as a membrane anchor for the lipoprotein. Some lipoproteins remain in the outer leaflet of the inner membrane, while others must cross the hydrophilic periplasmic space to the outer membrane. This sorting of lipoproteins to the outer membrane is achieved by the Lol system, which consists of five proteins (Fig. 1) (reviewed in reference 1). In this process, lipoproteins destined for the outer membrane are released from the inner membrane by the LolCDE complex, an inner membrane ABC transporter. LolCDE transfers the lipoproteins to LolA, a diffusible periplasmic chaperone (2, 3). LolA then transfers the lipoprotein to LolB, the outer membrane lipoprotein receptor, which incorporates these lipoproteins into the inner leaflet of the outer membrane (1, 4). This is in contrast to Grampositive bacteria, which have a single membrane bilayer; therefore, localization of lipoproteins to the cell surface requires only export through the cytoplasmic membrane and acylation (5).Infections c...

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Multidrug resistant Gram-negative bacterial infections are an increasing public health threat due to rapidly rising resistance toward β-lactam antibiotics. The hydrolytic enzymes called β-lactamases are responsible for a large proportion of the resistance phenotype. β-Lactamase inhibitors (BLIs) can be administered in combination with β-lactam antibiotics to negate the action of the β-lactamases, thereby restoring activity of the β-lactam. Newly developed BLIs offer some advantage over older BLIs in terms of enzymatic spectrum but are limited to the intravenous route of administration. Reported here is a novel, orally bioavailable diazabicyclooctane (DBO) β-lactamase inhibitor. This new DBO, ETX1317, contains an endocyclic carbon–carbon double bond and a fluoroacetate activating group and exhibits broad spectrum activity against class A, C, and D serine β-lactamases. The ester prodrug of ETX1317, ETX0282, is orally bioavailable and, in combination with cefpodoxime proxetil, is currently in development as an oral therapy for multidrug resistant and carbapenem-resistant Enterobacterales infections.