The unrelenting rise of antimicrobial-resistant bacteria has necessitated the search for novel antibiotic solutions. Herein we describe further mechanistic studies on a 2.0-nm-diameter gold nanoparticle-based antibiotic (designated LAL-32). This antibiotic exhibits bactericidal activity against the Gram-negative bacterium Escherichia coli at 1.0 μM, a concentration significantly lower than several clinically available antibiotics (such as ampicillin and gentamicin), and acute treatment with LAL-32 does not give rise to spontaneous resistant mutants. LAL-32 treatment inhibits cellular division, daughter cell separation, and twin-arginine translocation (Tat) pathway dependent shuttling of proteins to the periplasm. Furthermore, we have found that the cedA gene imparts increased resistance to LAL-32, and shown that an E. coli cedA transposon mutant exhibits increased susceptibility to LAL-32. Taken together, these studies further implicate cell division pathways as the target for this nanoparticle-based antibiotic and demonstrate that there may be inherently higher barriers for resistance evolution against nanoscale antibiotics in comparison to their small molecule counterparts.

Antibiotic resistance is one of the greatest current threats to human health, and without significant action we face the chilling prospect of a world without effective antibiotics. Although continued effort toward the development of new antibiotics, particularly those with novel mechanisms of action, remains crucial, this alone probably will not be enough to prevail, and it is imperative that additional approaches are also explored. One such approach is the identification of adjuvants that augment the activity of current antibiotics. This approach has the potential to render an antibiotic against which bacteria have developed resistance once again effective, to broaden the spectrum of an antibiotic, and to lower the required dose of an antibiotic. In this viewpoint we discuss some of the advantages and disadvantages of the use of adjuvants, and describe various approaches to their identification.

Bacterial resistance to polymyxin antibiotics has taken on a new and more menacing form. Common are genomically-encoded resistance mechanisms to polymyxins, specifically colistin (polymyxin E), however, the plasmid-borne mobile colistin resistance-1 (mcr-1) gene has recently been identified and poses a new threat to global public health. Within six months of initial identification in Chinese swine in November 2015, the first human clinical isolation in the US was reported (Apr. 2016). Herein we report successful reversion of mcr-1-driven colistin resistance in Acinetobacter baumannii, Klebsiella pneumoniae, and Escherichia coli with adjuvants we previously reported as modulators of chromosomally-encoded colistin resistance. Further screening of our in-house library of nitrogen-dense heterocycles has identified additional chemical scaffolds that actively attenuate colistin resistance. Ultimately, we present a diverse cohort of adjuvants that both sensitize colistin-resistant and colistin-susceptible bacteria to this antibiotic, thus providing a potential avenue to both reduce colistin dosage and toxicity, and overcome colistin resistance.Download high-res image (130KB)Download full-size image

With only two new classes of antibiotics developed in the last 40 years, novel antibiotics are desperately needed to combat the growing problem of multidrug-resistant and extensively drug resistant bacteria, particularly Gram-negative bacteria. Described in this letter is the synthesis and antibiotic activity of 1,2,4-triazolidine-3-thiones as narrow spectrum antibiotics. Optimization of the 1,2,4-triazolidine-3-thione scaffold identified a small molecule with potent antibiotic activity against multiple strains of multidrug-resistant and extensively drug-resistant Acinetobacter baumannii. This small molecule also shows single dose, in vivo activity in a Galleria mellonella infection model with A. baumannii and represents a promising start in the development of a class of drugs that can target this bacterial pathogen.Keywords: Acinetobacter baumannii; antibiotic resistance; antibiotics;

A small molecule library consisting of 45 compounds was synthesized based on the bacterial metabolite ethyl N-(2-phenethyl) carbamate. Screening of the compounds revealed a potent analogue capabale of inhibiting several strains of Methicillin Resistant S. aureus biofilms with low to moderate micromolar IC50 values.

A screen of 20 compounds identified small molecule adjuvants capable of potentiating antibiotic activity against Francisella philomiragia. Analogue synthesis of an initial hit compound led to the discovery of a potentially new class of small molecule adjuvants containing an indole core. The lead compound was able to lower the MIC of colistin by 32-fold against intrinsically resistant F. philomiragia.

We recently reported a 2-aminoimidazole-based antibiotic adjuvant that reverses colistin resistance in two species of Gram-negative bacteria. Mechanistic studies in Acinetobacter baumannii demonstrated that this compound downregulated the PmrAB two-component system and abolished a lipid A modification that is required for colistin resistance. We now report the synthesis and evaluation of two separate libraries of substituted 2-aminoimidazole analogues based on this parent compound. From these libraries, a new small molecule was identified that lowers the minimum inhibitory concentration of colistin by up to 32-fold greater than the parent compound while also displaying less inherent bacterial toxicity, thereby minimizing the likelihood of resistance evolution.

Rising antibiotic resistance means that alternative antibacterial strategies are sorely needed. In this issue, Zhu et al. (2015) report the use of a Caenorhabditis elegans model to validate the Pseudomonas aeruginosa virulence factor LasB as a potential therapeutic target and to identify a LasB inhibitor with in vivo efficacy.

The emergence of resistance to multiple antimicrobial agents by pathogenic bacteria has become a significant global public health threat. Multi-drug-resistant (MDR) Gram-negative bacteria have become particularly problematic, as no new classes of small-molecule antibiotics for Gram-negative bacteria have emerged in over two decades. We have developed a combinatorial screening process for identifying mixed ligand monolayer/gold nanoparticle conjugates (2.4 nm diameter) with antibiotic activity. The method previously led to the discovery of several conjugates with potent activity against the Gram-negative bacterium Escherichia coli. Here we show that these conjugates are also active against MDR E. coli and MDR Klebsiella pneumoniae. Moreover, we have shown that resistance to these nanoparticles develops significantly more slowly than to a commercial small-molecule drug. These results, combined with their relatively low toxicity to mammalian cells and biocompatibility in vivo, suggest that gold nanoparticles may be viable new candidates for the treatment of MDR Gram-negative bacterial infections.

Antimicrobial drug discovery has slowed considerably over the last few decades. One major cause for concern is the lack of innovative approaches to treat infections caused by mycobacteria such as TB. Herein we demonstrate that our Small Molecule Variable Ligand Display (SMLVD) method for nanoparticle antibiotic discovery can be expanded around a ligand feed ratio parameter space to identify gold nanoparticle conjugates that are potent inhibitors of mycobacteria growth, with our most potent inhibitor able to reduce growth by five orders of magnitude at 8 μM.

Recent efforts toward combating antibiotic resistance in bacteria have focused on Gram-positive bacteria; however, multidrug-resistant Gram-negative bacteria pose a significant risk to public health. An orthogonal approach to the development of new antibiotics is to develop adjuvant compounds that enhance the susceptibility of drug-resistant strains of bacteria to currently approved antibiotics. This paper describes the synthesis and biological activity of a library of aryl amide 2-aminoimidazoles based on a lead structure from an initial screen. A small molecule was identified from this library that is capable of lowering the minimum inhibitory concentration of β-lactam antibiotics by up to 64-fold.

Infections caused by multi-drug resistant bacteria, particularly Gram-negative bacteria, are an ever-increasing problem. While the development of new antibiotics remains one option in the fight against bacteria that have become resistant to currently available antibiotics, an attractive alternative is the development of adjuvant therapeutics that restore the efficacy of existing antibiotics. We report a small molecule adjuvant that suppresses colistin resistance in multidrug resistant Acinetobacter baumannii and Klebsiella pneumoniae by interfering with the expression of a two-component system. The compound downregulates the pmrCAB operon and reverses phosphoethanolamine modification of lipid A responsible for colistin resistance. Furthermore, colistin-susceptible and colistin-resistant bacteria do not evolve resistance to combination treatment. This represents the first definitive example of a compound that breaks antibiotic resistance by directly modulating two-component system activity.

Aberrant gene expression is responsible for a myriad of human diseases from infectious diseases to cancer. Precise regulation of these genes via specific interactions with the DNA double helix could pave the way for novel therapeutics. Pyrrole–imidazole polyamides are small molecules capable of binding to pre-determined DNA sequences up to 16 base pairs with affinity and specificity comparable to natural transcription factors. In the three decades since their development, great strides have been made relating to synthetic accessibility and improved sequence specificity and binding affinity. This perspective presents a brief history of early seminal developments in the field and highlights recent reports of the utility of polyamides as both genetic modulators and molecular probes.

Herein is described a method of accessing indole–triazole and benzothiophene–triazole analogues that selectively promote or inhibit biofilm formation by Gram-positive and Gram-negative bacteria. Structure/function studies revealed that the addition of a bromine atom at the 2-position of the indole–triazole scaffold altered activity against both Gram-negative and Gram-positive bacteria and could transform a biofilm inhibitor into a biofilm inducer. Isosteric replacement of the indole core by a benzothiophene significantly impaired anti-biofilm activity. A competition assay exposing Escherichia coli to the most potent biofilm inducer and an inhibitor of E. coli biofilm formation was performed. The inducer exhibited the ability to mute the effect of the anti-biofilm compound for this targeted bacterial population.

β-Lactam antibiotics are one of the most important antibiotic classes but are plagued by problems of resistance, and the development of new β-lactam antibiotics through side-chain modification of existing β-lactam classes is not keeping pace with resistance development. In this JOCSynopsis, we summarize small molecule strategies to overcome resistance to β-lactam antibiotics. These approaches include the development of β-lactamase inhibitors and compounds that interfere with the ability of the bacteria to sense an antibiotic threat and activate their resistance mechanisms.

The already considerable global public health threat of multidrug-resistant Gram-negative bacteria has become even more of a concern following the emergence of New Delhi metallo-β-lactamase (NDM-1) producing strains of Klebsiella pneumoniae and other Gram-negative bacteria. As an alternative approach to the traditional development of new bactericidal entities, we have identified a 2-aminoimidazole-derived small molecule that acts as an antibiotic adjuvant and is able to suppress resistance of a NDM-1 producing strain of K. pneumoniae to imipenem and meropenem, in addition to suppressing resistance of other β-lactam nonsusceptible K. pneumoniae strains. The small molecule is able to lower carbapenem minimum inhibitory concentrations by up to 16-fold, while exhibiting little bactericidal activity itself.Keywords: 2-aminoimidazole; antibiotic adjuvant; Klebsiella pneumoniae; NDM-1; resistance suppression

A diverse 23-compound library of N-1 substituted 2-aminobenzimidazoles was synthesized via an efficient three-step process. This small library produced several non-toxic biofilm modulators of two strains of MRSA. Preliminary mechanistic studies reveal a zinc-dependent mode of action for these compounds.

Indole signaling is one of the putative universal signaling networks in bacteria. We have investigated the use of desformylflustrabromine (dFBr) derivatives for the inhibition of biofilm formation through modulation of the indole-signaling network in Escherichia coli and Staphylococcus aureus. We have found dFBr derivatives that are 10–1000 times more active than indole itself, demonstrating that the flustramine family of indolic natural products represent a privileged scaffold for the design of molecules to control pathogenic bacterial behavior.

•Biofilms are inherently resistant to antibiotics and are a major health threat.•Quorum sensing antagonists inhibit biofilm formation.•Inhibition of two-component systems disrupts biofilm formation.•Approaches to target the extracellular matrix to disrupt biofilms are discussed.Infections caused by bacterial biofilms are a significant global health problem, causing considerable patient morbidity and mortality and contributing to the economic burden of infectious disease. This review describes diverse strategies to combat bacterial biofilms, focusing firstly on small molecule interference with bacterial communication and signaling pathways, including quorum sensing and two-component signal transduction systems. Secondly we discuss enzymatic approaches to the degradation of extracellular matrix components to effect biofilm dispersal. Both of these approaches are based upon non-microbicidal mechanisms of action, and thereby do not place a direct evolutionary pressure on the bacteria to develop resistance. Such approaches have the potential to, in combination with conventional antibiotics, play an important role in the eradication of biofilm based bacterial infections.

The increasing prevalence of infections caused by multidrug-resistant bacteria is a global health problem that has been exacerbated by the dearth of novel classes of antibiotics entering the clinic over the past 40 years. Herein, we describe recent developments toward combination therapies for the treatment of multidrug-resistant bacterial infections. These efforts include antibiotic–antibiotic combinations, and the development of adjuvants that either directly target resistance mechanisms such as the inhibition of β-lactamase enzymes, or indirectly target resistance by interfering with bacterial signaling pathways such as two-component systems (TCSs). We also discuss screening of libraries of previously approved drugs to identify nonobvious antimicrobial adjuvants.Highlights► Combination approaches to combat multidrug-resistant bacteria. ► The problem of drug-resistant bacterial infections continues to worsen. ► Bacteria will inevitably acquire resistance against new antibiotics. ► Antibiotic–adjuvant combinations are an attractive approach to treat infections.

Novel approaches that do not rely upon developing microbicidal compounds are sorely needed to combat multidrug resistant (MDR) bacteria. The potential of marine secondary metabolites to serve as a source of non-traditional anti-bacterial agents is demonstrated by showing that pyrrole-imidazole alkaloids inhibit biofilm formation and suppress antibiotic resistance.

A small molecule library consisting of 45 compounds was synthesized based on the bacterial metabolite ethyl N-(2-phenethyl) carbamate. Screening of the compounds revealed a potent analogue capabale of inhibiting several strains of Methicillin Resistant S. aureus biofilms with low to moderate micromolar IC50 values.

Antimicrobial drug discovery has slowed considerably over the last few decades. One major cause for concern is the lack of innovative approaches to treat infections caused by mycobacteria such as TB. Herein we demonstrate that our Small Molecule Variable Ligand Display (SMLVD) method for nanoparticle antibiotic discovery can be expanded around a ligand feed ratio parameter space to identify gold nanoparticle conjugates that are potent inhibitors of mycobacteria growth, with our most potent inhibitor able to reduce growth by five orders of magnitude at 8 μM.