Signed, Sealed, Delivered: Conjugate and Prodrug Strategies as Targeted Delivery Vectors for Antibiotics

Site-Specific Antimicrobial Activity of a Dual-Responsive Ciprofloxacin Prodrug

Bacterial infections create distinctive microenvironments with a unique mix of metabolites and enzymes compared with healthy tissues that can be used to trigger the activation of antibiotic prodrugs. Here, a single and dual prodrug masking the C3 carboxylate and C7 piperazine of the fluoroquinolone, ciprofloxacin, responsive to nitroreductase (NTR) and/or hydrogen sulfide (H2S), was developed. Masking both functional groups reduced the activity of the prodrug against Staphylococcus aureus and Escherichia coli, increasing its minimum inhibitory concentration (MIC) by ∼512-fold (S. aureus) and ∼8000-fold (E. coli strains), while masking a single group only increased the MIC by ∼128-fold. Bacteria subjected to prolonged prodrug exposure did not show any increase in resistance. Triggering assays demonstrated the conversion of prodrugs to ciprofloxacin, and in a murine infection model, responsive prodrugs showed antibacterial activity comparable to that of ciprofloxacin, suggesting in vivo activation of prodrugs. Thus, the potential for site-specific antibiotic treatment with reduced threat of resistance is demonstrated.

Strategies for the eradication of intracellular bacterial pathogens

Intracellular pathogens affect a significant portion of world population and cause millions of deaths each year. They can invade host cells and survive inside them and are extremely resistant to immune systems and antibiotics. Current treatments have limitations, and therefore, new effective therapies are needed to combat this ongoing health challenge. Active research efforts have been made to develop many new strategies to eradicate these intracellular pathogens. In this review, we focus on the intracellular bacterial pathogens and first introduce several representative intracellular bacteria and the diseases they cause. We then discuss the challenges in eradicating these bacteria and summarize the current therapeutics for intracellular bacteria. Finally, recent advances in intracellular bacteria eradication are highlighted.

Peptide-Drug Conjugates: An Emerging Direction for the Next Generation of Peptide Therapeutics

Building on recent advances in peptide science, medicinal chemists have developed a hybrid class of bioconjugates, called peptide-drug conjugates, that demonstrate improved efficacy compared to peptides and small molecules independently. In this Perspective, we discuss how the conjugation of synergistic peptides and small molecules can be used to overcome complex disease states and resistance mechanisms that have eluded contemporary therapies because of their multi-component activity. We highlight how peptide-drug conjugates display a multi-factor therapeutic mechanism similar to that of antibody-drug conjugates but also demonstrate improved therapeutic properties such as less-severe off-target effects and conjugation strategies with greater site-specificity. The many considerations that go into peptide-drug conjugate design and optimization, such as peptide/small-molecule pairing and chemo-selective chemistries, are discussed. We also examine several peptide-drug conjugate series that demonstrate notable activity toward complex disease states such as neurodegenerative disorders and inflammation, as well as viral and bacterial targets with established resistance mechanisms.

Multigram-scale chemoenzymatic synthesis of diverse aminopolycarboxylic acids as potential metallo-β-lactamase inhibitors

Toxin A, a precursor to naturally occurring aspergillomarasmine A, aspergillomarasmine B, lycomarasmine and related aminopolycarboxylic acids, was synthesized as the desired (2S,2'S)-diastereomer on a multigram-scale (>99% conversion, 82% isolated yield, dr > 95 : 5) from commercially available starting materials using the enzyme ethylenediamine-N,N'-disuccinic acid lyase. A single-step protection route of this chiral synthon was developed to aid N-sulfonylation/-alkylation and reductive amination at the terminal primary amine for easy derivatization, followed by global deprotection to give the corresponding toxin A derivatives, including lycomarasmine, in moderate to good yields (23-66%) and with high stereopurity (dr > 95 : 5). Furthermore, a chemoenzymatic route was developed to introduce a click handle on toxin A (yield 72%, dr > 95 : 5) and its cyclized congener for further analogue design. Finally, a chemoenzymatic route towards the synthesis of photocaged aspergillomarasmine B (yield 8%, dr > 95 : 5) was established, prompting further steps into smart prodrug design and precision delivery. These new synthetic methodologies have the prospective of facilitating research into the finding of more selective and potent metallo-β-lactamase (MBL) inhibitors, which are urgently needed to combat MBL-based infections.

Detection of intact vancomycin-arginine as the active antibacterial conjugate in E. coli by whole-cell solid-state NMR

The introduction of new and improved antibacterial agents based on facile synthetic modifications of existing antibiotics represents a promising strategy to deliver urgently needed antibacterial candidates to treat multi-drug resistant bacterial infections. Using this strategy, vancomycin was transformed into a highly active agent against antibiotic-resistant Gram-negative organisms in vitro and in vivo through the addition of a single arginine to yield vancomycin-arginine (V-R). Here, we report detection of the accumulation of V-R in E. coli by whole-cell solid-state NMR using 15N-labeled V-R. 15N CPMAS NMR revealed that the conjugate remained fully amidated without loss of arginine, demonstrating that intact V-R represents the active antibacterial agent. Furthermore, C{N}REDOR NMR in whole cells with all carbons at natural abundance 13C levels exhibited the sensitivity and selectivity to detect the directly bonded 13C-15N pairs of V-R within E. coli cells. Thus, we also present an effective methodology to directly detect and evaluate active drug agents and their accumulation within bacteria without the need for potentially perturbative cell lysis and analysis protocols.

Siderophores: Chemical Tools for Precise Antibiotic Delivery

The success of precision medicine coupled with the disappointing impact of broad-spectrum antibiotic use on microbiome stability and bacterial resistance, has triggered a shift in antibiotic design strategies toward precision antibiotics. This also includes the implementation of novel vectorization approaches directed to improve the internalization of antibacterial agents into deadly gram-negative pathogens through precise and well-defined mechanisms. The conjugation of antibiotics to siderophores (iron scavengers), which are compounds that are able to afford stable iron-complexes that facilitate the internalization into the cell by using bacterial iron uptake pathways as gateways, is a strategy that has begun to show excellent results with the commercialization of the first antibiotic based on this principle, cefiderocol. This digests review provides an overview of the molecular basis for this antibiotic-siderophore conjugation approach, along with recent successful examples and highlights future challenges facing this booming research area.

Fatty acids as molecular carriers in cleavable antifungal conjugates

Conjugates composed of C_2-18 fatty acid (FA) residues as a molecular carrier and 5-fluorocytosine (5-FC) as an active agent, released upon the action of intracellular esterases on the ester bond between FA and "trimethyl lock" intramolecular linker, demonstrate good in vitro activity against human pathogenic yeasts of Candida spp. The minimal inhibitory concentrations (MIC) values for the most active conjugates containing caprylic (C₈), capric (C₁₀), lauric (C₁₂), or myristic (C₁₄) acid residues were in the 2-64 μg mL^-1 range, except for these against the least susceptible Candida krusei. The least active conjugates containing C₂, C_16, or C₁₈ FA were slowly hydrolyzed by esterase and probably poorly taken up by Candida cells, as found for their analogs containing a fluorescent label, Nap-NH₂ instead of 5-FC.

Synthesis of an Antimicrobial Enterobactin-Muraymycin Conjugate for Improved Activity Against Gram-Negative Bacteria

Overcoming increasing antibiotic resistance requires the development of novel antibacterial agents that address new targets in bacterial cells. Naturally occurring nucleoside antibiotics (such as muraymycins) inhibit the bacterial membrane protein MraY, a clinically unexploited essential enzyme in peptidoglycan (cell wall) biosynthesis. Even though a range of synthetic muraymycin analogues has already been reported, they generally suffer from limited cellular uptake and a lack of activity against Gram-negative bacteria. We herein report an approach to overcome these hurdles: a synthetic muraymycin analogue has been conjugated to a siderophore, i. e. the enterobactin derivative Ent_KL , to increase the cellular uptake into Gram-negative bacteria. The resultant conjugate showed significantly improved antibacterial activity against an efflux-deficient E. coli strain, thus providing a proof-of-concept of this novel approach and a starting point for the future optimisation of such conjugates towards potent agents against Gram-negative pathogens.

Antibody-Antimicrobial Conjugates for Combating Antibiotic Resistance

As the development of new antibiotics lags far behind the emergence of drug-resistant bacteria, alternative strategies to resolve this dilemma are urgently required. Antibody-drug conjugate is a promising therapeutic platform to delivering cytotoxic payloads precisely to target cells for efficient disease treatment. Antibody-antimicrobial conjugates (AACs) have recently attracted considerable interest from researchers as they can target bacteria in the target sites and improve the effectiveness of drugs (i.e., reduced drug dosage and adverse effects), abating the upsurge of antimicrobial resistance. In this review, the selection and progress of three essential blocks that compose the AACs: antibodies, antimicrobial payloads, and linkers are discussed. The commonly used conjugation strategies and the latest applications of AACs in recent years are also summarized. The challenges and opportunities of this booming technology are also discussed at the end of this review.

Inhibitors of glucosamine-6-phosphate synthase as potential antimicrobials or antidiabetics - synthesis and properties

Glucosamine-6-phosphate synthase (GlcN-6-P synthase) is known as a promising target for antimicrobial agents and antidiabetics. Several compounds of natural or synthetic origin have been identified as inhibitors of this enzyme. This set comprises highly selective l-glutamine, amino sugar phosphate or transition state intermediate cis-enolamine analogues. Relatively low antimicrobial activity of these inhibitors, poorly penetrating microbial cell membranes, has been improved using the pro-drug approach. On the other hand, a number of heterocyclic and polycyclic compounds demonstrating antimicrobial activity have been presented as putative inhibitors of the enzyme, based on the results of molecular docking to GlcN-6-P synthase matrix. The most active compounds of this group could be considered promising leads for development of novel antimicrobial drugs or antidiabetics, provided their selective toxicity is confirmed.

Strategies and progresses for enhancing targeted antibiotic delivery

Antibiotic resistance is a global health issue and a potential risk for society. Antibiotics administered through conventional formulations are devoid of targeting effect and often spread to various undesired body sites, leading to sub-lethal concentrations at the site of action and thus resulting in emergence of resistance, as well as side effects. Moreover, we have a very slim antibiotic pipeline. Drug-delivery systems have been designed to control the rate, time, and site of drug release, and innovative approaches for antibiotic delivery provide a glint of hope for addressing these issues. This review elaborates different delivery strategies and approaches employed to overcome the limitations of conventional antibiotic therapy. These include antibiotic conjugates, prodrugs, and nanocarriers for local and targeted antibiotic release. In addition, a wide range of stimuli-responsive nanocarriers and biological carriers for targeted antibiotic delivery are discussed. The potential advantages and limitations of targeted antibiotic delivery strategies are described along with possible solutions to avoid these limitations. A number of antibiotics successfully delivered through these approaches with attained outcomes and potentials are reviewed.

Fluoroquinolone Derivatives in the Treatment of Mycobacterium tuberculosis Infection

Tuberculosis (TB) is currently one of the leading causes of death due to infective agents, and the growing rate of multidrug-resistant tuberculosis (MDR TB) cases poses an emergent public health threat. Fluoroquinolones are commonly used in the treatment of both MDR TB and drug-sensitive tuberculosis patients who are intolerant to first-line antitubercular agents. Unfortunately, these drugs have mild side effects, relevant to the prolonged treatment regimens and diminished bioavailability due to binding of metal ions. Moreover, the resistance to fluoroquinolones is also on the rise, a characteristic of extensively drug-resistant TB (XDR TB). Here, we developed esters as prodrugs of the fluoroquinolones levofloxacin and ciprofloxacin, with long-chain fatty alcohols. Both the alcohols and the quinolone have previously shown antimycobacterial activity and the aim was to develop esters with improved lipophilicity and capable of delivering the free acid inside mycobacterial cells. The carboxylic acid group of fluoroquinolones is essential to the mode of action but is also responsible for many of its side effects and metal-chelating properties. The synthesis, stability in biological media, and antibacterial activity were evaluated, the latter not only against Mycobacterium tuberculosis but also against other clinically relevant bacterial species, since the parent compounds display a broad spectrum of activity. The biological results show a reduction in the antitubercular activity of the synthesized derivatives, probably due to deficient activation of the ester prodrug. Despite this, it was found that the derivatives exhibit bioactivity against other fluoroquinolone-resistant bacteria, indicating a different mode of action and suggesting that it may be worthwhile to research further modifications to the carboxylic acid group. This might lead to new compounds that are efficient against resistant strains. This idea that the compounds may act by a different mechanism of action was further supported by a brief computer investigation that demonstrated the potential lack of selectivity of the esters to the fluoroquinolone target.

New Bioprecursor Prodrugs of Sulfadiazine: Synthesis, X-ray Structure and Hirshfeld Analysis

Sulphonamide motif is found extensively in numerous chemotherapeutic drug candidates, it acts by stopping the production of folate inside the bacterial cell. Current research has established the synthesis and characterization of new bioprecursor prodrugs of sulfadiazine. The first prodrug, 3, was synthesized via the coupling of diazonium salt of sulfadiazine with ethyl acetoacetate in AcONa at 0 °C. The second prodrug, sulfadiazine-pyrazole, 5, was furnished via cyclocondensation of the hydrazono derivative, 3, and 2-pyridyl hydrazine, 4. The generated data from the X-ray analysis is interpreted and refined to obtain the crystal structure of the target compound, 5. Density functional theory (DFT) method was used to calculate the optimized geometrical parameters, electronic state (HOMO–LUMO), and the electronic properties. Moreover, Hirshfeld analysis revealed that the most important contributions to the crystal packing of the prodrug 5 are H···H, O···H and H···C contacts.

An allosteric inhibitor of bacterial Hsp70 chaperone potentiates antibiotics and mitigates resistance

DnaK is the bacterial homolog of Hsp70, an ATP-dependent chaperone that helps cofactor proteins to catalyze nascent protein folding and salvage misfolded proteins. In the pathogen Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), DnaK and its cofactors are proposed antimycobacterial targets, yet few small-molecule inhibitors or probes exist for these families of proteins. Here, we describe the repurposing of a drug called telaprevir that is able to allosterically inhibit the ATPase activity of DnaK and to prevent chaperone function by mimicking peptide substrates. In mycobacterial cells, telaprevir disrupts DnaK- and cofactor-mediated cellular proteostasis, resulting in enhanced efficacy of aminoglycoside antibiotics and reduced resistance to the frontline TB drug rifampin. Hence, this work contributes to a small but growing collection of protein chaperone inhibitors, and it demonstrates that these molecules disrupt bacterial mechanisms of survival in the presence of different antibiotic classes.

Dual-targeted anti-CMV/anti-HIV-1 heterodimers

Despite the development of efficient anti-human immunodeficiency virus-1 (HIV-1) therapy, HIV-1 associated pathogens remain a major clinical problem. Human cytomegalovirus (CMV) is among the most common HIV-1 copathogens and one of the main causes of persistent immune activation associated with dysregulation of the immune system, cerebrovascular and cardiovascular pathologies, and premature aging. Here, we report on the development of dual-targeted drugs with activity against both HIV-1 and CMV. We synthesized seven compounds that constitute conjugates of molecules that suppress both pathogens. We showed that all seven compounds exhibit low cytotoxicity and efficiently inhibited both viruses in cell lines. Furthermore, we chose a representative compound and demonstrated that it efficiently suppressed replication of HIV-1 and CMV in human lymphoid tissue ex vivo coinfected with both viruses. Further development of such compounds may lead to the development of dual-targeted anti-CMV/HIV-1 drugs.

Biomimetic enterobactin analogue mediates iron-uptake and cargo transport into E. coli and P. aeruginosa

The design, synthesis and biological evaluation of the artificial enterobactin analogue EntKL and several fluorophore-conjugates thereof are described. EntKL provides an attachment point for cargos such as fluorophores or antimicrobial payloads. Corresponding conjugates are recognized by outer membrane siderophore receptors of Gram-negative pathogens and retain the natural hydrolyzability of the tris-lactone backbone. Initial density-functional theory (DFT) calculations of the free energies of solvation (ΔG(sol)) and relaxed Fe–O force constants of the corresponding [Fe-EntKL]3− complexes indicated a similar iron binding constant compared to natural enterobactin (Ent). The synthesis of EntKL was achieved via an iterative assembly based on a 3-hydroxylysine building block over 14 steps with an overall yield of 3%. A series of growth recovery assays under iron-limiting conditions with Escherichia coli and Pseudomonas aeruginosa mutant strains that are defective in natural siderophore synthesis revealed a potent concentration-dependent growth promoting effect of EntKL similar to natural Ent. Additionally, four cargo-conjugates differing in molecular size were able to restore growth of E. coli indicating an uptake into the cytosol. P. aeruginosa displayed a stronger uptake promiscuity as six different cargo-conjugates were found to restore growth under iron-limiting conditions. Imaging studies utilizing BODIPY_FL-conjugates, demonstrated the ability of EntKL to overcome the Gram-negative outer membrane permeability barrier and thus deliver molecular cargos via the bacterial iron transport machinery of E. coli and P. aeruginosa.

The design, synthesis and evaluation of the enterobactin derivative (AcO)EntKL is reported, which mediates iron uptake and cargo transport into E. coli and P. aeruginosa and was able to compete with human enterobactin and iron binding proteins.

Pyochelin Biosynthetic Metabolites Bind Iron and Promote Growth in Pseudomonads Demonstrating Siderophore-like Activity

Pseudomonads employ several strategies to sequester iron vital for their survival including the use of siderophores such as pyoverdine and pyochelin. Similar in structure but significantly less studied are pyochelin biosynthetic byproducts, dihydroaeruginoic acid, aeruginoic acid, aeruginaldehyde (IQS), and aeruginol, along with two other structurally related molecules, aerugine and pyonitrins A-D, which have all been isolated from numerous Pseudomonad extracts. Because of the analogous substructure of these compounds to pyochelin, we hypothesized that they may play a role in iron homeostasis or have a biological effect on other bacterial species. Herein, we discuss the physiochemical evaluation of these molecules and disclose, for the first time, their ability to bind iron and promote growth in Pseudomonads.

Synthetic strategies in construction of organic macromolecular carrier-drug conjugates

Many metabolic inhibitors, considered potential antimicrobial or anticancer drug candidates, exhibit very limited ability to cross the biological membranes of target cells. The restricted cellular penetration of those molecules is often due to their highhydrophilicity. One of the possible solutions to this problem is a conjugation of an inhibitor with a molecular organic nanocarrier. The conjugate thus formed should be able to penetrate the membrane(s) by direct translocation, endocytosis or active transport mechanisms and once internalized, the active component could reach its intracellular target, either after release from the conjugate or in an intact form. Several such nanocarriers have been proposed so far, including macromolecular systems, carbon nanotubes and dendrimers. Herein, we present a comprehensive review of the current status of rational design and synthesis of macromolecular organic nanocarrier-drug conjugates, with special attention focused on the mode of coupling of a nanocarrier moiety with a "cargo" molecule through linking fragments of non-cleavable or cleavable type.

Synthesis of Vitamin B12-Antibiotic Conjugates with Greatly Improved Activity against Gram-Negative Bacteria

There is an urgent need to discover new antibiotics and improve the efficacy of known antibiotics against Gram-negative bacteria. "Trojan horse" conjugates are novel and promising antibiotics. Herein we report the design and synthesis of vitamin-B₁₂-ampicillin conjugates, which exhibited more than 500 times improved activity against Escherichia coli compared with ampicillin itself. Our studies demonstrate that the vitamin-B₁₂ uptake pathway could be employed for effective antibiotic delivery and efficacy enhancement.

Metabolic Efficacy of Phosphate Prodrugs and the Remdesivir Paradigm

Drugs that contain phosphates (and phosphonates or phosphinates) have intrinsic absorption issues and are therefore often delivered in prodrug forms to promote their uptake. Effective prodrug forms distribute their payload to the site of the intended target and release it efficiently with minimal byproduct toxicity. The ability to balance unwanted payload release during transit with desired release at the site of action is critical to prodrug efficacy. Despite decades of research on prodrug forms, choosing the ideal prodrug form remains a challenge which is often solved empirically. The recent emergency use authorization of the antiviral remdesivir for COVID-19 exemplifies a new approach for delivery of phosphate prodrugs by parenteral dosing, which minimizes payload release during transit and maximizes tissue payload distribution. This review focuses on the role of metabolic activation in efficacy during oral and parenteral dosing of phosphate, phosphonate, and phosphinate prodrugs. Through examining prior structure–activity studies on prodrug forms and the choices that led to development of remdesivir and other clinical drugs and drug candidates, a better understanding of their ability to distribute to the planned site of action, such as the liver, plasma, PBMCs, or peripheral tissues, can be gained. The structure–activity relationships described here will facilitate the rational design of future prodrugs.

New strategies and structural considerations in development of therapeutics for carbapenem-resistant Enterobacteriaceae

Antimicrobial resistance poses a significant threat to our ability to treat infections. Especially concerning is the emergence of carbapenem-resistant Enterobacteriaceae (CRE). In the new 2019 United States Centers for Disease Control and Prevention Antibiotic Resistance Report, CRE remain in the most urgent antimicrobial resistance threat category. There is good reason for this concerning designation. In particular, the combination of several resistance elements in CRE can make these pathogens untreatable or effectively untreatable with our current armamentarium of anti-infective agents. This article reviews recently approved agents with activity against CRE and a range of modalities in the pipeline, from early academic investigation to those in clinical trials, with a focus on structural aspects of new antibiotics. Another article in this series addresses the need to incentive pharmaceutical companies to invest in CRE antimicrobial development and to encourage hospitals to make these agents available in their formularies. This article will also consider the need for change in requirements for antimicrobial susceptibility testing implementation in clinical laboratories to address practical roadblocks that impede our efforts to provide even existing CRE antibiotics to our patients.

Concanavalin A-Rose Bengal bioconjugate for targeted Gram-negative antimicrobial photodynamic therapy

Photodynamic therapy (PDT) is considered a very promising therapeutic modality for antimicrobial therapy. Although several studies have demonstrated that Gram-positive bacteria are very sensitive to PDT, Gram-negative bacteria are more resistant to photodynamic action. This difference is due to a different cell wall structure. Gram-negative bacteria have an outer cell membrane containing lipopolysaccharides (LPS) that hinder the binding of photosensitizer molecules, protecting the bacterial cells from chemical attacks. Combination of the lipopolysaccharides-binding activity of Concanavalin A (ConA) with the photodynamic properties of Rose Bengal (RB) holds the potential of an innovative protein platform for targeted photodynamic therapy against Gram-negative bacteria. A ConA-RB bioconjugate was synthesized and characterized. Approximately 2.4 RB molecules were conjugated per ConA monomer. The conjugation of RB to ConA determines a decrease of the singlet oxygen generation and an increase of superoxide and peroxide production. The photokilling efficacy of the ConA-RB bioconjugate was demonstrated in a planktonic culture of E. coli. Irradiation with white light from a LED lamp produced a dose-dependent photokilling of bacteria. ConA-RB conjugates exhibited a consistent improvement over RB (up to 117-fold). The improved uptake of the photosensitizer explains the enhanced PDT effect accompanying increased membrane damages induced by the ConA-RB conjugate. The approach can be readily generalized (i) using different photo/sonosensitizers, (ii) to target other pathogens characterized by cell membranes containing lipopolysaccharides (LPS).

Antibiotics in the clinical pipeline in October 2019

The development of new and effective antibacterial drugs to treat multi-drug resistant (MDR) bacteria, especially Gram-negative (G−ve) pathogens, is acknowledged as one of the world’s most pressing health issues; however, the discovery and development of new, nontoxic antibacterials is not a straightforward scientific task, which is compounded by a challenging economic model. This review lists the antibacterials, β-lactamase/β-lactam inhibitor (BLI) combinations, and monoclonal antibodies (mAbs) first launched around the world since 2009 and details the seven new antibiotics and two new β-lactam/BLI combinations launched since 2016. The development status, mode of action, spectra of activity, lead source, and administration route for the 44 small molecule antibacterials, eight β-lactamase/BLI combinations, and one antibody drug conjugate (ADC) being evaluated in worldwide clinical trials at the end of October 2019 are described. Compounds discontinued from clinical development since 2016 and new antibacterial pharmacophores are also reviewed. There has been an increase in the number of early stage clinical candidates, which has been fueled by antibiotic-focused funding agencies; however, there is still a significant gap in the pipeline for the development of new antibacterials with activity against β-metallolactamases, orally administered with broad spectrum G−ve activity, and new treatments for MDR Acinetobacter and gonorrhea.

Constructing and deconstructing the bacterial cell wall

The history of modern medicine cannot be written apart from the history of the antibiotics. Antibiotics are cytotoxic secondary metabolites that are isolated from Nature. The antibacterial antibiotics disproportionately target bacterial protein structure that is distinct from eukaryotic protein structure, notably within the ribosome and within the pathways for bacterial cell-wall biosynthesis (for which there is not a eukaryotic counterpart). This review focuses on a pre-eminent class of antibiotics-the β-lactams, exemplified by the penicillins and cephalosporins-from the perspective of the evolving mechanisms for bacterial resistance. The mechanism of action of the β-lactams is bacterial cell-wall destruction. In the monoderm (single membrane, Gram-positive staining) pathogen Staphylococcus aureus the dominant resistance mechanism is expression of a β-lactam-unreactive transpeptidase enzyme that functions in cell-wall construction. In the diderm (dual membrane, Gram-negative staining) pathogen Pseudomonas aeruginosa a dominant resistance mechanism (among several) is expression of a hydrolytic enzyme that destroys the critical β-lactam ring of the antibiotic. The key sensing mechanism used by P. aeruginosa is monitoring the molecular difference between cell-wall construction and cell-wall deconstruction. In both bacteria, the resistance pathways are manifested only when the bacteria detect the presence of β-lactams. This review summarizes how the β-lactams are sensed and how the resistance mechanisms are manifested, with the expectation that preventing these processes will be critical to future chemotherapeutic control of multidrug resistant bacteria.

Crossroads of Antibiotic Resistance and Biosynthesis

The biosynthesis of antibiotics and self-protection mechanisms employed by antibiotic producers are an integral part of the growing antibiotic resistance threat. The origins of clinically relevant antibiotic resistance genes found in human pathogens have been traced to ancient microbial producers of antibiotics in natural environments. Widespread and frequent antibiotic use amplifies environmental pools of antibiotic resistance genes and increases the likelihood for the selection of a resistance event in human pathogens. This perspective will provide an overview of the origins of antibiotic resistance to highlight the crossroads of antibiotic biosynthesis and producer self-protection that result in clinically relevant resistance mechanisms. Some case studies of synergistic antibiotic combinations, adjuvants, and hybrid antibiotics will also be presented to show how native antibiotic producers manage the emergence of antibiotic resistance.