Bacterial cell-wall recycling

A Marine Group A isolate relies on other growing bacteria for cell wall formation

Most of Earth's prokaryotes live under energy limitation, yet the full breadth of strategies that enable survival under such conditions remain poorly understood. Here we report the isolation of a bacterial strain, IA91, belonging to the candidate phylum Marine Group A (SAR406 or 'Candidatus Marinimicrobia') that is unable to synthesize the central cell wall compound peptidoglycan itself. Using cultivation experiments and microscopy, we show that IA91 growth and cell shape depend on other bacteria, deriving peptidoglycan, energy and carbon from exogenous muropeptide cell wall fragments released from growing bacteria. Reliance on exogenous muropeptides is traceable to the phylum's ancestor, with evidence of vertical inheritance across several classes. This dependency may be widespread across bacteria (16 phyla) based on the absence of key peptidoglycan synthesis genes. These results suggest that uptake of exogenous cell wall components could be a relevant and potentially common survival strategy in energy-limited habitats like the deep biosphere.

The purpose and ubiquity of turnover

Turnover-constant component production and destruction-is ubiquitous in biology. Turnover occurs across organisms and scales, including for RNAs, proteins, membranes, macromolecular structures, organelles, cells, hair, feathers, nails, antlers, and teeth. For many systems, turnover might seem wasteful when degraded components are often fully functional. Some components turn over with shockingly high rates and others do not turn over at all, further making this process enigmatic. However, turnover can address fundamental problems by yielding powerful properties, including regeneration, rapid repair onset, clearance of unpredictable damage and errors, maintenance of low constitutive levels of disrepair, prevention of stable hazards, and transitions. I argue that trade-offs between turnover benefits and metabolic costs, combined with constraints on turnover, determine its presence and rates across distinct contexts. I suggest that the limits of turnover help explain aging and that turnover properties and the basis for its levels underlie this fundamental component of life.

Releasing hidden MAMPs from precursor proteins in plants

The recognition of pathogens by plants at the cell surface is crucial for activating plant immunity. Plants employ pattern recognition receptors (PRRs) to detect microbe-associated molecular patterns (MAMPs). However, our knowledge of the release of peptide MAMPs from their precursor proteins is very limited. Here, we explore seven protein precursors of well-known MAMP peptides and discuss the likelihood of processing being required for their recognition based on structural models and public knowledge. This analysis indicates the existence of multiple extracellular events that are likely pivotal for pathogen perception but remain to be uncovered.

Designing phage cocktails to combat the emergence of bacteriophage-resistant mutants in multidrug-resistant Klebsiella pneumoniae

IMPORTANCE

In this study, we aimed to design a novel and effective bacteriophage cocktail that can target both wild-type bacteria and phage-resistant mutants. To achieve this goal, we isolated four phages (U2874, phi_KPN_H2, phi_KPN_S3, and phi_KPN_HS3) that recognized different bacterial surface molecules using phage-resistant bacteria. We constructed three phage cocktails and tested their phage resistance-suppressing ability against multidrug-resistant Klebsiella pneumoniae. We argue that the phage cocktail that induces resensitization of phage susceptibility exhibited superior phage resistance-suppressing ability. Moreover, we observed trade-off effects that manifested progressively in phage-resistant bacteria. We hypothesize that such trade-off effects can augment therapeutic efficacy. We also recommend collating phage host range data against phage-resistant mutants in addition to wild-type bacteria when establishing phage banks to improve the efficiency of phage therapy. Our study underscores the importance of phage host range data in constructing effective phage cocktails for clinical use.

Divergent genetic landscapes drive lower levels of AmpC induction and stable de-repression in Serratia marcescens compared to Enterobacter cloacae

The chromosomally encoded AmpC beta-lactamase is widely distributed throughout the Enterobacterales. When expressed at high levels through transient induction or stable de-repression, resistance to ceftriaxone, a commonly used antibiotic, can develop. Recent clinical guidance suggests, based on limited evidence, that resistance may be less likely to develop in Serratia marcescens compared to the better-studied Enterobacter cloacae and recommends that ceftriaxone may be used if the clinical isolate tests susceptible. We sought to generate additional data relevant to this recommendation. AmpC de-repression occurs predominantly because of mutation in the ampD peptidoglycan amidohydrolase. We find that, in contrast to E. cloacae, where deletion of ampD results in high-level ceftriaxone resistance (with ceftriaxone MIC = 96 µg/mL), in S. marcescens deletion of two amidohydrolases (ampD and amiD2) is necessary for AmpC de-repression, and the resulting ceftriaxone MIC is 1 µg/mL. Two mechanisms for this difference were identified. We find both a higher relative increase in ampC transcript level in E. cloacae ΔampD compared to S. marcescens ΔampDΔamiD2, as well as higher in vivo efficiency of ceftriaxone hydrolysis by the E. cloacae AmpC enzyme compared to the S. marcescens AmpC enzyme. We also observed higher relative levels of transient AmpC induction in E. cloacae vs S. marcescens when exposed to ceftriaxone. In time-kill curves, this difference translates into the survival of E. cloacae but not S. marcescens at clinically relevant ceftriaxone concentrations. In summary, our findings can explain the decreased propensity for on-treatment ceftriaxone resistance development in S. marcescens, thereby supporting recently issued clinical guidance.

Quantitative proteomics reveals unique responses to antimicrobial treatments in clinical Pseudomonas aeruginosa isolates

IMPORTANCE

Pseudomonas aeruginosa is an important pathogen often associated with hospital-acquired infections and chronic lung infections in people with cystic fibrosis. P. aeruginosa possesses a wide array of intrinsic and adaptive mechanisms of antibiotic resistance, and the regulation of these mechanisms is complex. Label-free quantitative proteomics is a powerful tool to compare susceptible and resistant strains of bacteria and their responses to antibiotic treatments. Here we compare the proteomes of three isolates of P. aeruginosa with different antibiotic resistance profiles in response to five challenge conditions. We uncover unique and shared proteome changes for the widely used laboratory strain PAO1 and two isolates of the Liverpool epidemic strain of P. aeruginosa, LESlike1 and LESB58. Our data set provides insight into antibiotic resistance in clinically relevant Pseudomonas isolates and highlights proteins, including those with uncharacterized functions, which can be further investigated for their role in adaptive responses to antibiotic treatments.

Catalytic process of anhydro-N-acetylmuramic acid kinase from Pseudomonas aeruginosa

The bacterial cell envelope is the structure with which the bacterium engages with, and is protected from, its environment. Within this envelop is a conserved peptidoglycan polymer which confers shape and strength to the cell envelop. The enzymatic processes that build, remodel, and recycle the chemical components of this cross-linked polymer are preeminent targets of antibiotics and exploratory targets for emerging antibiotic structures. We report a comprehensive kinetic and structural analysis for one such enzyme, the Pseudomonas aeruginosa anhydro-N-acetylmuramic acid (anhNAM) kinase (AnmK). AnmK is an enzyme in the peptidoglycan-recycling pathway of this pathogen. It catalyzes the pairing of hydrolytic ring opening of anhNAM with concomitant ATP-dependent phosphoryl transfer. AnmK follows a random-sequential kinetic mechanism with respect to its anhNAM and ATP substrates. Crystallographic analyses of four distinct structures (apo AnmK, AnmK:AMPPNP, AnmK:AMPPNP:anhNAM, and AnmK:ATP:anhNAM) demonstrate that both substrates enter the active site independently in an ungated conformation of the substrate subsites, with protein loops acting as gates for anhNAM binding. Catalysis occurs within a closed conformational state for the enzyme. We observe this state crystallographically using ATP-mimetic molecules. A remarkable X-ray structure for dimeric AnmK sheds light on the precatalytic and postcatalytic ternary complexes. Computational simulations in conjunction with the high-resolution X-ray structures reveal the full catalytic cycle. We further report that a P. aeruginosa strain with disrupted anmK gene is more susceptible to the β-lactam imipenem compared to the WT strain. These observations position AnmK for understanding the nexus among peptidoglycan recycling, susceptibility to antibiotics, and bacterial virulence.

Development of an aminotransferase-driven biocatalytic cascade for deracemization of d,l-phosphinothricin

2-oxo-4-[(hydroxy)(methyl)phosphinoyl]butyric acid (PPO) is the essential precursor keto acid for the asymmetric biosynthesis of herbicide l-phosphinothricin (l-PPT). Developing a biocatalytic cascade for PPO production with high efficiency and low cost is highly desired. Herein, a d-amino acid aminotransferase from Bacillus sp. YM-1 (Ym DAAT) with high activity (48.95 U/mg) and affinity (Km  = 27.49 mM) toward d-PPT was evaluated. To circumvent the inhibition of by-product d-glutamate (d-Glu), an amino acceptor (α-ketoglutarate) regeneration cascade was constructed as a recombinant Escherichia coli (E. coli D), by coupling Ym d-AAT, d-aspartate oxidase from Thermomyces dupontii (TdDDO) and catalase from Geobacillus sp. CHB1. Moreover, the regulation of the ribosome binding site was employed to overcome the limiting step of expression toxic protein TdDDO in E. coli BL21(DE3). The aminotransferase-driven whole-cell biocatalytic cascade (E. coli D) showed superior catalytic efficiency for the synthesis of PPO from d,l-phosphinothricin (d,l-PPT). It revealed the production of PPO exhibited high space-time yield (2.59 g L-1  h-1 ) with complete conversion of d-PPT to PPO at high substrate concentration (600 mM d,l-PPT) in 1.5 L reaction system. This study first provides the synthesis of PPO from d,l-PPT employing an aminotransferase-driven biocatalytic cascade.

Are Bacteria Leaky? Mechanisms of Metabolite Externalization in Bacterial Cross-Feeding

The metabolism of a bacterial cell stretches beyond its boundaries, often connecting with the metabolism of other cells to form extended metabolic networks that stretch across communities, and even the globe. Among the least intuitive metabolic connections are those involving cross-feeding of canonically intracellular metabolites. How and why are these intracellular metabolites externalized? Are bacteria simply leaky? Here I consider what it means for a bacterium to be leaky, and I review mechanisms of metabolite externalization from the context of cross-feeding. Despite common claims, diffusion of most intracellular metabolites across a membrane is unlikely. Instead, passive and active transporters are likely involved, possibly purging excess metabolites as part of homeostasis. Re-acquisition of metabolites by a producer limits the opportunities for cross-feeding. However, a competitive recipient can stimulate metabolite externalization and initiate a positive-feedback loop of reciprocal cross-feeding.

N-acetylmuramic acid recognition by MurK kinase from the MurNAc auxotrophic oral pathogen Tannerella forsythia

The bacterial cell wall consists of a three-dimensional peptidoglycan layer, composed of peptides linked to the sugars N-acetylmuramic acid (MurNAc) and GlcNAc. Unlike other bacteria, the pathogenic Tannerella forsythia, a member of the red complex group of bacteria associated with the late stages of periodontitis, lacks biosynthetic pathways for MurNAc production and therefore obtains MurNAc from the environment. Sugar kinases play a crucial role in the MurNAc recycling process, activating the sugar molecules by phosphorylation. In this study, we present the first crystal structures of a MurNAc kinase, called murein sugar kinase (MurK), in its unbound state as well as in complexes with the ATP analog β-γ-methylene adenosine triphosphate (AMP-PCP) and with MurNAc. We also determined the crystal structures of K1058, a paralogous MurNAc kinase of T. forsythia, in its unbound state and in complex with MurNAc. We identified the active site and residues crucial for MurNAc specificity as the less bulky side chains of S133, P134, and L135, which enlarge the binding cavity for the lactyl ether group, unlike the glutamate or histidine residues present in structural homologs. In establishing the apparent kinetic parameters for both enzymes, we showed a comparable affinity for MurNAc (Km 180 μM and 30 μM for MurK and K1058, respectively), with MurK being over two hundred times faster than K1058 (Vmax 80 and 0.34 μmol min-1 mg-1, respectively). These data might support a structure-guided approach to development of inhibitory MurNAc analogs for pathogen MurK enzymes.

Insights into the central role of N-acetyl-glucosamine-1-phosphate uridyltransferase (GlmU) in peptidoglycan metabolism and its potential as a therapeutic target

Several decades after the discovery of the first antibiotic (penicillin) microbes have evolved novel mechanisms of resistance; endangering not only our abilities to combat future bacterial pandemics but many other clinical challenges such as acquired infections during surgeries. Antimicrobial resistance (AMR) is attributed to the mismanagement and overuse of these medications and is complicated by a slower rate of the discovery of novel drugs and targets. Bacterial peptidoglycan (PG), a three-dimensional mesh of glycan units, is the foundation of the cell wall that protects bacteria against environmental insults. A significant percentage of drugs target PG, however, these have been rendered ineffective due to growing drug resistance. Identifying novel druggable targets is, therefore, imperative. Uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) is one of the key building blocks in PG production, biosynthesized by the bifunctional enzyme N-acetyl-glucosamine-1-phosphate uridyltransferase (GlmU). UDP-GlcNAc metabolism has been studied in many organisms, but it holds some distinctive features in bacteria, especially regarding the bacterial GlmU enzyme. In this review, we provide an overview of different steps in PG biogenesis, discuss the biochemistry of GlmU, and summarize the characteristic structural elements of bacterial GlmU vital to its catalytic function. Finally, we will discuss various studies on the development of GlmU inhibitors and their significance in aiding future drug discoveries.

Monomer and Oligomer Transition of Zinc Phthalocyanine Is Key for Photobleaching in Photodynamic Therapy

Photodynamic therapy (PDT) is recognized as a powerful method to inactivate cells. However, the photosensitizer (PS), a key component of PDT, has suffered from undesired photobleaching. Photobleaching reduces reactive oxygen species (ROS) yields, leading to the compromise of and even the loss of the photodynamic effect of the PS. Therefore, much effort has been devoted to minimizing photobleaching in order to ensure that there is no loss of photodynamic efficacy. Here, we report that a type of PS aggregate showed neither photobleaching nor photodynamic action. Upon direct contact with bacteria, the PS aggregate was found to fall apart into PS monomers and thus possessed photodynamic inactivation against bacteria. Interestingly, the disassembly of the bound PS aggregate in the presence of bacteria was intensified by illumination, generating more PS monomers and leading to an enhanced antibacterial photodynamic effect. This demonstrated that on a bacterial surface, the PS aggregate photo-inactivated bacteria via PS monomer during irradiation, where the photodynamic efficiency was retained without photobleaching. Further mechanistic studies showed that PS monomers disrupted bacterial membranes and affected the expression of genes related to cell wall synthesis, bacterial membrane integrity, and oxidative stress. The results obtained here are applicable to other types of PSs in PDT.

Impact of the host microbiota on fungal infections: new possibilities for intervention?

Many human fungal pathogens are opportunistic. They are primarily benign residents of the human body and only become infectious when the host's immunity and microbiome are compromised. Bacteria dominate the human microbiome, playing an essential role in keeping fungi harmless and acting as the first line of defense against fungal infection. The Human Microbiome Project, launched by NIH in 2007, has stimulated extensive investigation and significantly advanced our understanding of the molecular mechanisms governing the interaction between bacteria and fungi, providing valuable insights for developing future antifungal strategies by exploiting the interaction. This review summarizes recent progress in this field and discusses new possibilities and challenges. We must seize the opportunities presented by researching bacterial-fungal interplay in the human microbiome to address the global spread of drug-resistant fungal pathogens and the drying pipelines of effective antifungal drugs.

Masters of Misdirection: Peptidoglycan Glycosidases in Bacterial Growth

The dynamic composition of the peptidoglycan cell wall has been the subject of intense research for decades, yet how bacteria coordinate the synthesis of new peptidoglycan with the turnover and remodeling of existing peptidoglycan remains elusive. Diversity and redundancy within peptidoglycan synthases and peptidoglycan autolysins, enzymes that degrade peptidoglycan, have often made it challenging to assign physiological roles to individual enzymes and determine how those activities are regulated. For these reasons, peptidoglycan glycosidases, which cleave within the glycan strands of peptidoglycan, have proven veritable masters of misdirection over the years. Unlike many of the broadly conserved peptidoglycan synthetic complexes, diverse bacteria can employ unrelated glycosidases to achieve the same physiological outcome. Additionally, although the mechanisms of action for many individual enzymes have been characterized, apparent conserved homologs in other organisms can exhibit an entirely different biochemistry. This flexibility has been recently demonstrated in the context of three functions critical to vegetative growth: (i) release of newly synthesized peptidoglycan strands from their membrane anchors, (ii) processing of peptidoglycan turned over during cell wall expansion, and (iii) removal of peptidoglycan fragments that interfere with daughter cell separation during cell division. Finally, the regulation of glycosidase activity during these cell processes may be a cumulation of many factors, including protein-protein interactions, intrinsic substrate preferences, substrate availability, and subcellular localization. Understanding the true scope of peptidoglycan glycosidase activity will require the exploration of enzymes from diverse organisms with equally diverse growth and division strategies.

Bacterial virulence regulation through soluble peptidoglycan fragments sensing and response: knowledge gaps and therapeutic potential

Given the growing clinical-epidemiological threat posed by the phenomenon of antibiotic resistance, new therapeutic options are urgently needed, especially against top nosocomial pathogens such as those within the ESKAPE group. In this scenario, research is pushed to explore therapeutic alternatives and, among these, those oriented toward reducing bacterial pathogenic power could pose encouraging options. However, the first step in developing these antivirulence weapons is to find weak points in the bacterial biology to be attacked with the goal of dampening pathogenesis. In this regard, during the last decades some studies have directly/indirectly suggested that certain soluble peptidoglycan-derived fragments display virulence-regulatory capacities, likely through similar mechanisms to those followed to regulate the production of several β-lactamases: binding to specific transcriptional regulators and/or sensing/activation of two-component systems. These data suggest the existence of intra- and also intercellular peptidoglycan-derived signaling capable of impacting bacterial behavior, and hence likely exploitable from the therapeutic perspective. Using the well-known phenomenon of peptidoglycan metabolism-linked β-lactamase regulation as a starting point, we gather and integrate the studies connecting soluble peptidoglycan sensing with fitness/virulence regulation in Gram-negatives, dissecting the gaps in current knowledge that need filling to enable potential therapeutic strategy development, a topic which is also finally discussed.

Multicopy suppressor screens reveal convergent evolution of single-gene lysis proteins

Single-strand RNA (ssRNA) Fiersviridae phages cause host lysis with a product of single gene (sgl for single-gene lysis; product Sgl) that induces autolysis. Many different Sgls have been discovered, but the molecular targets of only a few have been identified. In this study, we used a high-throughput genetic screen to uncover genome-wide host suppressors of diverse Sgls. In addition to validating known molecular mechanisms, we discovered that the Sgl of PP7, an ssRNA phage of Pseudomonas aeruginosa, targets MurJ, the flippase responsible for lipid II export, previously shown to be the target of the Sgl of coliphage M. These two Sgls, which are unrelated and predicted to have opposite membrane topology, thus represent a case of convergent evolution. We extended the genetic screens to other uncharacterized Sgls and uncovered a common set of multicopy suppressors, suggesting that these Sgls act by the same or similar mechanism.

An NlpC/P60 protein catalyzes a key step in peptidoglycan recycling at the intersection of energy recovery, cell division and immune evasion in the intracellular pathogen Chlamydia trachomatis

The obligate intracellular Chlamydiaceae do not need to resist osmotic challenges and thus lost their cell wall in the course of evolution. Nevertheless, these pathogens maintain a rudimentary peptidoglycan machinery for cell division. They build a transient peptidoglycan ring, which is remodeled during the process of cell division and degraded afterwards. Uncontrolled degradation of peptidoglycan poses risks to the chlamydial cell, as essential building blocks might get lost or trigger host immune response upon release into the host cell. Here, we provide evidence that a primordial enzyme class prevents energy intensive de novo synthesis and uncontrolled release of immunogenic peptidoglycan subunits in Chlamydia trachomatis. Our data indicate that the homolog of a Bacillus NlpC/P60 protein is widely conserved among Chlamydiales. We show that the enzyme is tailored to hydrolyze peptidoglycan-derived peptides, does not interfere with peptidoglycan precursor biosynthesis, and is targeted by cysteine protease inhibitors in vitro and in cell culture. The peptidase plays a key role in the underexplored process of chlamydial peptidoglycan recycling. Our study suggests that chlamydiae orchestrate a closed-loop system of peptidoglycan ring biosynthesis, remodeling, and recycling to support cell division and maintain long-term residence inside the host. Operating at the intersection of energy recovery, cell division and immune evasion, the peptidoglycan recycling NlpC/P60 peptidase could be a promising target for the development of drugs that combine features of classical antibiotics and anti-virulence drugs.

Natural sanitizer potential of Cuminum cyminum and applicable approach for calculation of Kováts retention index of its compounds

The demand for natural agents instead of chemicals in terms of food and health safety is increasing day by day. This study aimed to investigate the potential of the methanolic extract of Cuminum cyminum (C. cyminum) in the fight against Escherichia coli (E. coli), Staphylococcus aureus (S. aureus)and Candida (C. albicans). The chemical composition of the methanolic extract of C. cyminum was analyzed using GS-MS. Also, Kováts retention indices were calculated for the detected compounds using an applicable formula. The most basic substance was cuminic aldehyde (27.86%) and p-(Dimethoxymethyl)-isopropylbenze (18.32%). The Minimum Inhibitory Concentration (MIC) of the extract was 0.1 g/mL for S. aureus and C. albicans while it was > 0.1 for E. coli. Although the methanol extract of C. cyminum acts against all three microorganisms, the most lasting effect was on S. aureus, indicating that it can be recommended as a strong antibacterial disinfectant for S. aureus.

Role of peptidoglycan recycling enzymes AmpD and AnmK in Acinetobacter baumannii virulence features

Acinetobacter baumannii is an important causative agent of hospital acquired infections. In addition to acquired resistance to many currently-available antibiotics, it is intrinsically resistant to fosfomycin. It has previously been shown that AmpD and AnmK contribute to intrinsic fosfomycin resistance in A. baumannii due to their involvement in the peptidoglycan recycling pathway. However, the role that these two enzymes play in the fitness and virulence of A. baumannii has not been studied. The aim of this study was to characterize several virulence-related phenotypic traits in A. baumannii mutants lacking AmpD and AnmK. Specifically, cell morphology, peptidoglycan thickness, membrane permeability, growth under iron-limiting conditions, fitness, resistance to disinfectants and antimicrobial agents, twitching motility and biofilm formation of the mutant strains A. baumannii ATCC 17978 ΔampD::Kan and ΔanmK::Kan were compared to the wild type strain. Our results demonstrate that bacterial growth and fitness of both mutants were compromised, especially in the ΔampD::Kan mutant. In addition, biofilm formation was decreased by up to 69%, whereas twitching movement was reduced by about 80% in both mutants. These results demonstrate that, in addition to increased susceptibility to fosfomycin, alteration of the peptidoglycan recycling pathway affects multiple aspects related to virulence. Inhibition of these enzymes could be explored as a strategy to develop novel treatments for A. baumannii in the future. Furthermore, this study establishes a link between intrinsic fosfomycin resistance mechanisms and bacterial fitness and virulence traits.

Application and mechanisms of metal-based nanoparticles in the control of bacterial and fungal crop diseases

Nanotechnology is a young branch of the discipline generated by nanomaterials. Its development has greatly contributed to technological progress and product innovation in the field of agriculture. The antimicrobial properties of nanoparticles (NPs) can be used to develop nanopesticides for plant protection. Plant diseases caused by bacterial and fungal infestations are the main types of crop diseases. Once infected, they will seriously threaten crop growth, reduce yield and quality, and affect food safety, posing a health risk to humans. We reviewed the application of metal-based nanoparticles in inhibiting plant pathogenic bacteria and fungi, and discuss the antibacterial mechanisms of metal-based nanoparticles from two aspects: the direct interaction between nanoparticles and pathogens, and the indirect effects of inducing plant resilience to disease. © 2022 Society of Chemical Industry.

Genome-wide identification of genes required for alternative peptidoglycan cross-linking in Escherichia coli revealed unexpected impacts of β-lactams

The D,D-transpeptidase activity of penicillin-binding proteins (PBPs) is the well-known primary target of β-lactam antibiotics that block peptidoglycan polymerization. β-lactam-induced bacterial killing involves complex downstream responses whose causes and consequences are difficult to resolve. Here, we use the functional replacement of PBPs by a β-lactam-insensitive L,D-transpeptidase to identify genes essential to mitigate the effects of PBP inactivation by β-lactams in actively dividing bacteria. The functions of the 179 conditionally essential genes identified by this approach extend far beyond L,D-transpeptidase partners for peptidoglycan polymerization to include proteins involved in stress response and in the assembly of outer membrane polymers. The unsuspected effects of β-lactams include loss of the lipoprotein-mediated covalent bond that links the outer membrane to the peptidoglycan, destabilization of the cell envelope in spite of effective peptidoglycan cross-linking, and increased permeability of the outer membrane. The latter effect indicates that the mode of action of β-lactams involves self-promoted penetration through the outer membrane.

σP-NagA-L1/L2 Regulatory Circuit Involved in ΔompA299-356-Mediated Increase in β-Lactam Susceptibility in Stenotrophomonas maltophilia

OmpA, the most abundant porin in Stenotrophomonas maltophilia KJ, exists as a two-domain structure with an N-terminal domain of β-barrel structure embedded in the outer membrane and a C-terminal domain collocated in the periplasm. KJΔOmpA299-356, an ompA mutant of S. maltophilia KJ with a truncated OmpA devoid of 299 to 356 amino acids (aa), was able to stably embed in the outer membrane. KJΔOmpA299-356 was more susceptible to β-lactams than wild-type KJ. We aimed to elucidate the mechanism underlying the ΔompA299-356-mediated increase in β-lactam susceptibility (abbreviated as "ΔOmpA299-356 phenotype"). KJΔOmpA299-356 displayed a lower ceftazidime (CAZ)-induced β-lactamase activity than KJ. Furthermore, KJ2, a L1/L2 β-lactamases-null mutant, and KJ2ΔOmpA299-356, a KJ2 mutant with truncated OmpA devoid of299 to 356 aa, had comparable β-lactam susceptibility. Both lines of evidence indicate that decreased β-lactamase activity contributes to the ΔOmpA299-356 phenotype. We analyzed the transcriptome results of KJ and KJΔOmpA299-356, focusing on PG homeostasis-associated genes. Among the 36 genes analyzed, the nagA gene was upregulated 4.65-fold in KJΔOmpA299-356. Deletion of the nagA gene from the chromosome of KJΔOmpA299-356 restored β-lactam susceptibility and CAZ-induced β-lactamase activity to wild-type levels, verifying that nagA-upregulation in KJΔOmpA299-356 contributes to the ΔOmpA299-356 phenotype. Furthermore, transcriptome analysis revealed that rpoE (Smlt3555) and rpoP (Smlt3514) were significantly upregulated in KJΔOmpA299-356. The deletion mutant construction, β-lactam susceptibility, and β-lactamase activity analysis demonstrated that σP, but not σE, was involved in the ΔOmpA299-356 phenotype. A real-time quantitative (qRT-PCR) assay confirmed that nagA is a member of the σP regulon. The involvement of the σP-NagA-L1/L2 regulatory circuit in the ΔOmpA299-356 phenotype was manifested. IMPORTANCE Porins of Gram-negative bacteria generally act as channels that allow the entry or extrusion of molecules. Moreover, the structural role of porins in stabilizing the outer membrane by interacting with peptidoglycan (PG) and the outer membrane has been proposed. The linkage between porin deficiency and antibiotic resistance increase has been reported widely, with a rationale for blocking antibiotic influx. In this study, a link between porin defects and β-lactam susceptibility increase was demonstrated. The underlying mechanism revealed that a novel σP-NagA-L1/L2 regulatory circuit is triggered due to the loss of the OmpA-PG interaction. This study extends the understanding on the porin defect and antibiotic susceptibility. Porin defects may cause opposite impacts on antibiotic susceptibility, which is dependent on the involvement of the defect. Blocking the porin channel role can increase antibiotic resistance; in contrast, the loss of porin structure role may increase antibiotic susceptibility.

Customized chitooligosaccharide production-controlling their length via engineering of rhizobial chitin synthases and the choice of expression system

Chitooligosaccharides (COS) have attracted attention from industry and academia in various fields due to their diverse bioactivities. However, their conventional chemical production is environmentally unfriendly and in addition, defined and pure molecules are both scarce and expensive. A promising alternative is the in vivo synthesis of desired COS in microbial platforms with specific chitin synthases enabling a more sustainable production. Hence, we examined the whole cell factory approach with two well-established microorganisms-Escherichia coli and Corynebacterium glutamicum-to produce defined COS with the chitin synthase NodC from Rhizobium sp. GRH2. Moreover, based on an in silico model of the synthase, two amino acids potentially relevant for COS length were identified and mutated to direct the production. Experimental validation showed the influence of the expression system, the mutations, and their combination on COS length, steering the production from originally pentamers towards tetramers or hexamers, the latter virtually pure. Possible explanations are given by molecular dynamics simulations. These findings pave the way for a better understanding of chitin synthases, thus allowing a more targeted production of defined COS. This will, in turn, at first allow better research of COS' bioactivities, and subsequently enable sustainable large-scale production of oligomers.

The mechanistic landscape of Lytic transglycosylase as targets for antibacterial therapy

Lytic transglycosylases (Ltgs) are glycan strand cleaving enzymes whose role is poorly understood in the genesis of the bacterial envelope. They play multiple roles in all stages of a bacterial life cycle, by creating holes in the peptidoglycan that is necessary for cell division and separation. Here, we review recent advances in understanding the suitability of Ltgs as antibacterial drug targets. We specifically highlight a known inhibitor bulgecin A that is able to inhibit the function of structurally diverse Ltgs, as well as synergize with beta-lactams to improve its efficacy in antibiotic insensitive strains. Discovery of new antibiotics or new targets has been challenging. These studies could provide a viable path toward designing broad-spectrum inhibitors that targets Ltgs.

Cefazolin and imipenem enhance AmpC expression and resistance in NagZ-dependent manner in Enterobacter cloacae complex

BACKGROUND

Enterobacter cloacae complex (ECC) is a common opportunistic pathogen and is responsible for causing various infections in humans. Owing to its inducible chromosomal AmpC β-lactamase (AmpC), ECC is inherently resistant to the 1st- and 2nd- generation cephalosporins. However, whether β-lactams antibiotics enhance ECC resistance remains unclear.

RESULTS

In this study, we found that subinhibitory concentrations (SICs) of cefazolin (CFZ) and imipenem (IMP) can advance the expression of AmpC and enhance its resistance towards β-lactams through NagZ in Enterobacter cloacae (EC). Further, AmpC manifested a substantial upregulation in EC in response to SICs of CFZ and IMP. In nagZ knockout EC (ΔnagZ), the resistance to β-lactam antibiotics was rather weakened and the effect of CFZ and IMP on AmpC induction was completely abrogated. NagZ ectopic expression can rescue the induction effects of CFZ and IMP on AmpC and increase ΔnagZ resistance. More importantly, CFZ and IMP have the potential to induce the expression of AmpR's target genes in a NagZ-dependent manner.

CONCLUSIONS

Our findings suggest that NagZ is a critical determinant for CFZ and IMP to promote AmpC expression and resistance and that CFZ and IMP should be used with caution since they may aggravate ECC resistance. At the same time, this study further improves our understanding of resistance mechanisms in ECC.

Role of Enzymatic Activity in the Biological Cost Associated with the Production of AmpC β-Lactamases in Pseudomonas aeruginosa

In the current scenario of growing antibiotic resistance, understanding the interplay between resistance mechanisms and biological costs is crucial for designing therapeutic strategies. In this regard, intrinsic AmpC β-lactamase hyperproduction is probably the most important resistance mechanism of Pseudomonas aeruginosa, proven to entail important biological burdens that attenuate virulence mostly under peptidoglycan recycling alterations. P. aeruginosa can acquire resistance to new β-lactam-β-lactamase inhibitor combinations (ceftazidime-avibactam and ceftolozane-tazobactam) through mutations affecting ampC and its regulatory genes, but the impact of these mutations on the associated biological cost and the role that β-lactamase activity plays per se in contributing to the above-mentioned virulence attenuation are unknown. The same questions remain unsolved for plasmid-encoded AmpC-type β-lactamases such as FOX enzymes, some of which also provide resistance to new β-lactam-β-lactamase inhibitor combinations. Here, we assessed from different perspectives the effects of changes in the active center and, thus, in the hydrolytic spectrum resistance to inhibitors of AmpC-type β-lactamases on the fitness and virulence of P. aeruginosa, using site-directed mutagenesis; the previously described AmpC variants T96I, G183D, and ΔG229-E247; and, finally, blaFOX-4 versus blaFOX-8. Our results indicate the essential role of AmpC activity per se in causing the reported full virulence attenuation (in terms of growth, motility, cytotoxicity, and Galleria mellonella larvae killing), although the biological cost of the above-mentioned AmpC-type variants was similar to that of the wild-type enzymes. This suggests that there is not an important biological burden that may limit the selection/spread of these variants, which could progressively compromise the future effectiveness of the above-mentioned drug combinations. IMPORTANCE The growing antibiotic resistance of the top nosocomial pathogen Pseudomonas aeruginosa pushes research to explore new therapeutic strategies, for which the resistance-versus-virulence balance is a promising source of targets. While resistance often entails significant biological costs, little is known about the bases of the virulence attenuations associated with a resistance mechanism as extraordinarily relevant as β-lactamase production. We demonstrate that besides potential energy and cell wall alterations, the enzymatic activity of the P. aeruginosa cephalosporinase AmpC is essential for causing the full attenuation associated with its hyperproduction by affecting different features related to pathogenesis, a fact exploitable from the antivirulence perspective. Less encouraging, we also show that the production of different chromosomal/plasmid-encoded AmpC derivatives conferring resistance to some of the newest antibiotic combinations causes no significantly increased biological burdens, which suggests a free way for the selection/spread of these types of variants, potentially compromising the future effectiveness of these antipseudomonal therapies.

Effects of a microbial muramidase on the growth performance, intestinal permeability, nutrient digestibility, and welfare of broiler chickens

The objective of these studies was to evaluate the inclusion of a microbial muramidase (MUR) in the diets of broiler chickens on the growth performance, intestinal permeability (IP), total blood carotenoid content, apparent ileal digestibility (AID), and foot pad dermatitis (FPD). In Experiment 1, a total of 1,000 one-day-old chicks were placed in floor-pens with reused litter, and randomly distributed into 4 treatments with 10 replicates each. Treatments were a basal diet (control), or basal diet supplemented with 15,000; 25,000 or 35,000 LSU (F)/kg of MUR. Feed intake (FI), body weight gain (BWG), and feed conversion ratio (FCR) were evaluated at d 21 and 43. Intestinal permeability was evaluated on d 35 by FITC-d, and FPD and AID on d 43. In Experiment 2, a total of 800 one-day-old chicks were placed in floor-pens with fresh litter, and randomly distributed into 4 treatments with 8 replicates each. Treatments were a basal diet (control), or basal diet supplemented with 25,000 or 35,000 LSU (F)/kg of MUR, and a fourth group where the basal diet was supplemented with enramycin. The birds were induced to a mild intestinal challenge. Feed intake, BWG, and FCR were evaluated on d 21 and d 42, and total blood concentration of carotenoids was evaluated on d 28. In experiment 1, 35,000 LSU (F)/kg of MUR promoted the best FCR (P < 0.05). Muramidase supplementation linearly increased the AID of dry matter, ash, and fat (P < 0.01), and regardless of the dose, MUR decreased the IP (P < 0.05). In Experiment 2, the supplementation of 35,000 LSU (F)/kg of MUR improved BWG and FCR in the entire cycle (1–42 d) and increased the concentration of carotenoids in the blood on d 28 compared to the control group (P < 0.05). These studies show that MUR improves growth performance of broilers by improving intestinal permeability, digestibility of dry matter, ash and fat, absorption of carotenoids, and reducing FPD.

Heavy isotope labeling and mass spectrometry reveal unexpected remodeling of bacterial cell wall expansion in response to drugs

Antibiotics of the β-lactam (penicillin) family inactivate target enzymes called D,D-transpeptidases or penicillin-binding proteins (PBPs) that catalyze the last cross-linking step of peptidoglycan synthesis. The resulting net-like macromolecule is the essential component of bacterial cell walls that sustains the osmotic pressure of the cytoplasm. In Escherichia coli, bypass of PBPs by the YcbB L,D-transpeptidase leads to resistance to these drugs. We developed a new method based on heavy isotope labeling and mass spectrometry to elucidate PBP- and YcbB-mediated peptidoglycan polymerization. PBPs and YcbB similarly participated in single-strand insertion of glycan chains into the expanding bacterial side wall. This absence of any transpeptidase-specific signature suggests that the peptidoglycan expansion mode is determined by other components of polymerization complexes. YcbB did mediate β-lactam resistance by insertion of multiple strands that were exclusively cross-linked to existing tripeptide-containing acceptors. We propose that this undocumented mode of polymerization depends upon accumulation of linear glycan chains due to PBP inactivation, formation of tripeptides due to cleavage of existing cross-links by a β-lactam-insensitive endopeptidase, and concerted cross-linking by YcbB.

Induction of AmpC-Mediated β-Lactam Resistance Requires a Single Lytic Transglycosylase in Agrobacterium tumefaciens

The remarkable ability of Agrobacterium tumefaciens to transfer DNA to plant cells has allowed the generation of important transgenic crops. One challenge of A. tumefaciens-mediated transformation is eliminating the bacteria after plant transformation to prevent detrimental effects to plants and the release of engineered bacteria to the environment. Here, we use a reverse-genetics approach to identify genes involved in ampicillin resistance, with the goal of utilizing these antibiotic-sensitive strains for plant transformations. We show that treating A. tumefaciens C58 with ampicillin led to increased β-lactamase production, a response dependent on the broad-spectrum β-lactamase AmpC and its transcription factor, AmpR. Loss of the putative ampD orthologue atu2113 led to constitutive production of AmpC-dependent β-lactamase activity and ampicillin resistance. Finally, one cell wall remodeling enzyme, MltB3, was necessary for the AmpC-dependent β-lactamase activity, and its loss elicited ampicillin and carbenicillin sensitivity in the A. tumefaciens C58 and GV3101 strains. Furthermore, GV3101 ΔmltB3 transforms plants with efficiency comparable to that of the wild type but can be cleared with sublethal concentrations of ampicillin. The functional characterization of the genes involved in the inducible ampicillin resistance pathway of A. tumefaciens constitutes a major step forward in efforts to reduce the intrinsic antibiotic resistance of this bacterium. IMPORTANCE Agrobacterium tumefaciens, a significant biotechnological tool for production of transgenic plant lines, is highly resistant to a wide variety of antibiotics, posing challenges for various applications. One challenge is the efficient elimination of A. tumefaciens from transformed plant tissue without using levels of antibiotics that are toxic to the plants. Here, we present the functional characterization of genes involved in β-lactam resistance in A. tumefaciens. Knowledge about proteins that promote or inhibit β-lactam resistance will enable the development of strains to improve the efficiency of Agrobacterium-mediated plant genetic transformations. Effective removal of Agrobacterium from transformed plant tissue has the potential to maximize crop yield and food production, improving the outlook for global food security.

Molecular basis for cell-wall recycling regulation by transcriptional repressor MurR in Escherichia coli

The cell-wall recycling process is important for bacterial survival in nutrient-limited conditions and, in certain cases, is directly involved in antibiotic resistance. In the sophisticated cell-wall recycling process in Escherichia coli, the transcriptional repressor MurR controls the expression of murP and murQ, which are involved in transporting and metabolizing N-acetylmuramic acid (MurNAc), generating N-acetylmuramic acid-6-phosphate (MurNAc-6-P) and N-acetylglucosamine-6-phosphate (GlcNAc-6-P). Here, we report that both MurNAc-6-P and GlcNAc-6-P can bind to MurR and weaken the DNA binding ability of MurR. Structural characterizations of MurR in complex with MurNAc-6-P or GlcNAc-6-P as well as in the apo form revealed the detailed ligand recognition chemistries. Further studies showed that only MurNAc-6-P, but not GlcNAc-6-P, is capable of derepressing the expression of murQP controlled by MurR in cells and clarified the substrate specificity through the identification of key residues responsible for ligand binding in the complex structures. In summary, this study deciphered the molecular mechanism of the cell wall recycling process regulated by MurR in E. coli.

WhyD tailors surface polymers to prevent premature bacteriolysis and direct cell elongation in Streptococcus pneumoniae

Penicillin and related antibiotics disrupt cell wall synthesis in bacteria causing the downstream misactivation of cell wall hydrolases called autolysins to induce cell lysis. Despite the clinical importance of this phenomenon, little is known about the factors that control autolysins and how penicillins subvert this regulation to kill cells. In the pathogen Streptococcus pneumoniae (Sp), LytA is the major autolysin responsible for penicillin-induced bacteriolysis. We recently discovered that penicillin treatment of Sp causes a dramatic shift in surface polymer biogenesis in which cell wall-anchored teichoic acids (WTAs) increase in abundance at the expense of lipid-linked teichoic acids (LTAs). Because LytA binds to both species of teichoic acids, this change recruits the enzyme to its substrate where it cleaves the cell wall and elicits lysis. In this report, we identify WhyD (SPD_0880) as a new factor that controls the level of WTAs in Sp cells to prevent LytA misactivation and lysis during exponential growth . We show that WhyD is a WTA hydrolase that restricts the WTA content of the wall to areas adjacent to active peptidoglycan (PG) synthesis. Our results support a model in which the WTA tailoring activity of WhyD during exponential growth directs PG remodeling activity required for proper cell elongation in addition to preventing autolysis by LytA.

XAC4296 Is a Multifunctional and Exclusive Xanthomonadaceae Gene Containing a Fusion of Lytic Transglycosylase and Epimerase Domains

Microorganisms have a limited and highly adaptable repertoire of genes capable of encoding proteins containing single or variable multidomains. The phytopathogenic bacteria Xanthomonas citri subsp. citri (X. citri) (Xanthomonadaceae family), the etiological agent of Citrus Canker (CC), presents a collection of multidomain and multifunctional enzymes (MFEs) that remains to be explored. Recent studies have shown that multidomain enzymes that act on the metabolism of the peptidoglycan and bacterial cell wall, belonging to the Lytic Transglycosylases (LTs) superfamily, play an essential role in X. citri biology. One of these LTs, named XAC4296, apart from the Transglycosylase SLT_2 and Peptidoglycan binding-like domains, contains an unexpected aldose 1-epimerase domain linked to the central metabolism; therefore, resembling a canonical MFE. In this work, we experimentally characterized XAC4296 revealing its role as an MFE and demonstrating its probable gene fusion origin and evolutionary history. The XAC4296 is expressed during plant-pathogen interaction, and the Δ4296 mutant impacts CC progression. Moreover, Δ4296 exhibited chromosome segregation and cell division errors, and sensitivity to ampicillin, suggesting not only LT activity but also that the XAC4296 may also contribute to resistance to β-lactams. However, both Δ4296 phenotypes can be restored when the mutant is supplemented with sucrose or glutamic acid as a carbon and nitrogen source, respectively; therefore, supporting the epimerase domain’s functional relationship with the central carbon and cell wall metabolism. Taken together, these results elucidate the role of XAC4296 as an MFE in X. citri, also bringing new insights into the evolution of multidomain proteins and antimicrobial resistance in the Xanthomonadaceae family.

Insulin resistance corresponds with a progressive increase in NOD1 in high fat diet-fed mice

PURPOSE

Innate immune components participate in obesity-induced inflammation, which can contribute to endocrine dysfunction during metabolic diseases. However, the chronological activation of specific immune proteins such as Nucleotide-binding oligomerization domain-containing protein 1 (NOD1) and relevance to cellular crosstalk during the progression of obesity-associated insulin resistance (IR) is not known.

METHODS

The NOD1 signaling in various insulin-sensitive metabolic tissues during the progression of diet-insulin resistance was assessed in C57BL/6J mice fed with 60% high-fat diet (HFD) for 4, 8, 12, and 16 weeks. Intestinal permeability was measured using FITC-dextran. NOD1 activating potential was analyzed using HEK-Blue mNOD1 cells.

RESULTS

HFD-fed mice showed progressive induction of glucose intolerance and impairment of insulin signaling in key metabolic tissues. We found a time-dependent increase in intestinal permeability coupled with transport and accumulation of NOD1 activating ligand in the serum of HFD-fed mice. We also observed a progressive accumulation of γ-D-glutamyl-meso-diaminopimelic acid (DAP), a microbial peptidoglycan ligand known to activate NOD1, in serum samples of the HFD-fed mice. There was also a progressive increase in transcripts levels of NOD1 in bone marrow-derived macrophages during HFD-feeding. In addition, skeletal muscle, adipose and liver, the key insulin sensitive metabolic tissues also had a time-dependent increase in transcripts of NOD1 and Rip2 and a corresponding activation of pro-inflammatory responses in these tissues.

CONCLUSION

These data highlight the correlation of inflammation and insulin resistance to NOD1 activation in the bone marrow derived macrophages and insulin responsive metabolic tissues during high fat diet feeding in mice.

β-Lactam Resistance in Azospirillum baldaniorum Sp245 Is Mediated by Lytic Transglycosylase and β-Lactamase and Regulated by a Cascade of RpoE7→RpoH3 Sigma Factors

Bacterial resistance to β-lactam antibiotics is often mediated by β-lactamases and lytic transglycosylases. Azospirillum baldaniorum Sp245 is a plant-growth-promoting rhizobacterium that shows high levels of resistance to ampicillin. Investigating the molecular basis of ampicillin resistance and its regulation in A. baldaniorum Sp245, we found that a gene encoding lytic transglycosylase (Ltg1) is organized divergently from a gene encoding an extracytoplasmic function (ECF) σ factor (RpoE7) in its genome. Inactivation of rpoE7 in A. baldaniorum Sp245 led to increased ability to form cell-cell aggregates and produce exopolysaccharides and biofilm, suggesting that rpoE7 might contribute to antibiotic resistance. Inactivation of ltg1 in A. baldaniorum Sp245, however, adversely affected its growth, indicating a requirement of Ltg1 for optimal growth. The expression of rpoE7, as well that of as ltg1, was positively regulated by RpoE7, and overexpression of RpoE7 conferred ampicillin sensitivity to both the rpoE7::km mutant and its parent. In addition, RpoE7 negatively regulated the expression of a gene encoding a β-lactamase (bla1). Out of the 5 paralogs of RpoH encoded in the genome of A. baldaniorum Sp245, RpoH3 played major roles in conferring ampicillin sensitivity and in the downregulation of bla1. The expression of rpoH3 was positively regulated by RpoE7. Collectively, these observations reveal a novel regulatory cascade of RpoE7-RpoH3 σ factors that negatively regulates ampicillin resistance in A. baldaniorum Sp245 by controlling the expression of a β-lactamase and a lytic transglycosylase. In the absence of a cognate anti-sigma factor, addressing how the activity of RpoE7 is regulated by β-lactams will unravel new mechanisms of regulation of β-lactam resistance in bacteria. IMPORTANCE Antimicrobial resistance is a global health problem that requires a better understanding of the mechanisms that bacteria use to resist antibiotics. Bacteria inhabiting the plant rhizosphere are a potential source of antibiotic resistance, but their mechanisms controlling antibiotic resistance are poorly understood. A. baldaniorum Sp245 is a rhizobacterium that is known for its characteristic resistance to ampicillin. Here, we show that an AmpC-type β-lactamase and a lytic transglycosylase mediate resistance to ampicillin in A. baldaniorum Sp245. While the gene encoding lytic transglycosylase is positively regulated by an ECF σ-factor (RpoE7), a cascade of RpoE7 and RpoH3 σ factors negatively regulates the expression of β-lactamase. This is the first evidence showing involvement of a regulatory cascade of σ factors in the regulation of ampicillin resistance in a rhizobacterium.

The AmiC/NlpD pathway dominates peptidoglycan breakdown in Neisseria meningitidis and affects cell separation, NOD1 agonist production, and infection

The human-restricted pathogen Neisseria meningitidis, which is best known for causing invasive meningococcal disease, has a nonpathogenic lifestyle as an asymptomatic colonizer of the human naso- and oropharyngeal space. N. meningitidis releases small peptidoglycan (PG) fragments during growth. It was demonstrated previously that N. meningitidis releases low levels of tripeptide PG monomer, which is an inflammatory molecule recognized by the human intracellular innate immune receptor NOD1. In this present study, we demonstrated that N. meningitidis released more PG-derived peptides compared to PG monomers. Using a reporter cell line overexpressing human NOD1, we showed that N. meningitidis activates NOD1 using PG-derived peptides. Generation of such peptides required the presence of the periplasmic N-acetylmuramyl-L-alanine amidase AmiC, and the outer membrane lipoprotein, NlpD. AmiC and NlpD were found to function in cell separation, and mutation of either amiC or nlpD resulted in large clumps of unseparated N. meningitidis cells instead of the characteristic diplococci. Using stochastic optical reconstruction microscopy, we demonstrated that FLAG epitope-tagged NlpD localized to the septum, while similarly-tagged AmiC was found at the septum in some diplococci but distributed around the cell in most cases. In a human whole blood infection assay, an nlpD mutant was severely attenuated and showed particular sensitivity to complement. Thus, in N. meningitidis the cell separation proteins AmiC and NlpD are necessary for NOD1 stimulation and for survival during infection of human blood.

Impact of Peptidoglycan Recycling Blockade and Expression of Horizontally Acquired β-Lactamases on Pseudomonas aeruginosa Virulence

In the current scenario of antibiotic resistance magnification, new weapons against top nosocomial pathogens like Pseudomonas aeruginosa are urgently needed. The interplay between β-lactam resistance and virulence is considered a promising source of targets to be attacked by antivirulence therapies, and in this regard, we previously showed that a peptidoglycan recycling blockade dramatically attenuated the pathogenic power of P. aeruginosa strains hyperproducing the chromosomal β-lactamase AmpC. Here, we sought to ascertain whether this observation could be applicable to other β-lactamases. To do so, P. aeruginosa wild-type or peptidoglycan recycling-defective strains (ΔampG and ΔnagZ) harboring different cloned β-lactamases (transferable GES, VIM, and OXA types) were used to assess their virulence in Galleria mellonella larvae by determining 50% lethal doses (LD50s). A mild yet significant LD50 increase was observed after peptidoglycan recycling disruption per se, whereas the expression of class A and B enzymes did not impact virulence. While the production of the narrow-spectrum class D OXA-2 entailed a slight attenuation, its extended-spectrum derivatives OXA-226 (W159R [bearing a change of W to R at position 159]), OXA-161 (N148D), and principally, OXA-539 (D149 duplication) were associated with outstanding virulence impairments, especially in recycling-defective backgrounds (with some LD50s being >1,000-fold that of the wild type). Although their exact molecular bases remain to be deciphered, these results suggest that mutations affecting the catalytic center and, therefore, the hydrolytic spectrum of OXA-2-derived enzymes also drastically impact the pathogenic power of P. aeruginosa. This work provides new and relevant knowledge to the complex topic of the interplay between the production of β-lactamases and virulence that could be useful to build future therapeutic strategies against P. aeruginosa. IMPORTANCE Pseudomonas aeruginosa is one of the leading nosocomial pathogens whose growing resistance makes the development of therapeutic options extremely urgent. The resistance-virulence interplay has classically aroused researchers' interest as a source of therapeutic targets. In this regard, we describe a wide array of virulence attenuations associated with different transferable β-lactamases, among which the production of OXA-2-derived extended-spectrum β-lactamases stood out as a dramatic handicap for pathogenesis, likely as a side effect of mutations causing the expansion of their hydrolytic spectrums. Moreover, our results confirm the validity of disturbing peptidoglycan recycling as a weapon to attenuate P. aeruginosa virulence in class C and D β-lactamase production backgrounds. In the current scenario of dissemination of horizontally acquired β-lactamases, this work brings out new data on the complex interplay between the production of specific enzymes and virulence attenuation that, if complemented with the characterization of the underlying mechanisms, will likely be exploitable to develop future virulence-targeting antipseudomonal strategies.

Induction of Broad β-lactam Resistance in Achromobacter ruhlandii by Exposure to Ticarcillin Is Primarily Linked to Substitutions in Murein Peptide Ligase Mpl

Achromobacter species are emerging pathogens in cystic fibrosis with inherent resistance to several classes of antimicrobial agents. We exposed strains with wild-type antimicrobial susceptibility to ticarcillin and generated mutants with broad β-lactam resistance. Within the detection limit of the assay, the capability to develop mutational resistance was strain-specific and reproducible. Mutational resistance was observed for all three tested strains of Achromobacter ruhlandii, for one of seven strains of Achromobacter xylosoxidans, and for none of five strains of Achromobacter insuavis. All mutants were resistant to piperacillin-tazobactam, while minimal inhibitory concentration of several other β-lactams increased 4–32-fold. Whole genome sequencing identified 1–4 non-synonymous mutations in known genes per mutant. All mutants encoded amino acid substitutions in cell wall recycling proteins, primarily Mpl, and the observed resistance is probably caused by hyperproduction of OXA-114-like β-lactamases. Related, but not identical substitutions were detected in clinical strains expressing acquired antimicrobial resistance.

Effects of Inactivation of d,d-Transpeptidases of Acinetobacter baumannii on Bacterial Growth and Susceptibility to β-Lactam Antibiotics

Resistance to β-lactams, the most used antibiotics worldwide, constitutes the major problem for the treatment of bacterial infections. In the nosocomial pathogen Acinetobacter baumannii, β-lactamase-mediated resistance to the carbapenem family of β-lactam antibiotics has resulted in the selection and dissemination of multidrug-resistant isolates, which often cause infections characterized by high mortality rates. There is thus an urgent demand for new β-lactamase-resistant antibiotics that also inhibit their targets, penicillin-binding proteins (PBPs). As some PBPs are indispensable for the biosynthesis of the bacterial cell wall and survival, we evaluated their importance for the growth of A. baumannii by performing gene inactivation studies of d,d-transpeptidase domains of high-molecular-mass (HMM) PBPs individually and in combination with one another. We show that PBP3 is essential for A. baumannii survival, as deletion mutants of this d,d-transpeptidase were not viable. The inactivation of PBP1a resulted in partial cell lysis and retardation of bacterial growth, and these effects were further enhanced by the additional inactivation of PBP2 but not PBP1b. Susceptibility to β-lactam antibiotics increased 4- to 8-fold for the A. baumannii PBP1a/PBP1b/PBP2 triple mutant and 2- to 4-fold for all remaining mutants. Analysis of the peptidoglycan structure revealed a significant change in the muropeptide composition of the triple mutant and demonstrated that the lack of d,d-transpeptidase activity of PBP1a, PBP1b, and PBP2 is compensated for by an increase in the l,d-transpeptidase-mediated cross-linking activity of LdtJ. Overall, our data showed that in addition to essential PBP3, the simultaneous inhibition of PBP1a and PBP2 or PBPs in combination with LdtJ could represent potential strategies for the design of novel drugs against A. baumannii.

Microbial cell surface engineering for high-level synthesis of bio-products

Microbial cell surface layers, which mainly include the cell membrane, cell wall, periplasmic space, outer membrane, capsules, S-layers, pili, and flagella, control material exchange between the cell and the extracellular environment, and have great impact on production titers and yields of various bio-products synthesized by microbes. Recent research work has made exciting achievements in metabolic engineering using microbial cell surface components as novel regulation targets without direct modifications of the metabolic pathways of the desired products. This review article will summarize the accomplishments obtained in this emerging field, and will describe various engineering strategies that have been adopted in bacteria and yeasts for the enhancement of mass transfer across the cell surface, improvement of protein expression and folding, modulation of cell size and shape, and re-direction of cellular resources, all of which contribute to the construction of more efficient microbial cell factories toward the synthesis of a variety of bio-products. The existing problems and possible future directions will also be discussed.

Role of AmpC-Inducing Genes in Modulating Other Serine Beta-Lactamases in Escherichia coli

The consistently mutating bacterial genotypes appear to have accelerated the global challenge with antimicrobial resistance (AMR); it is therefore timely to investigate certain less-explored fields of targeting AMR mechanisms in bacterial pathogens. One of such areas is beta-lactamase (BLA) induction that can provide us with a collection of prospective therapeutic targets. The key genes (ampD, ampE and ampG) to which the AmpC induction mechanism is linked are also involved in regulating the production of fragmented muropeptides generated during cell-wall peptidoglycan recycling. Although the involvement of these genes in inducing class C BLAs is apparent, their effect on serine beta-lactamase (serine-BLA) induction is little known. Here, by using ∆ampD and ∆ampE mutants of E. coli, we attempted to elucidate the effects of ampD and ampE on the expression of serine-BLAs originating from Enterobacteriaceae, viz., CTX-M-15, TEM-1 and OXA-2. Results show that cefotaxime is the preferred inducer for CTX-M-15 and amoxicillin for TEM-1, whereas oxacillin for OXA-2. Surprisingly, exogenous BLA expressions are elevated in ∆ampD and ∆ampE mutants but do not always alter their beta-lactam susceptibility. Moreover, the beta-lactam resistance is increased upon in trans expression of ampD, whereas the same is decreased upon ampE expression, indicating a differential effect of ampD and ampE overexpression. In a nutshell, depending on the BLA, AmpD amidase moderately facilitates a varying level of serine-BLA expression whereas AmpE transporter acts likely as a negative regulator of serine-BLA.

Imitate to illuminate: labeling of bacterial peptidoglycan with fluorescent and bio-orthogonal stem peptide-mimicking probes

Because of its high involvement in antibiotic therapy and the emergence of drug-resistance, the chemical structure and biosynthesis of bacterial peptidoglycan (PGN) have been some of the key topics in bacteriology for several decades. Recent advances in the development of fluorescent or bio-orthogonal stem peptide-mimicking probes for PGN-labeling have rekindled the interest of chemical biologists and microbiologists in this area. The structural designs, bio-orthogonal features and flexible uses of these peptide-based probes allow directly assessing, not only the presence of PGN in different biological systems, but also specific steps in PGN biosynthesis. In this review, we summarize the design rationales, functioning mechanisms, and microbial processes/questions involved in these PGN-targeting probes. Our perspectives on the limitations and future development of these tools are also presented.

By imitating the structures of stem peptide, many fluorescent and bio-orthogonal labeling probes have been designed and used in illuminating the peptidoglycan biosynthesis processes.

Horizontal-Acquisition of a Promiscuous Peptidoglycan-Recycling Enzyme Enables Aphids To Influence Symbiont Cell Wall Metabolism

During evolution, enzymes can undergo shifts in preferred substrates or in catalytic activities. An intriguing question is how enzyme function changes following horizontal gene transfer, especially for bacterial genes that have moved to animal genomes. Some insects have acquired genes that encode enzymes for the biosynthesis of bacterial cell wall components and that appear to function to support or control their obligate endosymbiotic bacteria. In aphids, the bacterial endosymbiont Buchnera aphidicola provides essential amino acids for aphid hosts but lacks most genes for remodeling of the bacterial cell wall. The aphid genome has acquired seven genes with putative functions in cell wall metabolism that are primarily expressed in the aphid cells harboring Buchnera. In analyses of aphid homogenates, we detected peptidoglycan (PGN) muropeptides indicative of the reactions of PGN hydrolases encoded by horizontally acquired aphid genes but not by Buchnera genes. We produced one such host enzyme, ApLdcA, and characterized its activity with both cell wall derived and synthetic PGN. Both ApLdcA and the homologous enzyme in Escherichia coli, which functions as an l,d-carboxypeptidase in the cytoplasmic PGN recycling pathway, exhibit turnover of PGN substrates containing stem pentapeptides and cross-linkages via l,d-endopeptidase activity, consistent with a potential role in cell wall remodeling. Our results suggest that ApLdcA derives its functions from the promiscuous activities of an ancestral LdcA enzyme, whose acquisition by the aphid genome may have enabled hosts to influence Buchnera cell wall metabolism as a means to control symbiont growth and division. IMPORTANCE Most enzymes are capable of performing biologically irrelevant side reactions. During evolution, promiscuous enzyme activities may acquire new biological roles, especially after horizontal gene transfer to new organisms. Pea aphids harbor obligate bacterial symbionts called Buchnera and encode horizontally acquired bacterial genes with putative roles in cell wall metabolism. Though Buchnera lacks cell wall endopeptidase genes, we found evidence of endopeptidase activity among peptidoglycan muropeptides purified from aphids. We characterized a multifunctional, aphid-encoded enzyme, ApLdcA, which displays l,d-endopeptidase activities considered promiscuous for the Escherichia coli homolog, for which these activities do not contribute to its native role in peptidoglycan recycling. These results exemplify the roles of enzyme promiscuity and horizontal gene transfer in enzyme evolution and demonstrate how aphids influence symbiont cell wall metabolism.

The transposable element-rich genome of the cereal pest Sitophilus oryzae

BACKGROUND

The rice weevil Sitophilus oryzae is one of the most important agricultural pests, causing extensive damage to cereal in fields and to stored grains. S. oryzae has an intracellular symbiotic relationship (endosymbiosis) with the Gram-negative bacterium Sodalis pierantonius and is a valuable model to decipher host-symbiont molecular interactions.

RESULTS

We sequenced the Sitophilus oryzae genome using a combination of short and long reads to produce the best assembly for a Curculionidae species to date. We show that S. oryzae has undergone successive bursts of transposable element (TE) amplification, representing 72% of the genome. In addition, we show that many TE families are transcriptionally active, and changes in their expression are associated with insect endosymbiotic state. S. oryzae has undergone a high gene expansion rate, when compared to other beetles. Reconstruction of host-symbiont metabolic networks revealed that, despite its recent association with cereal weevils (30 kyear), S. pierantonius relies on the host for several amino acids and nucleotides to survive and to produce vitamins and essential amino acids required for insect development and cuticle biosynthesis.

CONCLUSIONS

Here we present the genome of an agricultural pest beetle, which may act as a foundation for pest control. In addition, S. oryzae may be a useful model for endosymbiosis, and studying TE evolution and regulation, along with the impact of TEs on eukaryotic genomes.

Cell Wall Biology of Vibrio cholerae

Most bacteria are protected from environmental offenses by a cell wall consisting of strong yet elastic peptidoglycan. The cell wall is essential for preserving bacterial morphology and viability, and thus the enzymes involved in the production and turnover of peptidoglycan have become preferred targets for many of our most successful antibiotics. In the past decades, Vibrio cholerae, the gram-negative pathogen causing the diarrheal disease cholera, has become a major model for understanding cell wall genetics, biochemistry, and physiology. More than 100 articles have shed light on novel cell wall genetic determinants, regulatory links, and adaptive mechanisms. Here we provide the first comprehensive review of V. cholerae's cell wall biology and genetics. Special emphasis is placed on the similarities and differences with Escherichia coli, the paradigm for understanding cell wall metabolism and chemical structure in gram-negative bacteria.

The influence of the gut microbiome on BCG-induced trained immunity

BACKGROUND

The bacillus Calmette-Guérin (BCG) vaccine protects against tuberculosis and heterologous infections but elicits high inter-individual variation in specific and nonspecific, or trained, immune responses. While the gut microbiome is increasingly recognized as an important modulator of vaccine responses and immunity in general, its potential role in BCG-induced protection is largely unknown.

RESULTS

Stool and blood were collected from 321 healthy adults before BCG vaccination, followed by blood sampling after 2 weeks and 3 months. Metagenomics based on de novo genome assembly reveals 43 immunomodulatory taxa. The nonspecific, trained immune response is detected by altered production of cytokines IL-6, IL-1β, and TNF-α upon ex vivo blood restimulation with Staphylococcus aureus and negatively correlates with abundance of Roseburia. The specific response, measured by IFN-γ production upon Mycobacterium tuberculosis stimulation, is associated positively with Ruminococcus and Eggerthella lenta. The identified immunomodulatory taxa also have the strongest effects on circulating metabolites, with Roseburia affecting phenylalanine metabolism. This is corroborated by abundances of relevant enzymes, suggesting alternate phenylalanine metabolism modules are activated in a Roseburia species-dependent manner.

CONCLUSIONS

Variability in cytokine production after BCG vaccination is associated with the abundance of microbial genomes, which in turn affect or produce metabolites in circulation. Roseburia is found to alter both trained immune responses and phenylalanine metabolism, revealing microbes and microbial products that may alter BCG-induced immunity. Together, our findings contribute to the understanding of specific and trained immune responses after BCG vaccination.

Role of endopeptidases in peptidoglycan synthesis mediated by alternative cross-linking enzymes in Escherichia coli

Bacteria resist to the turgor pressure of the cytoplasm through a net-like macromolecule, the peptidoglycan, made of glycan strands connected via peptides cross-linked by penicillin-binding proteins (PBPs). We recently reported the emergence of β-lactam resistance resulting from a bypass of PBPs by the YcbB L,D-transpeptidase (LdtD), which form chemically distinct 3→3 cross-links compared to 4→3 formed by PBPs. Here we show that peptidoglycan expansion requires controlled hydrolysis of cross-links and identify among eight endopeptidase paralogues the minimum enzyme complements essential for bacterial growth with 4→3 (MepM) and 3→3 (MepM and MepK) cross-links. Purified Mep endopeptidases unexpectedly displayed a 4→3 and 3→3 dual specificity implying recognition of a common motif in the two cross-link types. Uncoupling of the polymerization of glycan chains from the 4→3 cross-linking reaction was found to facilitate the bypass of PBPs by YcbB. These results illustrate the plasticity of the peptidoglycan polymerization machinery in response to the selective pressure of β-lactams.

The Diadenylate Cyclase CdaA Is Critical for Borrelia turicatae Virulence and Physiology

Relapsing fever (RF), caused by spirochetes of the genus Borrelia, is a globally distributed, vector-borne disease with high prevalence in developing countries. To date, signaling pathways required for infection and virulence of RF Borrelia spirochetes are unknown. Cyclic di-AMP (c-di-AMP), synthesized by diadenylate cyclases (DACs), is a second messenger predominantly found in Gram-positive organisms that is linked to virulence and essential physiological processes. Although Borrelia is Gram-negative, it encodes one DAC (CdaA), and its importance remains undefined. To investigate the contribution of c-di-AMP signaling in the RF bacterium Borrelia turicatae, a cdaA mutant was generated. The mutant was significantly attenuated during murine infection, and genetic complementation reversed this phenotype. Because c-di-AMP is essential for viability in many bacteria, whole-genome sequencing was performed on cdaA mutants, and single-nucleotide polymorphisms identified potential suppressor mutations. Additionally, conditional mutation of cdaA confirmed that CdaA is important for normal growth and physiology. Interestingly, mutation of cdaA did not affect expression of homologs of virulence regulators whose levels are impacted by c-di-AMP signaling in the Lyme disease bacterium Borrelia burgdorferi Finally, the cdaA mutant had a significant growth defect when grown with salts, at decreased osmolarity, and without pyruvate. While the salt treatment phenotype was not reversed by genetic complementation, possibly due to suppressor mutations, growth defects at decreased osmolarity and in media lacking pyruvate could be attributed directly to cdaA inactivation. Overall, these results indicate CdaA is critical for B. turicatae pathogenesis and link c-di-AMP to osmoregulation and central metabolism in RF spirochetes.

A peptidoglycan storm caused by β-lactam antibiotic's action on host microbiota drives Candida albicans infection

The commensal fungus Candida albicans often causes life-threatening infections in patients who are immunocompromised with high mortality. A prominent but poorly understood risk factor for the C. albicans commensal‒pathogen transition is the use of broad-spectrum antibiotics. Here, we report that β-lactam antibiotics cause bacteria to release significant quantities of peptidoglycan fragments that potently induce the invasive hyphal growth of C. albicans. We identify several active peptidoglycan subunits, including tracheal cytotoxin, a molecule produced by many Gram-negative bacteria, and fragments purified from the cell wall of Gram-positive Staphylococcus aureus. Feeding mice with β-lactam antibiotics causes a peptidoglycan storm that transforms the gut from a niche usually restraining C. albicans in the commensal state to promoting invasive growth, leading to systemic dissemination. Our findings reveal a mechanism underlying a significant risk factor for C. albicans infection, which could inform clinicians regarding future antibiotic selection to minimize this deadly disease incidence.

The Building Blocks of Antimicrobial Resistance in Pseudomonas aeruginosa: Implications for Current Resistance-Breaking Therapies

P. aeruginosa is classified as a priority one pathogen by the World Health Organisation, and new drugs are urgently needed, due to the emergence of multidrug-resistant (MDR) strains. Antimicrobial-resistant nosocomial pathogens such as P. aeruginosa pose unwavering and increasing threats. Antimicrobial stewardship has been a challenge during the COVID-19 pandemic, with a majority of those hospitalized with SARS-CoV2 infection given antibiotics as a safeguard against secondary bacterial infection. This increased usage, along with increased handling of sanitizers and disinfectants globally, may further accelerate the development and spread of cross-resistance to antibiotics. In addition, P. aeruginosa is the primary causative agent of morbidity and mortality in people with the life-shortening genetic disease cystic fibrosis (CF). Prolonged periods of selective pressure, associated with extended antibiotic treatment and the actions of host immune effectors, results in widespread adaptive and acquired resistance in P. aeruginosa found colonizing the lungs of people with CF. This review discusses the arsenal of resistance mechanisms utilized by P. aeruginosa, how these operate under high-stress environments such as the CF lung and how their interconnectedness can result in resistance to multiple antibiotic classes. Intrinsic, adaptive and acquired resistance mechanisms will be described, with a focus on how each layer of resistance can serve as a building block, contributing to multi-tiered resistance to antimicrobial activity. Recent progress in the development of anti-resistance adjuvant therapies, targeting one or more of these building blocks, should lead to novel strategies for combatting multidrug resistant P. aeruginosa. Anti-resistance adjuvant therapy holds great promise, not least because resistance against such therapeutics is predicted to be rare. The non-bactericidal nature of anti-resistance adjuvants reduce the selective pressures that drive resistance. Anti-resistance adjuvant therapy may also be advantageous in facilitating efficacious use of traditional antimicrobials, through enhanced penetration of the antibiotic into the bacterial cell. Promising anti-resistance adjuvant therapeutics and targets will be described, and key remaining challenges highlighted. As antimicrobial stewardship becomes more challenging in an era of emerging and re-emerging infectious diseases and global conflict, innovation in antibiotic adjuvant therapy can play an important role in extending the shelf-life of our existing antimicrobial therapeutic agents.