Genomewide Profiling of the Enterococcus faecalis Transcriptional Response to Teixobactin Reveals CroRS as an Essential Regulator of Antimicrobial Tolerance

Novel Teixobactin Analogues Show Promising In Vitro Activity on Biofilm Formation by Staphylococcus aureus and Enterococcus faecalis

The treatment of infections caused by biofilm-forming organisms is challenging. The newly discovered antibiotic teixobactin shows activity against a wide range of biofilm-forming bacteria. However, the laborious and low-yield chemical synthesis of teixobactin complicates its further development for clinical application. The use of more easily synthesized teixobactin analogues may offer promise in this regard. In this article, three newly developed analogues were tested for efficacy against Staphylococcus aureus and Enterococcus faecalis. Minimum inhibitory and -bactericidal concentrations were investigated. MIC values for S. aureus and E. faecalis ranged from 0.5-2 and 2-4 μg/mL, respectively. Moreover, the ability of the analogues to prevent biofilm formation and to inactivate bacterial cells in already established S. aureus biofilm on medical grade materials (PVC and PTFE) used in the production of infusion tubing and catheters were also tested. The analogues showed an ability to prevent biofilm formation and inactivate bacterial cells in established biofilms at concentrations as low as 1-2 μg/mL. Confocal laser scanning microscopy showed that the most promising analogue (TB3) inactivated S. aureus cells in a preformed biofilm and gave a reduction in biovolume. The relative ease of synthesis of the analogues and their in vitro efficacy, makes them promising candidates for pharmaceutical development.

Guarding the walls: the multifaceted roles of Bce modules in cell envelope stress sensing and antimicrobial resistance

Bacteria have developed diverse strategies for defending their cell envelopes from external threats. In Firmicutes, one widespread strategy is to use Bce modules-membrane protein complexes that unite a peptide-detoxifying ABC transporter with a stress response coordinating two-component system. These modules provide specific, front-line defense for a wide variety of antimicrobial peptides and small molecule antibiotics as well as coordinate responses for heat, acid, and oxidative stress. Because of these abilities, Bce modules play important roles in virulence and the development of antibiotic resistance in a variety of pathogens, including Staphylococcus, Streptococcus, and Enterococcus species. Despite their importance, Bce modules are still poorly understood, with scattered functional data in only a small number of species. In this review, we will discuss Bce module structure in light of recent cryo-electron microscopy structures of the B. subtilis BceABRS module and explore the common threads and variations-on-a-theme in Bce module mechanisms across species. We also highlight the many remaining questions about Bce module function. Understanding these multifunctional membrane complexes will enhance our understanding of bacterial stress sensing and may point toward new therapeutic targets for highly resistant pathogens.

Arginine impacts aggregation, biofilm formation, and antibiotic susceptibility in Enterococcus faecalis

Enterococcus faecalis is a commensal bacterium in the gastrointestinal tract (GIT) of humans and other organisms. E. faecalis also causes infections in root canals, wounds, the urinary tract, and on heart valves. E. faecalis metabolizes arginine through the arginine deiminase (ADI) pathway, which converts arginine to ornithine and releases ATP, ammonia, and CO2. E. faecalis arginine metabolism also affects virulence of other pathogens during co-culture. E. faecalis may encounter elevated levels of arginine in the GIT or the oral cavity, where arginine is used as a dental therapeutic. Little is known about how E. faecalis responds to growth in arginine in the absence of other bacteria. To address this, we used RNAseq and additional assays to measure growth, gene expression, and biofilm formation in E. faecalis OG1RF grown in arginine. We demonstrate that arginine decreases E. faecalis biofilm production and causes widespread differential expression of genes related to metabolism, quorum sensing, and polysaccharide synthesis. Growth in arginine also increases aggregation of E. faecalis and promotes decreased susceptibility to the antibiotics ampicillin and ceftriaxone. This work provides a platform for understanding of how the presence of arginine in biological niches affects E. faecalis physiology and virulence of surrounding microbes.

CroR Regulates Expression of pbp4(5) to Promote Cephalosporin Resistance in Enterococcus faecalis

Enterococcus faecalis is an opportunistic pathogen and a major cause of severe nosocomial infections. Treatment options against enterococcal infections are declining due to the resistance of enterococci to numerous antibiotics. A key risk factor for developing enterococcal infections is treatment with cephalosporin antibiotics, to which enterococci are intrinsically resistant. For susceptible organisms, cephalosporins inhibit bacterial growth by acylating the active site of penicillin-binding proteins (PBPs), key enzymes that catalyze peptidoglycan cross-linking. Two specific PBPs of enterococci, Pbp4(5) and PbpA(2b), exhibit low reactivity toward cephalosporins, allowing these PBPs to cross-link peptidoglycan in the presence of cephalosporins to drive resistance in enterococci, but the mechanisms by which these PBPs are regulated are poorly understood. The CroS/R two-component signal transduction system (TCS) is also required for cephalosporin resistance. Activation of CroS/R by cephalosporins leads to CroR-dependent changes in gene expression. However, the specific genes regulated by CroS/R that are responsible for cephalosporin resistance remain largely unknown. In this study, we characterized CroR-dependent transcriptome remodeling by RNA-seq, identifying pbp4(5) as a CroR regulon member in multiple, diverse lineages of E. faecalis. Through genetic analysis of the pbp4(5) and croR promoters, we uncovered a CroR-dependent regulatory motif. Mutations in this motif to disrupt CroR-dependent upregulation of pbp4(5) in the presence of cell wall stress resulted in a reduction of resistance to cephalosporins in E. faecalis, demonstrating that enhanced production of Pbp4(5) and likely other proteins involved in peptidoglycan biogenesis by the CroS/R system drives enterococcal cephalosporin resistance. IMPORTANCE Investigation into molecular mechanisms used by enterococci to subvert cephalosporin antibiotics is imperative for preventing and treating life-threatening infections. In this study, we used genetic means to investigate the functional output of the CroS/R TCS required for enterococcal resistance to cephalosporins. We found that enhanced production of the penicillin-binding protein Pbp4(5) upon exposure to cell wall stress was mediated by CroS/R and was critical for intrinsic cephalosporin resistance of E. faecalis.

Antimicrobial Resistance and Its Spread Is a Global Threat

Antimicrobial resistance (AMR) is a challenge to human wellbeing the world over and is one of the more serious public health concerns. AMR has the potential to emerge as a serious healthcare threat if left unchecked, and could put into motion another pandemic. This establishes the need for the establishment of global health solutions around AMR, taking into account microdata from different parts of the world. The positive influences in this regard could be establishing conducive social norms, charting individual and group behavior practices that favor global human health, and lastly, increasing collective awareness around the need for such action. Apart from being an emerging threat in the clinical space, AMR also increases treatment complexity, posing a real challenge to the existing guidelines around the management of antibiotic resistance. The attribute of resistance development has been linked to many genetic elements, some of which have complex transmission pathways between microbes. Beyond this, new mechanisms underlying the development of AMR are being discovered, making this field an important aspect of medical microbiology. Apart from the genetic aspects of AMR, other practices, including misdiagnosis, exposure to broad-spectrum antibiotics, and lack of rapid diagnosis, add to the creation of resistance. However, upgrades and innovations in DNA sequencing technologies with bioinformatics have revolutionized the diagnostic industry, aiding the real-time detection of causes of AMR and its elements, which are important to delineating control and prevention approaches to fight the threat.

Antimicrobial tolerance and its role in the development of resistance: Lessons from enterococci

Bacteria have developed resistance against every antimicrobial in clinical use at an alarming rate. There is a critical need for more effective use of antimicrobials to both extend their shelf life and prevent resistance from arising. Significantly, antimicrobial tolerance, i.e., the ability to survive but not proliferate during antimicrobial exposure, has been shown to precede the development of bona fide antimicrobial resistance (AMR), sparking a renewed and rapidly increasing interest in this field. As a consequence, problematic infections for the first time are now being investigated for antimicrobial tolerance, with increasing reports demonstrating in-host evolution of antimicrobial tolerance. Tolerance has been identified in a wide array of bacterial species to all bactericidal antimicrobials. Of particular interest are enterococci, which contain the opportunistic bacterial pathogens Enterococcus faecalis and Enterococcus faecium. Enterococci are one of the leading causes of hospital-acquired infection and possess intrinsic tolerance to a number of antimicrobial classes. Persistence of these infections in the clinic is of growing concern, particularly for the immunocompromised. Here, we review current known mechanisms of antimicrobial tolerance, and include an in-depth analysis of those identified in enterococci with implications for both the development and prevention of AMR.

Antimicrobial Effects of L-Chg10-Teixobactin against Enterococcus faecalis In Vitro

Objective: Teixobactin and its analogues are a new class of antibiotics that have no detectable bacterial resistance. This study was designed to determine the antibacterial and antibiofilm activities of a novel teixobactin analogue, _L-Chg₁₀-teixobactin, against two strains of Enterococcus faecalis (E. faecalis). Materials and Methods: The efficacy of _L-Chg₁₀-teixobactin against two strains of E. faecalis (ATCC 29212 and 47077) was determined using Clinical and Laboratory Standards Institute methods. _L-Chg₁₀-teixobactin was prepared at a stock concentration of 1 mg/mL in 5% DMSO. The minimum inhibitory concentration (MIC) was calculated using a two-fold serial broth dilution method, utilizing a 96-well plate. The minimum bactericidal concentration (MBC) was determined by plating the bacteria onto agar to define the concentration that resulted in 99.9% of bacterial death. Ampicillin was used as the control. The effect of _L-Chg₁₀-teixobactin on the inhibition of ATCC 47077 strain biofilm formation was determined by measuring the minimum biofilm inhibitory concentration (MBIC) using the safranin assay, while the eradication of the preformed biofilm was determined by measuring the minimum biofilm eradication concentration (MBEC) using the XTT assay. For nonlinear data, the log dose–response curve was plotted to calculate the optimum concentration using Excel (version 16.51, Microsoft^® excel. 2021, Microsoft Corporation, Reymond, WA, USA). The data are presented as mean ± standard deviation (SD). Results: The MIC and MBC values of _L-Chg₁₀-teixobactin against both strains of E. faecalis were 0.8 μg/mL. The MIC of ampicillin was 1.25 μg/mL for ATCC 29212 and ranged from 1.25 to 5 μg/mL for ATCC 47077. The MBC of ampicillin for ATCC 29212 and ATCC 47077 was 10 and 20 μg/mL, respectively. The MIC and MBC of ampicillin were much higher compared with those of _L-Chg₁₀-teixobactin. The MBEC₈₀ of _L-Chg₁₀-teixobactin was 4.60 μg/mL for ATCC 47077, which was much lower than that of ampicillin (20 μg/mL). Conclusions:_L-Chg₁₀-teixobactin demonstrated potent antibacterial and antibiofilm effects against E. faecalis, suggesting its potential role an effective antibacterial and antibiofilm agent in endodontic treatment.

IreK-Mediated, Cell Wall-Protective Phosphorylation in Enterococcus faecalis

Enterococcus faecalis is a Gram-positive bacterium that is a major cause of hospital-acquired infections due, in part, to its intrinsic resistance to cell wall-active antimicrobials. One critical determinant of this resistance is the transmembrane kinase IreK, which belongs to the penicillin-binding protein and serine/threonine kinase-associated kinase family of bacterial signaling proteins involved with the regulation of cell wall homeostasis. The activity of IreK is enhanced in response to cell wall stress, but direct substrates of IreK phosphorylation, leading to antimicrobial resistance, are largely unknown. To better understand stress-modulated phosphorylation events contributing to antimicrobial resistance, wild type E. faecalis cells treated with cell wall-active antimicrobials, chlorhexidine or ceftriaxone, were examined via phosphoproteomics. Among the most prominent changes was increased phosphorylation of divisome components after both treatments, suggesting that E. faecalis modulates cell division in response to cell wall stress. Phosphorylation mediated by IreK was then determined via a similar analysis with a E. faecalis ΔireK mutant strain, revealing potential IreK substrates involved with the regulation of peptidoglycan biosynthesis and within the E. faecalis CroS/R two-component system, another signal transduction pathway that promotes antimicrobial resistance. These results reveal critical insights into the biological functions of IreK.

From cloning to mutant in 5 days: rapid allelic exchange in Staphylococcus aureus

In the last 10 years, the barriers preventing the uptake of foreign DNA by clinical Staphylococcus aureus isolates have been identified and powerful mutagenesis techniques such as allelic exchange are now possible in most genotypes. However, these targeted approaches can still be cumbersome, and the construction of unmarked deletions/point mutations may take many weeks or months. Here, we introduce a streamlined allelic exchange protocol using IMxxB Escherichia coli and the plasmid pIMAY-Z. With this optimized approach, a site-specific mutation can be introduced into S. aureus in 5 days, from the start of cloning to isolation of genomic DNA for confirmatory whole-genome sequencing. This streamlined protocol considerably reduces the time required to introduce a specific, unmarked mutation in S. aureus and should dramatically improve the scalability of gene-function studies.

Bioinformatics Approaches to the Understanding of Molecular Mechanisms in Antimicrobial Resistance

Antimicrobial resistance (AMR) is a major health concern worldwide. A better understanding of the underlying molecular mechanisms is needed. Advances in whole genome sequencing and other high-throughput unbiased instrumental technologies to study the molecular pathogenicity of infectious diseases enable the accumulation of large amounts of data that are amenable to bioinformatic analysis and the discovery of new signatures of AMR. In this work, we review representative methods published in the past five years to define major approaches developed to-date in the understanding of AMR mechanisms. Advantages and limitations for applications of these methods in clinical laboratory testing and basic research are discussed.