Inhibition of cell wall turnover and autolysis by vancomycin in a highly vancomycin-resistant mutant of Staphylococcus aureus

Genomic investigation and clinical correlates of the in vitro β-lactam: NaHCO3 responsiveness phenotype among methicillin-resistant Staphylococcus aureus isolates from a randomized clinical trial

NaHCO3 responsiveness is a novel phenotype where some methicillin-resistant Staphylococcus aureus (MRSA) isolates exhibit significantly lower minimal inhibitory concentrations (MIC) to oxacillin and/or cefazolin in the presence of NaHCO3. NaHCO3 responsiveness correlated with treatment response to β-lactams in an endocarditis animal model. We investigated whether treatment of NaHCO3-responsive strains with β-lactams was associated with faster clearance of bacteremia. The CAMERA2 trial (Combination Antibiotics for Methicillin-Resistant Staphylococcus aureus) randomly assigned participants with MRSA bloodstream infections to standard therapy, or to standard therapy plus an anti-staphylococcal β-lactam (combination therapy). For 117 CAMERA2 MRSA isolates, we determined by broth microdilution the MIC of cefazolin and oxacillin, with and without 44 mM of NaHCO3. Isolates exhibiting ≥4-fold decrease in the MIC to cefazolin or oxacillin in the presence of NaHCO3 were considered "NaHCO3-responsive" to that agent. We compared the rate of persistent bacteremia among participants who had infections caused by NaHCO3-responsive and non-responsive strains, and that were assigned to combination treatment with a β-lactam. Thirty-one percent (36/117) and 25% (21/85) of MRSA isolates were NaHCO3-responsive to cefazolin and oxacillin, respectively. The NaHCO3-responsive phenotype was significantly associated with sequence type 93, SCCmec type IVa, and mecA alleles with substitutions in positions -7 and -38 in the regulatory region. Among participants treated with a β-lactam, there was no association between the NaHCO3-responsive phenotype and persistent bacteremia (cefazolin, P = 0.82; oxacillin, P = 0.81). In patients from a randomized clinical trial with MRSA bloodstream infection, isolates with an in vitro β-lactam-NaHCO3-responsive phenotype were associated with distinctive genetic signatures, but not with a shorter duration of bacteremia among those treated with a β-lactam.

Staphylococcus aureus response and adaptation to vancomycin

Antibiotic resistance is an increasing challenge for the human pathogen Staphylococcus aureus. Methicillin-resistant S. aureus (MRSA) clones have spread globally, and a growing number display decreased susceptibility to vancomycin, the favoured antibiotic for treatment of MRSA infections. These vancomycin-intermediate S. aureus (VISA) or heterogeneous vancomycin-intermediate S. aureus (hVISA) strains arise from accumulation of a variety of point mutations, leading to cell wall thickening and reduced vancomycin binding to the cell wall building block, Lipid II, at the septum. They display only minor changes in vancomycin susceptibility, with varying tolerance between cells in a population, and therefore, they can be difficult to detect. In this review, we summarize current knowledge of VISA and hVISA. We discuss the role of genetic strain background or epistasis for VISA development and the possibility of strains being 'transient' VISA with gene expression changes mediated by, for example, VraTSR, GraXSR, or WalRK signal transduction systems, leading to temporary vancomycin tolerance. Additionally, we address collateral susceptibility to other antibiotics than vancomycin. Specifically, we estimate how mutations in rpoB, encoding the β-subunit of the RNA polymerase, affect overall protein structure and compare changes with rifampicin resistance. Ultimately, such in-depth analysis of VISA and hVISA strains in terms of genetic and transcriptional changes, as well as changes in protein structures, may pave the way for improved detection and guide antibiotic therapy by revealing strains at risk of VISA development. Such tools will be valuable for keeping vancomycin an asset also in the future.

Unveil of the role of fungal taxa in iron(III) reduction in paddy soil

Hitherto, research on iron(III)-reduction has mainly focused on bacteria rather than fungal communities. To acquire insight into fungi involved in iron(III) reduction, typical organic matters (containing cellulose, glucose, lactate, and acetate) and ferrihydrite were used as electron donors and acceptors, respectively, in the presence of antibiotics. After antibiotic addition, microbial iron(III) reduction was still detected at quite high rates. In comparison, rates of iron(III) reduction were significantly lower in cellulose-amended groups than those with glucose, lactate, and acetate under the antibiotic-added condition. Patterns of intermediate (e.g., acetate, pyruvate, glucose) turnover were markedly different between treatments with and without antibiotics during organic degradation. A total of 20 genera of potential respiratory and fermentative iron(III)-reducing fungi were discovered based on ITS sequencing and genome annotation. This study provided an insight into the diversity of iron(III)-reducing fungi, indicating the underestimated contribution of fungi to iron and the coupled carbon biogeochemical cycling in environments.

Antimicrobial pharmacodynamics of vancomycin and disulfiram (Antabuse®) in Staphylococcus aureus

Introduction

Intravenous vancomycin (VAN) is the primary treatment for systemic infections due to methicillin-resistant Staphylococcus aureus (MRSA). Pharmacokinetic/pharmacodynamic target (PK/PD) indices for VAN therapies are more difficult to achieve for MRSA isolates with a minimum inhibitory concentration (MIC) greater than 1 µg mL^-1. This research investigated the in vitro antimicrobial PD interaction of disulfiram (DSF) with VAN as a potential adjuvant therapy for infections due to these bacteria.

Methods

The antimicrobial interaction was assessed by differential analysis using checkerboard titration testing, time-kill studies, flow cytometry, and the post-antibiotic effect (PAE) experiment. Ten MRSA strains with MICs ranging from 1 to >256 µg mL^-1 for VAN were evaluated. A comprehensive PD assessment of the VAN/DSF interaction was performed using the VAN-intermediate (VISA) strain Mu50 (MIC 8 µg mL^-1).

Results

The addition of DSF lowered the MIC and minimum bactericidal concentration (MBC) of VAN in either a synergistic or additive manner for the MRSA panel. Optimal bactericidal effects and suppression of VISA Mu50 growth were observed with a 4/8 µg mL^-1 combination of VAN/DSF, but not the individual drugs. Flow cytometry further confirmed the enhanced killing action on a cellular level; however, the addition of DSF had an overall antagonistic effect on the PAEs for VAN.

Discussion

This research established that DSF exhibits additive to synergistic killing action with VAN for MRSA. Conversely, antagonism was observed on the PAE of VAN with DSF addition for the Mu50 strain. Flow cytometry further confirmed the enhanced bactericidal effect on a cellular level while revealing that DSF may counteract the muropeptide fortification mechanism against VAN in VISA.

Knockout of ykcB, a Putative Glycosyltransferase, Leads to Reduced Susceptibility to Vancomycin in Bacillus subtilis

Vancomycin resistance of Gram-positive bacteria poses a serious health concern around the world. In this study, we searched for vancomycin-tolerant mutants from a gene deletion library of a model Gram-positive bacterium, Bacillus subtilis, to elucidate the mechanism of vancomycin resistance. We found that knockout of ykcB, a glycosyltransferase that is expected to utilize C55-P-glucose to glycosylate cell surface components, caused reduced susceptibility to vancomycin in B. subtilis. Knockout of ykcB altered the susceptibility to multiple antibiotics, including sensitization to β-lactams and increased the pathogenicity to silkworms. Furthermore, the ykcB-knockout mutant had (i) a decreased amount of lipoteichoic acid, (ii) decreased biofilm formation, and (iii) an increased content of diglucosyl diacylglycerol, a glycolipid that shares a precursor with C55-P-glucose. These phenotypes and vancomycin tolerance were abolished by knockout of ykcC, a gene in the same operon with ykcB probably involved in C55-P-glucose synthesis. Overexpression of ykcC enhanced vancomycin tolerance in both the parent strain and the ykcB-knockout mutant. These findings suggest that ykcB deficiency induces structural changes of cell surface molecules depending on the ykcC function, leading to reduced susceptibility to vancomycin, decreased biofilm formation, and increased pathogenicity to silkworms. IMPORTANCE Although vancomycin is effective against Gram-positive bacteria, vancomycin-resistant bacteria are a major public health concern. While the vancomycin-resistance mechanisms of clinically important bacteria such as Staphylococcus aureus, Enterococcus faecium, and Streptococcus pneumoniae are well studied, they remain unclear in other Gram-positive bacteria. In the present study, we searched for vancomycin-tolerant mutants from a gene deletion library of a model Gram-positive bacterium, Bacillus subtilis, and found that knockout of a putative glycosyltransferase, ykcB, caused vancomycin tolerance in B. subtilis. Notably, unlike the previously reported vancomycin-resistant bacterial strains, ykcB-deficient B. subtilis exhibited increased virulence while maintaining its growth rate. Our results broaden the fundamental understanding of vancomycin-resistance mechanisms in Gram-positive bacteria.

Multidrug-Resistant Bacteria: Their Mechanism of Action and Prophylaxis

In the present scenario, resistance to antibiotics is one of the crucial issues related to public health. Earlier, such resistance to antibiotics was limited to nosocomial infections, but it has now become a common phenomenon. Several factors, like extensive development, overexploitation of antibiotics, excessive application of broad-spectrum drugs, and a shortage of target-oriented antimicrobial drugs, could be attributed to this condition. Nowadays, there is a rise in the occurrence of these drug-resistant pathogens due to the availability of a small number of effective antimicrobial agents. It has been estimated that if new novel drugs are not discovered or formulated, there would be no effective antibiotic available to treat these deadly resistant pathogens by 2050. For this reason, we have to look for the formulation of some new novel drugs or other options or substitutes to treat such multidrug-resistant microorganisms (MDR). The current review focuses on the evolution of the most common multidrug-resistant bacteria and discusses how these bacteria escape the effects of targeted antibiotics and become multidrug resistant. In addition, we also discuss some alternative mechanisms to prevent their infection as well.

Pharmacokinetics and pharmacodynamics of peptide antibiotics

Antimicrobial resistance is a major global health challenge. As few new efficacious antibiotics will become available in the near future, peptide antibiotics continue to be major therapeutic options for treating infections caused by multidrug-resistant pathogens. Rational use of antibiotics requires optimisation of the pharmacokinetics and pharmacodynamics for the treatment of different types of infections. Toxicodynamics must also be considered to improve the safety of antibiotic use and, where appropriate, to guide therapeutic drug monitoring. This review focuses on the pharmacokinetics/pharmacodynamics/toxicodynamics of peptide antibiotics against multidrug-resistant Gram-negative and Gram-positive pathogens. Optimising antibiotic exposure at the infection site is essential for improving their efficacy and minimising emergence of resistance.

Synergy Between Beta-Lactams and Lipo-, Glyco-, and Lipoglycopeptides, Is Independent of the Seesaw Effect in Methicillin-Resistant Staphylococcus aureus

Methicillin-resistant S. aureus (MRSA) are resistant to beta-lactams, but synergistic activity between beta-lactams and glycopeptides/lipopeptides is common. Many have attributed this synergy to the beta-lactam-glycopeptide seesaw effect; however, this association has not been rigorously tested. The objective of this study was to determine whether the seesaw effect is necessary for synergy and to measure the impact of beta-lactam exposure on lipid metabolism. We selected for three isogenic strains with reduced susceptibility to vancomycin, daptomycin, and dalbavancin by serial passaging the MRSA strain N315. We used whole genome sequencing to identify genetic variants that emerged and tested for synergy between vancomycin, daptomycin, or dalbavancin in combination with 6 beta-lactams with variable affinity for staphylococcal penicillin binding proteins (PBPs), including nafcillin, meropenem, ceftriaxone, ceftaroline, cephalexin, and cefoxitin, using time-kills. We observed that the seesaw effect with each beta-lactam was variable and the emergence of the seesaw effect for a particular beta-lactam was not necessary for synergy between that beta-lactam and vancomycin, daptomycin, or dalbavancin. Synergy was more commonly observed with vancomycin and daptomycin based combinations than dalbavancin in time-kills. Among the beta-lactams, cefoxitin and nafcillin were the most likely to exhibit synergy using the concentrations tested, while cephalexin was the least likely to exhibit synergy. Synergy was more common among the resistant mutants than the parent strain. Interestingly N315-D1 and N315-DAL0.5 both had mutations in vraTSR and walKR despite their differences in the seesaw effect. Lipidomic analysis of all strains exposed to individual beta-lactams at subinhibitory concentrations suggested that in general, the abundance of cardiolipins (CLs) and most free fatty acids (FFAs) positively correlated with the presence of synergistic effects while abundance of phosphatidylglycerols (PGs) and lysylPGs mostly negatively correlated with synergistic effects. In conclusion, the beta-lactam-glycopeptide seesaw effect and beta-lactam-glycopeptide synergy are distinct phenomena. This suggests that the emergence of the seesaw effect may not have clinical importance in terms of predicting synergy. Further work is warranted to characterize strains that don't exhibit beta-lactam synergy to identify which strains should be targeted with combination therapy and which ones cannot and to further investigate the potential role of CLs in mediating synergy.

Dosing Considerations for Combination Antistaphylococcal β-Lactam and Glyco/lipopeptide Salvage Therapy for Resistant Gram-Positive Infections: A Systematic Review

OBJECTIVE

The objective of this systematic review is to evaluate dosing regimens of combination salvage regimens used as part of infectious disease pharmacotherapy.

DATA SOURCES

A systematic review was conducted on PubMed, MEDLINE, Scopus, ProQuest Central, and CINAHL through March 2021 using the following terminology: "combination" OR "Seesaw" OR "see-saw" OR "salvage" AND "infection" OR "resistant infection" OR "Gram-positive" AND "beta-lactam" OR "cephalosporin" OR "carbapenem" OR "monobactam" OR "glycopeptide" OR "lipopeptide."

STUDY SELECTION AND DATA EXTRACTION

Following the application of inclusion and exclusion criteria, 8 pieces of literature were ultimately included in this review.

DATA SYNTHESIS

Vancomycin in combination with another agent was most commonly prescribed as initial or empirical therapy. The most common combination salvage therapy regimen consisted of daptomycin in doses up to 12 mg/kg IV every 24 hours with ceftaroline 200 to 600 mg IV every 8 to 12 hours. Although the duration of combination salvage therapy varied drastically, blood culture clearance was typically observed within 24 hours.

RELEVANCE TO PATIENT CARE AND CLINICAL PRACTICE

Antimicrobial-resistant Gram-positive organisms have posed an emergent threat to antimicrobial stewardship initiatives. Utilizing either a glycopeptide or lipopeptide antibiotic in combination with an antistaphylococcal β-lactam antibiotic has demonstrated efficacy in treating resistant bacteria. This work describes the heterogeneity of dosing regimens and seeks to define an optimal dose, duration, and combination of antibiotics.

CONCLUSIONS

Combination salvage therapy has demonstrated efficacy and safety in treatment of resistant Gram-positive infections. It appears the combination of daptomycin and ceftaroline can clear resistant infections expeditiously.

Lytic bacteriophages facilitate antibiotic sensitization of Enterococcus faecium

Enterococcus faecium, a commensal of the human intestine, has emerged as a hospital-adapted, multi-drug resistant (MDR) pathogen. Bacteriophages (phages), natural predators of bacteria, have regained attention as therapeutics to stem the rise of MDR bacteria. Despite their potential to curtail MDR E. faecium infections, the molecular events governing E. faecium-phage interactions remain largely unknown. Such interactions are important to delineate because phage selective pressure imposed on E. faecium will undoubtedly result in phage resistance phenotypes that could threaten the efficacy of phage therapy. In an effort to understand the emergence of phage resistance in E. faecium, three newly isolated lytic phages were used to demonstrate that E. faecium phage resistance is conferred through an array of cell wall-associated molecules, including secreted antigen A (SagA), enterococcal polysaccharide antigen (Epa), wall teichoic acids, capsule, and an arginine-aspartate-aspartate (RDD) protein of unknown function. We find that capsule and Epa are important for robust phage adsorption and that phage resistance mutations in sagA, epaR, and epaX enhance E. faecium susceptibility to ceftriaxone, an antibiotic normally ineffective due to its low affinity for enterococcal penicillin binding proteins. Consistent with these findings, we provide evidence that phages potently synergize with cell wall (ceftriaxone and ampicillin) and membrane-acting (daptomycin) antimicrobials to slow or completely inhibit the growth of E. faecium Our work demonstrates that the evolution of phage resistance comes with fitness defects resulting in drug sensitization and that lytic phages could serve as effective antimicrobials for the treatment of E. faecium infections.

Dalbavancin, Vancomycin and Daptomycin Alone and in Combination with Cefazolin against Resistant Phenotypes of Staphylococcus aureus in a Pharmacokinetic/Pharmacodynamic Model

The most efficacious antimicrobial therapy to aid in the successful elimination of resistant S. aureus infections is unknown. In this study, we evaluated varying phenotypes of S. aureus against dalbavancin (DAL), vancomycin (VAN), and daptomycin (DAP) alone and in combination with cefazolin (CFZ). The objective of this study was to observe whether there was a therapeutic improvement in adding a beta-lactam to a glycopeptide, lipopeptide, or a lipoglycopeptide. We completed a series of in vitro tests including minimum inhibitory concentration testing (MIC) of the antimicrobials in combination, time-kill analysis (TKA), and a 168 h (7-day) one-compartment pharmacokinetic/pharmacodynamic (PK/PD) model on two daptomycin non-susceptible (DNS), vancomycin intermediate S. aureus strains (VISA), D712 and 6913. Results from our MIC testing demonstrated a minimum 2-fold and a maximum 32-fold reduction in MIC values for DAL, VAN, and DAP in combination with CFZ, in contrast to either agent used alone. The TKAs completed on four strains paralleled the enhanced activity demonstrated via the combination MICs. In the one-compartment PK/PD models, the combination of DAP plus CFZ or VAN plus CFZ resulted in a significant (p < 0.001) improvement in bactericidal activity and overall reduction in CFU/ml over the 7-day period. While the addition of CFZ to DAL improved time to bactericidal activity, DAL alone demonstrated equal and more sustained overall activity compared to all other treatments. The use of DAL alone, with or without CFZ and the combinations of VAN or DAP with CFZ appear to result in increased bactericidal activity against various recalcitrant S. aureus phenotypes.

Dalbavancin exposure in vitro selects for dalbavancin-non-susceptible and vancomycin-intermediate strains of methicillin-resistant Staphylococcus aureus

OBJECTIVES

Dalbavancin is a lipoglycopeptide active against methicillin-resistant Staphylococcus aureus (MRSA). Its long half-life (8.5-16 days) allows for once-weekly or single-dose treatments but could prolong the mutant selection window, promoting resistance and cross-resistance to related antimicrobials such as vancomycin. The objective of this study was to evaluate the capacity of post-distributional pharmacokinetic exposures of dalbavancin to select for resistance and cross-resistance in MRSA.

METHODS

We simulated average, post-distributional exposures of single-dose (1500 mg) dalbavancin (fCmax 9.9 μg/mL, β-elimination t_1/2 204 h) in an in vitro pharmacokinetic/pharmacodynamic (PK/PD) model for 28 days (672 h) against five MRSA strains and one methicillin-susceptible strain (MSSA). Samples were collected at least daily, and surviving colonies were enumerated and screened for resistance on drug-free and dalbavancin-supplemented medium respectively. Isolates from resistance screening plates were subjected to whole-genome sequencing (WGS) and susceptibly testing against dalbavancin, vancomycin, daptomycin, and six β-lactams with varying penicillin-binding protein (PBP) affinities.

RESULTS

Dalbavancin was bactericidal against most strains for days 1-4 before regrowth of less susceptible subpopulations occurred. Isolates with eight-fold increases in dalbavancin MIC were detected as early as day 4 but increased 64-128-fold in all models by day 28. Vancomycin and daptomycin MICs increased 4-16-fold, exceeding the susceptibly breakpoints for both antibiotics; β-lactam MICs generally decreased by two-to eight-fold, suggesting a dalbavancin-β-lactam seesaw effect, but increased by eight-fold or more in certain isolates. Resistant isolates carried mutations in a variety of genes, most commonly walKR, apt, stp1, and atl.

CONCLUSIONS

In our in vitro system, post-distributional dalbavancin exposures selected for stable mutants with reduced susceptibility to dalbavancin, vancomycin, and daptomycin, and generally increased susceptibility to β-lactams in all strains of MRSA tested. The clinical significance of these findings remains unclear, but created an opportunity to genotype a unique collection of dalbavancin-resistant strains for the first time. Mutations involved genes previously associated with vancomycin intermediate susceptibility and daptomycin non-susceptibility, most commonly walKR-associated genes.

Antimicrobial O-Alkyl Derivatives of Naringenin and Their Oximes Against Multidrug-Resistant Bacteria

New antimicrobial agents are needed to address infections caused by multidrug-resistant bacteria. Here, we are reporting novel O-alkyl derivatives of naringenin and their oximes, including novel compounds with a naringenin core and O-hexyl chains, showing activity against clinical strains of clarithromycin-resistant Helicobacter pylori, vancomycin-resistant Enterococcus faecalis, methicillin-resistant Staphylococcus aureus, and beta-lactam-resistant Acinetobacter baumannii and Klebsiella pneumoniae. The minimum inhibitory concentrations (MICs), which provide a quantitative measure of antimicrobial activity, were in the low microgram range for the selected compounds. Checkerboard assays for the most active compounds in combination with antibiotics revealed interactions that varied from synergistic to neutral.

Combinations of (lipo)glycopeptides with β-lactams against MRSA: susceptibility insights

Background

Increasing application of vancomycin due to the high prevalence of MRSA infections has led to the emergence of vancomycin intermediate-resistant Staphylococcus aureus (VISA) and heterogeneous VISA (hVISA). Consequently, the need for alternative therapies that target MRSA has become evident.

Objectives

To evaluate the synergy between (lipo)glycopeptides (LGP/GPs) (vancomycin, teicoplanin, telavancin, dalbavancin and oritavancin) and β-lactams (ceftaroline, cefepime, cefazolin and oxacillin) against MRSA, hVISA, VISA and daptomycin non-susceptible (DNS) phenotypes.

Methods

Twenty randomly selected clinical MRSA strains (i.e. 5 MRSA, 5 hVISA, 5 VISA and 5 DNS) were assessed versus LGP/GPs alone and LGP/GPs in combination with β-lactams for MICs. Although verification of antibiotic potency against bacterial strains is assessed by the microbroth dilution (MBD) MIC method recommended by the CLSI, some antibiotics need modified assay conditions in order to demonstrate their optimal activity.

Results

Addition of β-lactams reduced MIC values of LGP/GPs against all strains (up to 160-fold reduction). In general, LGPs (dalbavancin, oritavancin and telavancin) were more active (significant differences in MIC values, up to 8-fold) compared with vancomycin and teicoplanin. The majority of these combinations were bactericidal and superior to any single agent.

Conclusions

This report has examined the susceptibility patterns of LGP/GPs and their combination with β-lactams. Of interest, the impact of susceptibility tests (in terms of MIC plates and their surface area) on the synergistic activity in 24 h time–kill experiments was apparent for LGPs. Further clinical research is required to investigate synergy with LGP/GPs and β-lactams against these Staphylococcus strains.

Resistance of Gram-Positive Bacteria to Current Antibacterial Agents and Overcoming Approaches

The discovery of antibiotics has created a turning point in medical interventions to pathogenic infections, but unfortunately, each discovery was consistently followed by the emergence of resistance. The rise of multidrug-resistant bacteria has generated a great challenge to treat infections caused by bacteria with the available antibiotics. Today, research is active in finding new treatments for multidrug-resistant pathogens. In a step to guide the efforts, the WHO has published a list of the most dangerous bacteria that are resistant to current treatments and requires the development of new antibiotics for combating the resistance. Among the list are various Gram-positive bacteria that are responsible for serious healthcare and community-associated infections. Methicillin-resistant Staphylococcus aureus, vancomycin-resistant Enterococcus faecium, and drug-resistant Streptococcus pneumoniae are of particular concern. The resistance of bacteria is an evolving phenomenon that arises from genetic mutations and/or acquired genomes. Thus, antimicrobial resistance demands continuous efforts to create strategies to combat this problem and optimize the use of antibiotics. This article aims to provide a review of the most critical resistant Gram-positive bacterial pathogens, their mechanisms of resistance, and the new treatments and approaches reported to circumvent this problem.

Focusing the Lens on the CAMERA Concepts: Early Combination β-Lactam and Vancomycin Therapy in Methicillin-Resistant Staphylococcus aureus Bacteremia

Methicillin-resistant Staphylococcus aureus (MRSA) has grown to become a major burden on health care systems. The cumulation of limited therapeutic options and worsened patient outcomes with persistent MRSA bacteremia has driven research in optimizing its initial management. The guidelines published by the Infectious Diseases Society of America currently recommend combination therapy for refractory MRSA bacteremia, but the utility of combining antibiotics from the start of therapy is under investigation. The alternative strategy of early use of β-lactam antibiotics in combination with vancomycin upon initial MRSA bacteremia detection has shown promise. While this concept has gained international attention, providers should give this strategy serious consideration prior to implementation. The objective of this review is to examine retrospective and prospective evidence for early combination with vancomycin and β-lactam antibiotics, as well as explore potential consequences of combination therapy.

The Emerging Role of β-Lactams in the Treatment of Methicillin-Resistant Staphylococcus aureus Bloodstream Infections

Methicillin-resistant Staphylococcus aureus (MRSA) bloodstream infections (BSI) are associated with substantial morbidity and mortality. Monotherapy with first-line antimicrobials such as vancomycin (VAN; glycopeptide) and daptomycin (DAP; lipopeptide) are inadequate in some cases due to reduced antibiotic susceptibilities or therapeutic failure. In recent years, β-lactam antibiotics have emerged as a potential option for combination therapy with VAN and DAP that may meet an unmet therapeutic need for MRSA BSI. Ceftaroline (CPT), the only commercially available β-lactam in the United States with intrinsic in vitro activity against MRSA, has been increasingly studied in the setting of VAN and DAP failures. Novel combinations of first-line agents (VAN and DAP) with β-lactams have been the subject of many recent investigations due to in vitro findings such as the "seesaw effect," where β-lactam susceptibility may be improved in the presence of decreased glycopeptide and lipopeptide susceptibility. The combination of CPT and DAP, in particular, has become the focus of many scientific evaluations, due to intrinsic anti-MRSA activities and potent in vitro synergistic activity against various MRSA strains. This article reviews the available literature describing these innovative therapeutic approaches for MRSA BSI, focusing on preclinical and clinical studies, and evaluates the potential benefits and limitations of each strategy.

The Staphylococcal Cell Wall

Dating back to the 1960s, initial studies on the staphylococcal cell wall were driven by the need to clarify the mode of action of the first antibiotics and the resistance mechanisms developed by the bacteria. During the following decades, the elucidation of the biosynthetic path and primary composition of staphylococcal cell walls was propelled by advances in microbial cell biology, specifically, the introduction of high-resolution analytical techniques and molecular genetic approaches. The field of staphylococcal cell wall gradually gained its own significance as the complexity of its chemical structure and involvement in numerous cellular processes became evident, namely its versatile role in host interactions, coordination of cell division and environmental stress signaling.This chapter includes an updated description of the anatomy of staphylococcal cell walls, paying particular attention to information from the last decade, under four headings: high-resolution analysis of the Staphylococcus aureus peptidoglycan; variations in peptidoglycan composition; genetic determinants and enzymes in cell wall synthesis; and complex functions of cell walls. The latest contributions to a more precise picture of the staphylococcal cell envelope were possible due to recently developed state-of-the-art microscopy and spectroscopy techniques and to a wide combination of -omics approaches, that are allowing to obtain a more integrative view of this highly dynamic structure.

Comparison of in vitro static and dynamic assays to evaluate the efficacy of an antimicrobial drug combination against Staphylococcus aureus

An easily implementable strategy to reduce treatment failures in severe bacterial infections is to combine already available antibiotics. However, most in vitro combination assays are performed by exposing standard bacterial inocula to constant concentrations of antibiotics over less than 24h, which can be poorly representative of clinical situations. The aim of this study was to assess the ability of static and dynamic in vitro Time-Kill Studies (TKS) to identify the potential benefits of an antibiotic combination (here, amikacin and vancomycin) on two different inoculum sizes of two S. aureus strains. In the static TKS (sTKS), performed by exposing both strains over 24h to constant antibiotic concentrations, the activity of the two drugs combined was not significantly different the better drug used alone. However, the dynamic TKS (dTKS) performed over 5 days by exposing one strain to fluctuating concentrations representative of those observed in patients showed that, with the large inoculum, the activities of the drugs, used alone or in combination, significantly differed over time. Vancomycin did not kill bacteria, amikacin led to bacterial regrowth whereas the combination progressively decreased the bacterial load. Thus, dTKS revealed an enhanced effect of the combination on a large inoculum not observed in sTKS. The discrepancy between the sTKS and dTKS results highlights that the assessment of the efficacy of a combination for severe infections associated with a high bacterial load could be demanding. These situations probably require the implementation of dynamic assays over the entire expected treatment duration rather than the sole static assays performed with steady drug concentrations over 24h.

Adaptive Metabolism in Staphylococci: Survival and Persistence in Environmental and Clinical Settings

Staphylococci are highly successful at colonizing a variety of dynamic environments, both nonpathogenic and those of clinical importance, and comprise the list of pathogens of global public health significance. Their remarkable survival and persistence can be attributed to a host of strategies, one of which is metabolic versatility—their ability to rapidly alter their metabolism in the presence of transient or long-term bacteriostatic and bactericidal conditions and facilitate cellular homeostasis. These attributes contribute to their widespread dissemination and challenging eradication particularly from clinical settings. The study of microbial behaviour at the metabolite level provides insight into mechanisms of survival and persistence under defined environmental and clinical conditions. This paper reviews the range of metabolic modulations that facilitate staphylococcal acclimatization and persistence in varying terrestrial and host conditions, and their public health ramifications in these settings.

Vancomycin plus nafcillin salvage for the treatment of persistent methicillin-resistant Staphylococcus aureus bacteremia following daptomycin failure: a case report and literature review

Background:

Evidence supporting beta-lactam plus vancomycin synergy for methicillin-resistant Staphylococcus aureus (MRSA) continues to grow. Current in vivo evidence demonstrates that combination therapy is associated with shorter time to blood sterilization than vancomycin monotherapy. However, this combination has not been reported as salvage therapy for persistent MRSA bacteremia.

Case report:

We report a case of an 81-year-old male who was successfully treated with vancomycin plus nafcillin after failing vancomycin monotherapy, daptomycin monotherapy, and daptomycin plus gentamicin combination therapy. The patient originally presented with sepsis from a suspected urinary tract infection. Blood cultures drawn on days 1, 3, 5, 15, 19, 23, and 28 remained positive for MRSA despite multiple antimicrobial therapy changes. On day 29, therapy was changed to vancomycin plus nafcillin. Blood cultures drawn on day 32 remained negative. After 11 days, nafcillin was changed to piperacillin–tazobactam due to an infected decubitus ulcer. The combination was continued for 42 days after achieving blood sterility, 71 days after the patient originally presented. Evidence regarding salvage therapy for persistent bacteremia is sparse and is limited to case reports and case series.

Conclusion:

This case report supports that vancomycin plus an anti-staphylococcal beta-lactam combination should be further studied as salvage therapy for persistent MRSA bacteremia.

β-Lactam Combinations with Vancomycin Show Synergistic Activity against Vancomycin-Susceptible Staphylococcus aureus, Vancomycin-Intermediate S. aureus (VISA), and Heterogeneous VISA

Increasing utilization of vancomycin due to the high prevalence of methicillin-resistant Staphylococcus aureus (MRSA) infections has led to the emergence of vancomycin-intermediate S. aureus (VISA) and heterogeneous VISA (hVISA) strains. In vitro data suggest the potential for potent synergy between several beta-lactams and vancomycin. The objective of this study is to evaluate the synergy between beta-lactams and vancomycin against MRSA that is vancomycin susceptible, vancomycin-susceptible Staphylococcus aureus (VSSA), hVISA, and VISA. Fifty randomly selected clinical MRSA strains with various susceptibility levels to vancomycin were evaluated for vancomycin alone and vancomycin in combination with various concentrations of cefazolin (CFZ), cefepime (FEP), ceftaroline (CPT), and nafcillin (NAF). The potential for synergy was assessed by 24-h time-kill studies. Beta-lactams reduced vancomycin MIC values against all strains (4- to 16-fold reduction). In time-kill studies against MRSA, CFZ, FEP, CPT, and NAF all demonstrated similar degrees of killing at 24 h, and all showed synergistic activity with vancomycin against VSSA, hVISA, and VISA. Each of these combinations was also superior to any single agent against isolates of all three phenotypes, and each was bactericidal (P < 0.001 for all comparisons). All single-agent exposures demonstrated no activity at 24 h. The combination of vancomycin and beta-lactams significantly improved antibacterial activity against VSSA, hVISA, and VISA strains compared to the activity of any agent alone, supporting the potential use of vancomycin-beta-lactam combination therapy in infections caused by MRSA. Further clinical research is warranted to investigate the synergy of vancomycin against these Staphylococcus strains.

Peptidoglycan Compositional Analysis of Enterococcus faecalis Biofilm by Stable Isotope Labeling by Amino Acids in a Bacterial Culture

Peptidoglycan (PG) is a major component of the cell wall in Enterococcus faecalis. Accurate analysis of PG composition provides crucial insights into the bacterium's cellular functions and responses to external stimuli, but this analysis remains challenging because of various chemical modifications to PG-repeat subunits. We characterized changes to the PG composition of E. faecalis grown as planktonic bacteria and biofilm by developing "stable isotope labeling by amino acids in bacterial culture" (SILAB), optimized for bacterial cultures with incomplete amino acid labeling. This comparative analysis by mass spectrometry was performed by labeling E. faecalis in biofilm with heavy Lys (l-[¹³C₆,²D₉,¹⁵N₂]Lys) and planktonic bacteria with natural abundance l-Lys, then mixing equal amounts of bacteria from each condition, and performing cell wall isolation and mutanolysin digestion necessary for liquid chromatography and mass spectrometry. An analytical method was developed to determine muropeptide abundances using correction factors to compensate for incomplete heavy Lys isotopic enrichment (98.33 ± 0.05%) and incorporation (83.23 ± 1.16%). Forty-seven pairs of PG fragment ions from isolated cell walls of planktonic and biofilm samples were selected for SILAB analysis. We found that the PG in biofilm showed an increased level of PG cross-linking, an increased level of N-deacetylation of GlcNAc, a decreased level of O-acetylation of MurNAc, and an increased number of stem modifications by d,d- and l,d-carboxypeptidases.

Metabolic Mitigation of Staphylococcus aureus Vancomycin Intermediate-Level Susceptibility

Staphylococcus aureus is a major human pathogen whose infections are increasingly difficult to treat due to increased antibiotic resistance, including resistance to vancomycin. Vancomycin-intermediate S. aureus (VISA) strains develop resistance to vancomycin through adaptive changes that are incompletely understood. Central to this adaptation are metabolic changes that permit growth in the presence of vancomycin. To define the metabolic changes associated with adaptive resistance to vancomycin in S. aureus, the metabolomes of a vancomycin-sensitive and VISA strain pair isolated from the same patient shortly after vancomycin therapy began and following vancomycin treatment failure were analyzed. The metabolic adaptations included increases in acetogenesis, carbon flow through the pentose phosphate pathway, wall teichoic acid and peptidoglycan precursor biosynthesis, purine biosynthesis, and decreased tricarboxylic acid (TCA) cycle activity. The significance of these metabolic pathways for vancomycin-intermediate susceptibility was determined by assessing the synergistic potential of human-use-approved inhibitors of these pathways in combination with vancomycin against VISA strains. Importantly, inhibitors of amino sugar and purine biosynthesis acted synergistically with vancomycin to kill a diverse set of VISA strains, suggesting that combinatorial therapy could augment the efficacy of vancomycin even in patients infected with VISA strains.

5-Benzylidene-4-oxazolidinones potently inhibit biofilm formation in Methicillin-resistant Staphylococcus aureus

Investigation into the biological function of 5-benzylidene-4-oxazolidinones revealed dose-dependent inhibition of biofilm formation in Methicillin-resistant S. aureus (MRSA). This structurally unusual class of small molecules inhibit up to 89% of biofilm formation with IC50 values as low as 0.78 μM, and disperse pre-formed biofilms with IC50 values as low as 4.7 μM. Together, these results suggest that 4-oxazolidinones represent new chemotypes to enable the study of bacterial biofilms with small molecule chemical probes.

Vancomycin Resistance in Staphylococcus aureus

The evolution of Staphylococcus aureus during the modern antibiotic era has been delineated by distinct strain emergence events, many of which include acquisition of antibiotic resistance. The relative high burden of methicillin-resistant S. aureus (MRSA) in healthcare and community settings is a major concern worldwide. Vancomycin, a glycopeptide antibiotic that inhibits cell wall biosynthesis, remains a drug of choice for treatment of severe MRSA infections. S. aureus strains exhibiting increased resistance to vancomycin, known as vancomycin intermediate-resistant S. aureus (VISA) (MIC = 4-8 µg/mL), were discovered in the 1990s. The molecular basis of resistance in VISA is polygenic and involves stepwise mutations in genes encoding molecules predominantly involved in cell envelope biosynthesis. S. aureus isolates with complete resistance to vancomycin (MIC ≥ 16 µg/mL) are termed vancomycin-resistant S. aureus (VRSA)-they were first reported in the U.S. in 2002. Resistance in VRSA is conferred by the vanA gene and operon, which is present on a plasmid. Although treatment of VRSA infections is challenging, the total number of human VRSA infections to date is limited (14 in the U.S.). By comparison, the burden of VISA is relatively high and the molecular mechanisms of resistance are less well-defined. VISA are associated with persistent infections, vancomycin treatment failure, and poor clinical outcomes. Here, we review in brief progress made toward understanding the acquisition of antibiotic resistance in S. aureus, with an emphasis on the molecular mechanisms underlying vancomycin resistance.

In vivo characterization of an Hfq protein encoded by the Bacillus anthracis virulence plasmid pXO1

Background

Bacterial Hfq proteins post-transcriptionally regulate gene expression, primarily by mediating the interaction between sRNAs (small RNAs) and their target mRNAs. The role of Hfq-based regulation has been well defined in Gram-negative bacteria, but comparatively less is known about the impact of Hfq proteins in Gram-positive species. The Gram-positive pathogen Bacillus anthracis (causative agent of anthrax) is distinct in that it expresses three homologs of Hfq: Hfq1 and Hfq2 from the chromosome, and Hfq3 from the pXO1 virulence plasmid.

Results

In this study, we utilized overexpression as a strategy to examine the impact of Hfq3 on B. anthracis physiology. The increase in Hfq3 protein levels led to anomalous cell shape and chain formation, which manifested as a severe growth defect. This phenotype was specific to B. anthracis, as Hfq3 expression in B. subtilis at similar levels was not toxic. Toxicity was dependent on residues on the distal face of Hfq3 that are involved in mRNA binding in other bacterial species.

Conclusions

Thus, we hypothesize that Hfq3 interacts with RNA(s) involved in essential functions in the B. anthracis cell, leading to increased binding upon overexpression that either sequesters or accelerates degradation of RNAs important for growth. These results not only aid in elucidating the role of Hfq proteins in B. anthracis, but also contribute to our current understanding of Hfq in Gram-positive bacteria.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-017-0973-y) contains supplementary material, which is available to authorized users.

Effectiveness of a polyhexanide irrigation solution on methicillin-resistant Staphylococcus aureus biofilms in a porcine wound model

Irrigation and removal of necrotic debris can be beneficial for proper healing. It is becoming increasingly evident that wounds colonized with biofilm forming bacteria, such as Staphylococcus aureus (SA), can be more difficult to eradicate. Here we report our findings of the effects of an irrigation solution containing propyl-betaine and polyhexanide (PHMB) on methicillin-resistant Staphylococcus aureus (MRSA) biofilms in a porcine wound model. Thirty-nine deep partial thickness wounds were created with six wounds assigned to one of six treatment groups: (i) PHMB, (ii) Ringer's solution, (iii) hypochlorous acid/sodium hypochlorite, (iv) sterile water, (v) octenidine dihydrochloride, and (vi) octenilin. Wounds were inoculated with MRSA and covered with a polyurethane dressing for 24 hours to allow biofilm formation. The dressings were then removed and the wounds were irrigated twice daily for 3 days with the appropriate solution. MRSA from four wounds were recovered from each treatment group at 3 days and 6 days hours after initial treatment. Irrigation of wounds with the PHMB solution resulted in 97·85% and 99·64% reductions of MRSA at the respective 3 days and 6 days assessment times when compared to the untreated group. Both of these reductions were statistically significant compared to all other treatment groups (P values <0·05).

Approved Glycopeptide Antibacterial Drugs: Mechanism of Action and Resistance

The glycopeptide antimicrobials are a group of natural product and semisynthetic glycosylated peptides that show antibacterial activity against Gram-positive organisms through inhibition of cell-wall synthesis. This is achieved primarily through binding to the d-alanyl-d-alanine terminus of the lipid II bacterial cell-wall precursor, preventing cross-linking of the peptidoglycan layer. Vancomycin is the foundational member of the class, showing both clinical longevity and a still preferential role in the therapy of methicillin-resistant Staphylococcus aureus and of susceptible Enterococcus spp. Newer lipoglycopeptide derivatives (telavancin, dalbavancin, and oritavancin) were designed in a targeted fashion to increase antibacterial activity, in some cases through secondary mechanisms of action. Resistance to the glycopeptides emerged in delayed fashion and occurs via a spectrum of chromosome- and plasmid-associated elements that lead to structural alteration of the bacterial cell-wall precursor substrates.

Glycopeptide resistance: Links with inorganic phosphate metabolism and cell envelope stress

Antimicrobial resistance is a critical health issue today. Many pathogens have become resistant to many or all available antibiotics and limited new antibiotics are in the pipeline. Glycopeptides are used as a 'last resort' antibiotic treatment for many bacterial infections, but worryingly, glycopeptide resistance has spread to very important pathogens such as Enterococcus faecium and Staphylococcus aureus. Bacteria confront multiple stresses in their natural environments, including nutritional starvation and the action of cell-wall stressing agents. These stresses impact bacterial susceptibility to different antimicrobials. This article aims to review the links between glycopeptide resistance and different stresses, especially those related with cell-wall biosynthesis and inorganic phosphate metabolism, and to discuss promising alternatives to classical antibiotics to avoid the problem of antimicrobial resistance.

SpoVG Regulates Cell Wall Metabolism and Oxacillin Resistance in Methicillin-Resistant Staphylococcus aureus Strain N315

Increasing cases of infections caused by methicillin-resistant Staphylococcus aureus (MRSA) strains in healthy individuals have raised concerns worldwide. MRSA strains are resistant to almost the entire family of β-lactam antibiotics due to the acquisition of an extra penicillin-binding protein, PBP2a. Studies have shown that spoVG is involved in oxacillin resistance, while the regulatory mechanism remains elusive. In this study, we have found that SpoVG plays a positive role in oxacillin resistance through promoting cell wall synthesis and inhibiting cell wall degradation in MRSA strain N315. Deletion of spoVG in strain N315 led to a significant decrease in oxacillin resistance and a dramatic increase in Triton X-100-induced autolytic activity simultaneously. Real-time quantitative reverse transcription-PCR revealed that the expression of 8 genes related to cell wall metabolism or oxacillin resistance was altered in the spoVG mutant. Electrophoretic mobility shift assay indicated that SpoVG can directly bind to the putative promoter regions of lytN (murein hydrolase), femA, and lytSR (the two-component system). These findings suggest a molecular mechanism in which SpoVG modulates oxacillin resistance by regulating cell wall metabolism in MRSA.

Synergy of β-Lactams with Vancomycin against Methicillin-Resistant Staphylococcus aureus: Correlation of Disk Diffusion and Checkerboard Methods

Modified disk diffusion (MDD) and checkerboard tests were employed to assess the synergy of combinations of vancomycin and β-lactam antibiotics for 59 clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) and Mu50 (ATCC 700699). Bacterial inocula equivalent to 0.5 and 2.0 McFarland standard were inoculated on agar plates containing 0, 0.5, 1, and 2 μg/ml of vancomycin. Oxacillin-, cefazolin-, and cefoxitin-impregnated disks were applied to the surface, and the zones of inhibition were measured at 24 h. The CLSI-recommended checkerboard method was used as a reference to detect synergy. The MICs for vancomycin were determined using the Etest method, broth microdilution, and the Vitek 2 automated system. Synergy was observed with the checkerboard method in 51% to 60% of the isolates when vancomycin was combined with any β-lactam. The fractional inhibitory concentration indices were significantly lower in MRSA isolates with higher vancomycin MIC combinations (P < 0.05). The overall agreement between the MDD and checkerboard methods to detect synergy in MRSA isolates with bacterial inocula equivalent to McFarland standard 0.5 were 33.0% and 62.5% for oxacillin, 45.1% and 52.4% for cefazolin, and 43.1% and 52.4% for cefoxitin when combined with 0.5 and 2 μg/ml of vancomycin, respectively. Based on our study, the simple MDD method is not recommended as a replacement for the checkerboard method to detect synergy. However, it may serve as an initial screening method for the detection of potential synergy when it is not feasible to perform other labor-intensive synergy tests.

Phenotypic and genomic comparisons of highly vancomycin-resistant Staphylococcus aureus strains developed from multiple clinical MRSA strains by in vitro mutagenesis

The development of vancomycin (VCM) resistance in Staphylococcus aureus threatens global health. Studies of the VCM-resistance mechanism and alternative therapeutic strategies are urgently needed. We mutagenized S. aureus laboratory strains and methicillin-resistant S. aureus (MRSA) with ethyl methanesulfonate, and isolated mutants that exhibited high resistance to VCM (minimum inhibitory concentration = 32 μg/ml). These VCM-resistant strains were sensitive to linezolid and rifampicin, and partly to arbekacin and daptomycin. Beta-lactams had synergistic effects with VCM against these mutants. VCM-resistant strains exhibited a 2-fold increase in the cell wall thickness. Several genes were commonly mutated among the highly VCM-resistant mutants. These findings suggest that MRSA has a potential to develop high VCM resistance with cell wall thickening by the accumulation of mutations.

The Three Bacterial Lines of Defense against Antimicrobial Agents

Antimicrobial agents target a range of extra- and/or intracellular loci from cytoplasmic wall to membrane, intracellular enzymes and genetic materials. Meanwhile, many resistance mechanisms employed by bacteria to counter antimicrobial agents have been found and reported in the past decades. Based on their spatially distinct sites of action and distribution of location, antimicrobial resistance mechanisms of bacteria were categorized into three groups, coined the three lines of bacterial defense in this review. The first line of defense is biofilms, which can be formed by most bacteria to overcome the action of antimicrobial agents. In addition, some other bacteria employ the second line of defense, the cell wall, cell membrane, and encased efflux pumps. When antimicrobial agents permeate the first two lines of defense and finally reach the cytoplasm, many bacteria will make use of the third line of defense, including alterations of intracellular materials and gene regulation to protect themselves from harm by bactericides. The presented three lines of defense theory will help us to understand the bacterial resistance mechanisms against antimicrobial agents and design efficient strategies to overcome these resistances.

Intravenous Antibiotics Used in the Treatment of Methicillin-Resistant Staphylococcus Aureus

Methicillin-resistant Staphylococcus aureus (MRSA) continues to cause significant morbidity and mortality. Despite advances in medical care, the prevalence of both community-acquired and hospital-acquired MRSA has progressively increased. Community-acquired MRSA typically occurs in patients without recent illness or hospitalization, presents as acute skin and soft tissue infections, and is usually not multidrug resistant. Hospital-acquired MRSA, however, presents in patients recently hospitalized or treated in long-term care settings and in those who have had medical procedures and is usually associated with multidrug-resistant strains. Both types of infections, if not properly treated, have the potential to become invasive. This article discusses current intravenous antibiotics that are available for the empiric treatment of MRSA infections along with a newer phenomenon known as the "seesaw effect."

Intravenous Antibiotics Used in the Treatment of Methicillin-Resistant Staphylococcus Aureus

Methicillin-resistant Staphylococcus aureus (MRSA) continues to cause significant morbidity and mortality. Despite advances in medical care, the prevalence of both community-acquired and hospital-acquired MRSA has progressively increased. Community-acquired MRSA typically occurs in patients without recent illness or hospitalization, presents as acute skin and soft tissue infections, and is usually not multidrug resistant. Hospital-acquired MRSA, however, presents in patients recently hospitalized or treated in long-term care settings and in those who have had medical procedures and is usually associated with multidrug-resistant strains. Both types of infections, if not properly treated, have the potential to become invasive. This article discusses current intravenous antibiotics that are available for the empiric treatment of MRSA infections along with a newer phenomenon known as the “seesaw effect.”

Alternative Pathway to a Glycopeptide-Resistant Cell Wall in the Balhimycin Producer Amycolatopsis balhimycina

Balhimycin, a vancomycin-type glycopeptide, is a lipid II targeting antibiotic produced by Amycolatopsis balhimycina. A. balhimycina has developed a self-resistance mechanism based on the synergistic action of different enzymes resulting in modified peptidoglycan. The canonical resistance mechanism against glycopeptides is the synthesis of peptidoglycan precursors ending with acyl-d-alanyl-d-lactate (d-Ala-d-Lac) rather than acyl-d-alanyl-d-alanine (d-Ala-d-Ala). This reprogramming is the result of the enzymes VanH, VanA, and VanX. VanH and VanA are required to produce d-Ala-d-Lac; VanX cleaves cytosolic pools of d-Ala-d-Ala, thereby ensuring that peptidoglycan is enriched in d-Ala-d-Lac. In A. balhimycina, the ΔvanHAXAb mutant showed a reduced glycopeptide resistance in comparison to the wild type. Nevertheless, ΔvanHAXAb was paradoxically still able to produce d-Ala-d-Lac containing resistant cell wall precursors suggesting the presence of a novel alternative glycopeptide resistance mechanism. In silico analysis, inactivation studies, and biochemical assays led to the characterization of an enzyme, Ddl1Ab, as a paraloguous chromosomal d-Ala-d-Lac ligase able to complement the function of VanAAb in the ΔvanHAXAb mutant. Furthermore, A. balhimycina harbors a vanYAb gene encoding a d,d-carboxypeptidase. Transcriptional analysis revealed an upregulated expression of vanYAb in the ΔvanHAXAb mutant. VanYAb cleaves the endstanding d-Ala from the pentapeptide precursors, reducing the quantity of sensitive cell wall precursors in the absence of VanXAb. These findings represent an unprecedented coordinated layer of resistance mechanisms in a glycopeptide antibiotic producing bacterium.

Genomic, Transcriptomic and Metabolomic Studies of Two Well-Characterized, Laboratory-Derived Vancomycin-Intermediate Staphylococcus aureus Strains Derived from the Same Parent Strain

Complete genome comparisons, transcriptomic and metabolomic studies were performed on two laboratory-selected, well-characterized vancomycin-intermediate Staphylococcus aureus (VISA) derived from the same parent MRSA that have changes in cell wall composition and decreased autolysis. A variety of mutations were found in the VISA, with more in strain 13136p(-)m⁺V20 (vancomycin MIC = 16 µg/mL) than strain 13136p(-)m⁺V5 (MIC = 8 µg/mL). Most of the mutations have not previously been associated with the VISA phenotype; some were associated with cell wall metabolism and many with stress responses, notably relating to DNA damage. The genomes and transcriptomes of the two VISA support the importance of gene expression regulation to the VISA phenotype. Similarities in overall transcriptomic and metabolomic data indicated that the VISA physiologic state includes elements of the stringent response, such as downregulation of protein and nucleotide synthesis, the pentose phosphate pathway and nutrient transport systems. Gene expression for secreted virulence determinants was generally downregulated, but was more variable for surface-associated virulence determinants, although capsule formation was clearly inhibited. The importance of activated stress response elements could be seen across all three analyses, as in the accumulation of osmoprotectant metabolites such as proline and glutamate. Concentrations of potential cell wall precursor amino acids and glucosamine were increased in the VISA strains. Polyamines were decreased in the VISA, which may facilitate the accrual of mutations. Overall, the studies confirm the wide variability in mutations and gene expression patterns that can lead to the VISA phenotype.

A mutant Escherichia coli that attaches peptidoglycan to lipopolysaccharide and displays cell wall on its surface

The lipopolysaccharide (LPS) forms the surface-exposed leaflet of the outer membrane (OM) of Gram-negative bacteria, an organelle that shields the underlying peptidoglycan (PG) cell wall. Both LPS and PG are essential cell envelope components that are synthesized independently and assembled by dedicated transenvelope multiprotein complexes. We have identified a point-mutation in the gene for O-antigen ligase (WaaL) in Escherichia coli that causes LPS to be modified with PG subunits, intersecting these two pathways. Synthesis of the PG-modified LPS (LPS*) requires ready access to the small PG precursor pool but does not weaken cell wall integrity, challenging models of precursor sequestration at PG assembly machinery. LPS* is efficiently transported to the cell surface without impairing OM function. Because LPS* contains the canonical vancomycin binding site, these surface-exposed molecules confer increased vancomycin-resistance by functioning as molecular decoys that titrate the antibiotic away from its intracellular target. This unexpected LPS glycosylation fuses two potent pathogen-associated molecular patterns (PAMPs).

DOI:http://dx.doi.org/10.7554/eLife.05334.001

Tiny Gram-negative bacteria are one of humankind's deadliest foes, causing infections of wounds and the bloodstream that are very hard to treat. Many Gram-negative bacteria are resistant to several common antibiotics, and the few treatments available that can successfully kill the bacteria are often also toxic to the patients. Understanding how these bacteria elude antibiotics could help scientists develop better, less toxic treatments.

Most bacteria are surrounded by a cell wall that helps protect the bacteria and gives them structure. Many broad-spectrum antibiotics, including penicillin and vancomycin, work by interfering with how this protective wall is built from molecules called peptidoglycans. However, Gram-negative bacteria have an outer membrane that prevents many antibiotics from reaching the cell wall, and so the antibiotics are unable to kill the bacteria.

The outer membrane of Gram-negative bacteria is made up of sugars and fatty molecules called lipids. Recently, scientists discovered a mutation that interferes with the movement of the lipid and sugar molecules that make up the outer membrane, which compromises this protective layer and makes the bacteria more susceptible to antibiotics.

To learn more about how this mutation interferes with the defenses of the Gram-negative bacteria Escherichia coli, Grabowicz et al. searched for compensating mutations that can counteract it and restore the antibiotic resistance of these mutant bacteria. The search revealed that a mutation in a gene called waaL increases E. coli's resistance to vancomycin, but not to other antibiotics. The gene encodes an enzyme, and the mutant form of the enzyme attaches some peptidoglycans to the surface of the outer membrane instead of incorporating them into the cell wall. The stray peptidoglycans on the cell's surface act as decoys, binding to vancomycin and keeping the drug from reaching its true target—the cell wall. The decoy strategy is similar to a mechanism used by Gram-positive bacteria—which lack a protective outer membrane—to resist vancomycin treatment, which also involves creating sites that bind the drug and keep it from its target.

Vancomycin is not currently used clinically to treat E. coli or other Gram-negative infections because these bacteria are naturally quite resistant for other reasons. However, Grabowicz et al.'s findings do demonstrate how quickly bacteria can adapt and produce new defenses to antibiotics when old strategies fail.

DOI:http://dx.doi.org/10.7554/eLife.05334.002

Self-resistance mechanisms of actinomycetes producing lipid II-targeting antibiotics

Glycopeptides and several lantibiotics are lipid II-targeting antibiotics produced by actinomycetes. To protect themselves from their own product, antibiotic producers developed self-resistance mechanisms. Inspection of different producer strains revealed that their resistance is not only based on a single determinant but on the synergistic action of different factors. Glycopeptide producers possess different ways to synthesize a modified peptidoglycan to prevent the binding of the glycopeptide antibiotic. One possible modification is the synthesis of peptidoglycan precursors terminating with a D-alanyl-D-lactate (D-Ala-D-Lac) rather than with a D-alanyl-D-alanine (D-Ala-D-Ala) resulting in a 1000-fold decreased binding affinity of the glycopeptide to its target. The reprogramming of the peptidoglycan precursor biosynthesis is based on the action of VanHAX or paralogous enzymes as it was shown for Amycolatopsis balhimycina. A second peptidoglycan modification resulting in glycopeptide resistance was investigated in the glycopeptide A40926 producer Nonomuraea ATCC 39727. Nonomuraea eliminates the glycopeptide target by synthesizing a peptidoglycan with 3-3 cross-linked peptide stems. The carboxypeptidase VanYn provides tetrapeptides which serve as substrates for the L,D-transpeptidase catalyzing the formation of 3-3 cross-links. The occurrence of 3-3 cross-linked dimers is also an important feature of the lantibiotic NAI-107 producer Microbispora ATCC PTA-5024. Moreover, the D-Ala in the fourth position in the acceptor peptide of muropeptides is exchanged to glycine or serine in Microbispora, a side reaction of the L,D-transpeptidase. Together with the lipoprotein MlbQ, the ABC transporter MlbYZ and the transmembrane protein MlbJ it might contribute to the self-resistance in Microbispora ATCC PTA-5024.

Vancomycin and oxacillin co-resistance of commensal staphylococci

Background:

Many disease conditions including Staphylococcal infections are becoming increasingly difficult to treat in South Africa due to the surge of vancomycin-oxacillin resistant strains. How widespread this phenomenon is in commensal isolates in the Nkonkobe municipality in the Eastern Cape Province of South Africa is not known, and considering the high level of immunocompromised individuals in the province, this study couldn’t have come at a better time.

Objectives:

The objective of this study is to evaluate the prevalence of vancomycin and oxacillin co-resistance in methicillin-resistant commensal staphylococci in Nkonkobe municipality, South Africa as part of our larger study on the surveillance of reservoirs of antibiotic resistance in South Africa.

Materials and Methods:

Staphylococcus species were isolated from domestic animals of Nkonkobe municipality, in the Eastern Cape Province of South Africa. The isolates were evaluated for antibiotic susceptibility against a panel of several relevant antibiotics. Specific primer sets were also used for the polymerase chain reaction assay to detect the presence of mecA gene as well as vanA and vanB genes in the genome of resistant Staphylococcus species.

Results:

A total of 120 Staphylococcus isolates were screened, out of which, 32 (26%) were susceptible to both methicillin and vancomycin, while 12 (10%) had co-resistance to the antibiotics, which is still on the high side, both clinically and epidemiologically. Gentamicin (an aminoglycoside) had a relatively high potency against the isolates with 107 (89.17%) of the bacteria being susceptible to it, while 10 (8.33%) were resistant. On the other hand, erythromycin (a macrolide) was active against 72 (60%) of the isolates, while 5 (4.17%) and 74 (61.67%) of them yielded intermediate and resistant responses, respectively. Similarly, 51 (42.5%) of the isolates were susceptible to rifampicin, while 1 (0.83%) and 17 (14.17%) were intermediate and resistant, respectively.

Conclusions:

Ten percent of the isolates were positive for mecA gene among the vancomycin-oxacillin resistant strains, while van gene was not detected in any of the isolates. The data obtained would be useful in clinical control of resistant staphylococcal strains.

Generation of a vancomycin-intermediate Staphylococcus aureus (VISA) strain by two amino acid exchanges in VraS

OBJECTIVES

Staphylococcus aureus is a notorious bacterial pathogen and antibiotic-resistant isolates complicate current treatment strategies. We characterized S. aureus VC40, a laboratory mutant that shows full resistance to glycopeptides (vancomycin and teicoplanin MICs ≥32 mg/L) and daptomycin (MIC = 4 mg/L), to gain deeper insights into the underlying resistance mechanisms.

METHODS

Genomics and transcriptomics were performed to characterize changes that might contribute to development of resistance. The mutations in vraS were reconstituted into a closely related parental background. In addition, antimicrobial susceptibility testing, growth analyses, transmission electron microscopy, lysostaphin-induced lysis and autolysis assays were performed to characterize the phenotype of resistant strains.

RESULTS

Genome sequencing of strain VC40 revealed 79 mutations in 75 gene loci including genes encoding the histidine kinases VraS and WalK that control cell envelope-related processes. Transcriptomics indicated the increased expression of their respective regulons. Although not reaching the measured MIC for VC40, reconstitution of the L114S and D242G exchanges in VraS(VC40) into the susceptible parental background (S. aureus NCTC 8325) resulted in increased resistance to glycopeptides and daptomycin. The expression of VraS(VC40) led to increased transcription of the cell wall stress stimulon, a thickened cell wall, a decreased growth rate, reduced autolytic activity and increased resistance to lysostaphin-induced lysis in the generated mutant.

CONCLUSIONS

We show that a double mutation of a single gene locus, namely vraS, is sufficient to convert the vancomycin-susceptible strain S. aureus NCTC 8325 into a vancomycin-intermediate S. aureus.

"Slow VISA," a novel phenotype of vancomycin resistance, found in vitro in heterogeneous vancomycin-intermediate Staphylococcus aureus strain Mu3

Heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA) clinical strain Mu3 spontaneously generates VISA strains at an extremely high frequency (≥1×10(-6)). The generated VISA strains usually grow more slowly than does the parent hVISA strain, but they form colonies on vancomycin-containing agar plates before 48 h of incubation. However, we noticed a curious group of VISA strains, designated "slow VISA" (sVISA), whose colonies appear only after 72 h of incubation. They have extremely prolonged doubling times but have vancomycin MICs of 8 to ∼24 mg/liter when determined after 72 to ∼144 h of incubation. We established strain Mu3-6R-P (6R-P), which has a vancomycin MIC of 16 mg/liter (at 72 h), as a representative sVISA strain. Its cell wall was thickened and autolytic activity was decreased compared to the respective qualities of the parent hVISA strain Mu3. Whole-genome sequencing of 6R-P revealed only one mutation, encoded by rpoB (R512P), which replaced the 512th arginine of the RNA polymerase β-subunit with proline. Its VISA phenotype was unstable, and the strain frequently reverted to hVISA with concomitant losses of pinpoint colony morphology and cell wall thickness and reduced autolytic activity. Sequencing of the rpoB genes of the phenotypic revertant strains revealed mutations affecting the 512th codon, where the proline of 6R-P was replaced with leucine, serine, or histidine. Slow VISA generated in the tissues of an infected patient serves as a temporary shelter for hVISA to survive vancomycin therapy. The sVISA strain spontaneously returns to hVISA when the threat of vancomycin is lifted. The rpoB(R512P) mutation may be regarded as a regulatory mutation that switches the reversible phenotype of sVISA on and off.

A current perspective on daptomycin for the clinical microbiologist

Daptomycin is a lipopeptide antimicrobial with in vitro bactericidal activity against Gram-positive bacteria that was first approved for clinical use in 2004 in the United States. Since this time, significant data have emerged regarding the use of daptomycin for the treatment of serious infections, such as bacteremia and endocarditis, caused by Gram-positive pathogens. However, there are also increasing reports of daptomycin nonsusceptibility, in Staphylococcus aureus and, in particular, Enterococcus faecium and Enterococcus faecalis. Such nonsusceptibility is largely in the context of prolonged treatment courses and infections with high bacterial burdens, but it may occur in the absence of prior daptomycin exposure. Nonsusceptibility in both S. aureus and Enterococcus is mediated by adaptations to cell wall homeostasis and membrane phospholipid metabolism. This review summarizes the data on daptomycin, including daptomycin's unique mode of action and spectrum of activity and mechanisms for nonsusceptibility in key pathogens, including S. aureus, E. faecium, and E. faecalis. The challenges faced by the clinical laboratory in obtaining accurate susceptibility results and reporting daptomycin MICs are also discussed.

Observation of "seesaw effect" with vancomycin, teicoplanin, daptomycin and ceftaroline in 150 unique MRSA strains

Introduction

Vancomycin (VAN) failures associated with the treatment of complicated methicillin-resistant Staphylococcus aureus (MRSA) infections have been well described. The reported “seesaw effect” demonstrates improved β-lactam activity when VAN and/or daptomycin (DAP) susceptibility decreases. However, there are minimal data comparing ceftaroline (CPT) susceptibility with these agents or teicoplanin (TEI). Therefore, to further explore the seesaw effect, we evaluated the relationship between CPT and VAN, TEI, and DAP susceptibilities.

Methods

One hundred and fifty clinical MRSA isolates from the Anti-Infective Research Laboratory (Detroit, MI, USA) from 2008 to 2012 were analyzed. VAN, TEI, DAP and CPT minimum inhibitory concentrations (MIC) were determined via Etest methodology. MIC₅₀ and MIC₉₀ were calculated for each antibiotic. Additionally, four isogenic strain pairs were randomly selected for evaluation by time–kill methodology for the potential of enhanced killing by CPT as MICs increased to VAN, TEI, and DAP.

Results

CPT MICs were inversely correlated with VAN, DAP, and TEI MICs with correlation coefficients of −0.535, −0.483, and −0.386, respectively (P ≤ 0.05). Comparison of the MIC relationship for glycopeptides and lipopeptides resulted in a positive correlation for all agent combinations. In time–kill evaluations, CPT demonstrated greater reductions in log₁₀ colony-forming unit (CFU)/mL against mutant strains (3.73 ± 0.67) versus parents (2.79 ± 0.75) despite no change in CPT MIC (P = 0.112).

Conclusion

This study demonstrated a marked “seesaw effect” whereby CPT displayed increased susceptibility as the VAN, DAP, and TEI MICs increased. Additionally, we observed a positive linear correlation between VAN, DAP, and TEI MICs for all agent combinations. Enhanced activity was noted with CPT in mutant strains versus the parent strains despite no change in MIC. Based upon the enhanced CPT activity observed against strains with decreased susceptibility to VAN, DAP and TEI, CPT may provide an option for infections with reduced susceptibility to glycopeptides or lipopeptides. Further evaluation is warranted to investigate the clinical implications of the seesaw effect.

Electronic supplementary material

The online version of this article (doi:10.1007/s40121-014-0023-0) contains supplementary material, which is available to authorized users.

Emergence of daptomycin resistance in daptomycin-naïve rabbits with methicillin-resistant Staphylococcus aureus prosthetic joint infection is associated with resistance to host defense cationic peptides and mprF polymorphisms

Background

Previous studies of both clinically-derived and in vitro passage-derived daptomycin–resistant (DAP-R) Staphylococcus aureus strains demonstrated the coincident emergence of increased DAP MICs and resistance to host defense cationic peptides (HDP-R).

Methods

In the present investigation, we studied a parental DAP-susceptible (DAP-S) methicillin-resistant Staphylococcus aureus (MRSA) strain and three isogenic variants with increased DAP MICs which were isolated from both DAP-treated and DAP-untreated rabbits with prosthetic joint infections. These strains were compared for: in vitro susceptibility to distinct HDPs differing in size, structure, and origin; i.e.; thrombin-induced platelet microbicidal proteins [tPMPs] and human neutrophil peptide-1 [hNP-1]; cell membrane (CM) phospholipid and fatty acid content; CM order; envelope surface charge; cell wall thickness; and mprF single nucleotide polymorphisms (SNPs) and expression profiles.

Results

In comparison with the parental strain, both DAP-exposed and DAP-naive strains exhibited: (i) significantly reduced susceptibility to each HDP (P<0.05); (ii) thicker cell walls (P<0.05); (iii) increased synthesis of CM lysyl-phosphatidylglycerol (L-PG); (iv) reduced content of CM phosphatidylglycerol (PG); and (v) SNPs within the mprF locus No significant differences were observed between parental or variant strains in outer CM content of L-PG, CM fluidity, CM fatty acid contents, surface charge, mprF expression profiles or MprF protein content. An isolate which underwent identical in vivo passage, but without evolving increased DAP MICs, retained parental phenotypes and genotype.

Conclusions

These results suggest: i) DAP MIC increases may occur in the absence of DAP exposures in vivo and may be triggered by organism exposure to endogenous HDPs: and ii) gain-in-function SNPs in mprF may contribute to such HDP-DAP cross-resistance phenotypes, although the mechanism of this relationship remains to be defined.

Current and prospective treatments for multidrug-resistant gram-positive infections

INTRODUCTION

Staphylococcus aureus and Enterococcus spp. are two of the most common organisms causing nosocomial infections today; and are consistently associated with high mortality rates (approximately 20 and 44%, respectively). Resistance among these pathogens to first line agents such as methicillin and vancomycin continues to rise while isolates with reduced susceptibility to newer agents including linezolid and daptomycin continue to emerge, representing a serious concern for clinicians.

AREAS COVERED

Mechanisms of action and resistance as well as in vitro and clinical experience in the treatment of resistant staphylococci and enterococci with currently available agents are discussed. Additionally, novel combination regimens showing enhanced efficacy and available data pertaining to prospective therapies including solithromycin, tedizolid, dalbavancin and oritavancin will be covered.

EXPERT OPINION

With an increase in organisms displaying reduced susceptibility to vancomycin and the associated treatment failures, the significance of alternative therapies such as daptomycin, linezolid, ceftaroline, and prospective anti-gram-positive agents is on the rise. As our understanding of antimicrobial pharmacokinetic-pharmacodynamics principles continues to evolve, the selection of highly effective agents and optimization of dosages may lead to improved patient outcomes and delay the development of resistance.