Linezolid update: stable in vitro activity following more than a decade of clinical use and summary of associated resistance mechanisms

Quantitative proteomic view associated with resistance to clinically important antibiotics in Gram-positive bacteria: a systematic review

The increase of methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE) poses a worldwide and serious health threat. Although new antibiotics, such as daptomycin and linezolid, have been developed for the treatment of infections of Gram-positive pathogens, the emergence of daptomycin-resistant and linezolid-resistant strains during therapy has now increased clinical treatment failures. In the past few years, studies using quantitative proteomic methods have provided a considerable progress in understanding antibiotic resistance mechanisms. In this review, to understand the resistance mechanisms to four clinically important antibiotics (methicillin, vancomycin, linezolid, and daptomycin) used in the treatment of Gram-positive pathogens, we summarize recent advances in studies on resistance mechanisms using quantitative proteomic methods, and also examine proteins playing an important role in the bacterial mechanisms of resistance to the four antibiotics. Proteomic researches can identify proteins whose expression levels are changed in the resistance mechanism to only one antibiotic, such as LiaH in daptomycin resistance and PrsA in vancomycin resistance, and many proteins simultaneously involved in resistance mechanisms to various antibiotics. Most of resistance-related proteins, which are simultaneously associated with resistance mechanisms to several antibiotics, play important roles in regulating bacterial envelope biogenesis, or compensating for the fitness cost of antibiotic resistance. Therefore, proteomic data confirm that antibiotic resistance requires the fitness cost and the bacterial envelope is an important factor in antibiotic resistance.

Detection of a New cfr-Like Gene, cfr(B), in Enterococcus faecium Isolates Recovered from Human Specimens in the United States as Part of the SENTRY Antimicrobial Surveillance Program

Two linezolid-resistant Enterococcus faecium isolates (MICs, 8 μg/ml) from unique patients of a medical center in New Orleans were included in this study. Isolates were initially investigated for the presence of mutations in the V domain of 23S rRNA genes and L3, L4, and L22 ribosomal proteins, as well as cfr. Isolates were subjected to pulsed-field gel electrophoresis (just one band difference), and one representative strain was submitted to whole-genome sequencing. Gene location was also determined by hybridization, and cfr genes were cloned and expressed in a Staphylococcus aureus background. The two isolates had one out of six 23S rRNA alleles mutated (G2576T), had wild-type L3, L4, and L22 sequences, and were positive for a cfr-like gene. The sequence of the protein encoded by the cfr-like gene was most similar (99.7%) to that found in Peptoclostridium difficile, which shared only 74.9% amino acid identity with the proteins encoded by genes previously identified in staphylococci and non-faecium enterococci and was, therefore, denominated Cfr(B). When expressed in S. aureus, the protein conferred a resistance profile similar to that of Cfr. Two copies of cfr(B) were chromosomally located and embedded in a Tn6218 similar to the cfr-carrying transposon described in P. difficile. This study reports the first detection of cfr genes in E. faecium clinical isolates in the United States and characterization of a new cfr variant, cfr(B). cfr(B) has been observed in mobile genetic elements in E. faecium and P. difficile, suggesting potential for dissemination. However, further analysis is necessary to access the resistance levels conferred by cfr(B) when expressed in enterococci.

Optimizing therapy for vancomycin-resistant enterococcal bacteremia in children

PURPOSE OF REVIEW

Uncertainties exist regarding the optimal treatment for vancomycin-resistant enterococcal (VRE) bloodstream infections, particularly in settings in which ampicillin cannot be used.

RECENT FINDINGS

Quinupristin-dalfopristin, linezolid, and daptomycin, all approved between 1999 and 2003, represent the mainstays of therapy for VRE bacteremia, although only linezolid has been specifically approved by the United States Food and Drug Administration for this indication. The main objective of this review is to compare the relative efficacies, dosing strategies, and side-effect profiles of quinupristin-dalfopristin, linezolid, and daptomycin for VRE bacteremia in the pediatric population. A brief description of recently approved broad-spectrum Gram-positive agents that may have a role in the management of VRE bacteremia in upcoming years is also provided.

SUMMARY

Linezolid, despite its bacteriostatic activity against VRE, may be the most versatile of the available drugs. It has activity against both Enterococcus faecalis and E. faecium, can be administered orally, and resistance appears to be less of a concern with linezolid compared with the other agents. Additionally, the results of two recent meta-analyses demonstrate more favorable outcomes with linezolid compared with daptomycin for the treatment of VRE bacteremia. The clinical pharmacokinetics of linezolid have been well described in children. The most notable concern with linezolid, however, is toxicities associated with prolonged use. Until more prospective data are available, we favor linezolid as first-line therapy for the treatment of VRE bacteremia in children.

What's new in the treatment of serious MRSA infection?

PURPOSE OF REVIEW

Vancomycin has been the cornerstone of treatment for methicillin-resistant Staphylococcus aureus (MRSA) infections. This review describes new MRSA-active antibiotics that have recently been introduced and highlights emerging resistance.

RECENT FINDINGS

Elevations in the vancomycin minimum inhibitory concentration within the susceptible range are associated with treatment failure and mortality in the treatment of MRSA infections. Ceftaroline and ceftobiprole are anti-MRSA cephalosporins and are noninferior to comparator agents in the treatment of acute bacterial skin and skin structure infections (ABSSSIs) and pneumonia. Tedizolid is more potent than linezolid, has improved pharmacokinetics and reduced toxicity and is active against cfr-containing S. aureus. Telavancin now has approval for treatment of hospital-acquired pneumonia, and recent phase 2 trial data showed similar cure rates in S. aureus bacteremia. Dalbavancin and oritavancin are administered once weekly and are noninferior to comparators for acute bacterial skin and skin structure infections. Resistance has emerged against many new anti-MRSA antimicrobials including ceftaroline. Combination therapy of β-lactams with vancomycin or daptomycin is increasing.

SUMMARY

Several new MRSA-active agents are now approved for use, although much of the data is derived from treatment of acute bacterial skin and skin structure infections or pneumonia. Further studies are required for more invasive infections, such as bacteremia and endocarditis.

Bacterial Infections in Hematopoietic Stem Cell Transplant Recipients

Bacterial infections are major complications after Hematopoietic Stem Cell Transplant (HSCT). They consist mainly of bloodstream infections (BSI), followed by pneumonia and gastrointestinal infections, including typhlitis and Clostridium difficile infection. Microbiological data come mostly from BSI. Coagulase negative staphylococci and Enterobacteriaceae are the most frequent pathogens causing approximately 25% of BSI each, followed by enterococci, P. aeruginosa and viridans streptococci. Bacterial pneumonia is frequent after HSCT, and Gram-negatives are predominant. Clostridium difficile infection affects approximately 15% of HSCT recipients, being more frequent in case of allogeneic than autologous HSCT. The epidemiology and the prevalence of resistant strains vary significantly between transplant centres. In some regions, multi-drug resistant (MDR) Gram-negative rods are increasingly frequent. In others, vancomycin-resistant enterococci are predominant. In the era of increasing resistance to antibiotics, the efficacy of fluoroquinolone prophylaxis and standard treatment of febrile neutropenia have been questioned. Therefore, a thorough evaluation of local epidemiology is mandatory to decide the need for prophylaxis and the choice of the best regimen for empirical treatment of febrile neutropenia. For the latter, individualised approach has been proposed, consisting of either escalation or de-escalation strategy. De-escalation strategy is recommended since resistant bacteria should be covered upfront, mainly in patients with severe clinical presentation and previous infection or colonisation with a resistant pathogen. Non-pharmacological interventions, such as screening for resistant bacteria, applying isolation and contact precautions should be put in place to limit the spread of MDR bacteria. Antimicrobial stewardship program should be implemented in transplant centres.

Clinical pharmacological approach for balancing the use of daptomycin and linezolid in comparison with that of vancomycin in the treatment of MRSA-related infections

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most challenging bacterial pathogens responsible for severe infections among hospitalized patients. In recent years there is increasing evidence that the clinical efficacy of vancomycin is progressively decreasing. Although daptomycin and linezolid are valuable alternatives to vancomycin for the treatment of MRSA-related bloodstream infections and pneumonia, respectively, a great deal of debate exists about their role in daily clinical practice due to cost-effectiveness issues. In this article we put into perspective the importance of pharmacokinetic/pharmacodynamic (PK/PD) considerations based on recent experimental and clinical data to argue whether they could be helpful in identifying clinical conditions in which these agents could be advantageous as compared to vancomycin.

Vancomycin in the treatment of methicillin-resistant Staphylococcus aureus - a clinician's guide to the science informing current practice

Clinicians treating an infection assess a patient in terms of disease manifestation, causative organism and available antibiotic options with the aim of devising a therapeutic strategy under the creed of 'first, do no harm'. It is often only when treatment is failing or options are limited, as in the scenario of multidrug-resistant organisms, that consideration is given to the interplay that occurs between the microbe and the host. The emergence of Staphylococcus aureus with reduced susceptibility to vancomycin provides a prime example of these dynamic interactions. This review shall explore these concepts in relation to vancomycin for the treatment of methicillin-resistant S. aureus, with the aim of providing an informed approach to the utilization of this drug.

Increased frequency of linezolid resistance among clinical Enterococcus faecium isolates from German hospital patients

Linezolid is an antibiotic of last resort for the treatment of infections with vancomycin-resistant enterococci (VRE). Here we report the increasing prevalence of linezolid resistance among clinical Enterococcus faecium strains from German hospital patients. Linezolid minimum inhibitory concentrations (MICs) were determined for 4461 clinical E. faecium strains isolated between 2008 and 2014. Isolates originated from the network of diagnostic laboratories collaborating with the National Reference Centre (NRC) for Staphylococci and Enterococci covering all German federal states. All linezolid-resistant isolates were determined by broth microdilution and confirmed by Etest as well as by analysing the 23S rDNA for putative mutations. Marker genes were determined by PCR. Genotyping was performed by SmaI macrorestriction analysis in pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST) for selected isolates. An increase in linezolid resistance was observed, from <1% in 2008 to >9% in 2014. Occasionally, outbreaks with linezolid-resistant VRE (ST117) were observed. In total, 232 (92.4%) of 251 linezolid-resistant E. faecium isolates (including 61 vanA and 29 vanB) contained the G2576T 23S rDNA mutation and showed a varying mixture of wild-type and mutated alleles per genome sufficient to confer linezolid resistance. In vitro growth experiments revealed a stable linezolid MIC. Of the 251 linezolid-resistant isolates, 5 were cfr-positive. In conclusion, these NRC data identified a country-wide ongoing trend of increasing linezolid resistance among clinical E. faecium isolates within the last 5 years.

Linezolid resistance in clinical isolates of Staphylococcus epidermidis from German hospitals and characterization of two cfr-carrying plasmids

OBJECTIVES

This study was a detailed investigation of Staphylococcus epidermidis clinical isolates exhibiting linezolid resistance.

METHODS

Thirty-six linezolid-resistant S. epidermidis from eight German hospitals, including isolates from suspected hospital-associated outbreaks between January 2012 and April 2013, were analysed with respect to their antimicrobial susceptibility and the presence of cfr and/or mutations in the 23S rRNA, rplC, rplD and rplV genes. Relatedness of isolates was estimated by MLST and SmaI macrorestriction analysis. Characterization of cfr plasmids was carried out by means of Illumina sequencing.

RESULTS

The MICs of linezolid varied substantially between the isolates. No apparent correlation was detected between the level of resistance, the presence of cfr and ribosomal target site mutations. S. epidermidis isolates from two hospitals were confirmed as clonally related, indicating the spread of the respective clone over a period of 1 year. Next-generation sequencing revealed two different categories of cfr-expressing plasmids, both of them varying in genetic arrangement and composition from previously published cfr plasmids: p12-00322-like plasmids showed incorporation of cfr into a pGO1-like backbone and displayed capabilities for intra- and inter-species conjugational transfer.

CONCLUSIONS

To date, linezolid-resistant S. epidermidis have rarely been isolated from human clinical sources in Germany. Here, we describe the emergence and outbreaks of these strains. We detected previously described and novel point mutations in the 23S ribosomal genes. The cfr gene was only present in six isolates. However, this is the first known description of cfr incorporation into conjugative vectors; under selective pressure, these vectors could give reasonable cause for concern.

Update on linezolid in vitro activity through the Zyvox Annual Appraisal of Potency and Spectrum Program, 2013

Linezolid showed MIC50s and MIC90s of 1 μg/ml (for both) against Staphylococcus aureus. Two S. aureus strains exhibited higher MICs (4 to 8 μg/ml) caused by cfr and/or target site mutations, including the first detection of cfr in Poland. Linezolid (MIC50 and MIC90, 0.5 and 1 μg/ml) had potent MICs against coagulase-negative staphylococci (CoNS). Four CoNS had MICs of 16 to 128 μg/ml due to alterations in 23S rRNA and/or L3/L4. Linezolid inhibited all enterococci and streptococci at ≤2 μg/ml, except for one Enterococcus faecium strain (MIC, 8 μg/ml; G2576T [Escherichia coli numbering] mutation).