A potential role for daptomycin in enterococcal infections: what is the evidence?

Comparing antibiotic resistance and virulence profiles of Enterococcus faecium, Klebsiella pneumoniae, and Pseudomonas aeruginosa from environmental and clinical settings

Antibiotic resistance and virulence profiles of Enterococcus faecium, Klebsiella pneumoniae, and Pseudomonas aeruginosa, isolated from water sources collected in informal settlements, were compared to clinical counterparts. Cluster analysis using repetitive extragenic palindromic sequence-based polymerase chain reaction (REP-PCR) indicated that, for each respective species, low genetic relatedness was observed between most of the clinical and environmental isolates, with only one clinical P. aeruginosa (PAO1) and one clinical K. pneumoniae (P2) exhibiting high genetic similarity to the environmental strains. Based on the antibiograms, the clinical E. faecium Ef CD1 was extensively drug resistant (XDR); all K. pneumoniae isolates (n = 12) (except K. pneumoniae ATCC 13883) were multidrug resistant (MDR), while the P. aeruginosa (n = 16) isolates exhibited higher susceptibility profiles. The tetM gene (tetracycline resistance) was identified in 47.4 % (n = 6 environmental; n = 3 clinical) of the E. faecium isolates, while the blaKPC gene (carbapenem resistance) was detected in 52.6 % (n = 7 environmental; n = 3 clinical) and 15.4 % (n = 2 environmental) of the E. faecium and K. pneumoniae isolates, respectively. The E. faecium isolates were predominantly poor biofilm formers, the K. pneumoniae isolates were moderate biofilm formers, while the P. aeruginosa isolates were strong biofilm formers. All E. faecium and K. pneumoniae isolates were gamma (γ)-haemolytic, non-gelatinase producing (E. faecium only), and non-hypermucoviscous (K. pneumoniae only), while the P. aeruginosa isolates exhibited beta (β)-haemolysis and produced gelatinase. The fimH (type 1 fimbriae adhesion) and ugE (uridine diphosphate galacturonate 4-epimerase synthesis) virulence genes were detected in the K. pneumoniae isolates, while the P. aeruginosa isolates possessed the phzM (phenazine production) and algD (alginate biosynthesis) genes. Similarities in antibiotic resistance and virulence profiles of environmental and clinical E. faecium, K. pneumoniae, and P. aeruginosa, thus highlights the potential health risks posed by using environmental water sources for daily water needs in low-and-middle-income countries.

Multidrug Resistance (MDR) and Collateral Sensitivity in Bacteria, with Special Attention to Genetic and Evolutionary Aspects and to the Perspectives of Antimicrobial Peptides-A Review

Antibiotic poly-resistance (multidrug-, extreme-, and pan-drug resistance) is controlled by adaptive evolution. Darwinian and Lamarckian interpretations of resistance evolution are discussed. Arguments for, and against, pessimistic forecasts on a fatal “post-antibiotic era” are evaluated. In commensal niches, the appearance of a new antibiotic resistance often reduces fitness, but compensatory mutations may counteract this tendency. The appearance of new antibiotic resistance is frequently accompanied by a collateral sensitivity to other resistances. Organisms with an expanding open pan-genome, such as Acinetobacter baumannii, Pseudomonas aeruginosa, and Klebsiella pneumoniae, can withstand an increased number of resistances by exploiting their evolutionary plasticity and disseminating clonally or poly-clonally. Multidrug-resistant pathogen clones can become predominant under antibiotic stress conditions but, under the influence of negative frequency-dependent selection, are prevented from rising to dominance in a population in a commensal niche. Antimicrobial peptides have a great potential to combat multidrug resistance, since antibiotic-resistant bacteria have shown a high frequency of collateral sensitivity to antimicrobial peptides. In addition, the mobility patterns of antibiotic resistance, and antimicrobial peptide resistance, genes are completely different. The integron trade in commensal niches is fortunately limited by the species-specificity of resistance genes. Hence, we theorize that the suggested post-antibiotic era has not yet come, and indeed might never come.

Daptomycin in the treatment of enterococcal bloodstream infections and endocarditis: a EUCAST position paper

SCOPE

This position paper describes the view adopted by EUCAST on the role of daptomycin in the treatment of serious infections caused by Enterococcus species.

BACKGROUND

High-dose daptomycin is considered effective in the treatment of enterococcal bloodstream infection (BSI) and endocarditis, although published clinical experience with the latter condition is limited.

METHODS

EUCAST reviewed the available published data on pharmacokinetics-pharmacodynamics (PK-PD), resistance selection, clinical efficacy and safety for the use of 10-12 mg/kg/day of daptomycin for these conditions, noting that the doses licensed by the European Medicines Agency are only 4-6 mg/kg/day, and only for infections caused by Staphylococcus aureus.

FINDINGS AND RECOMMENDATIONS

The PK-PD evidence shows that, even with doses of 10-12 mg/kg/day, it is not possible to treat infections caused by isolates at the upper end of the wild-type distributions of Enterococcus faecalis (with MICs of 4 mg/L) and E. faecium (with MICs of 4 or 8 mg/L). For this reason, and because there are ongoing issues with the reliability of laboratory testing, EUCAST lists daptomycin breakpoints for Enterococcus species as "IE"-insufficient evidence. EUCAST advises increased vigilance in the use of high-dose of daptomycin to treat enterococcal BSI and endocarditis. Additional PK-PD studies and prospective efficacy and safety studies of serious Enterococcal infections treated with high-dose daptomycin may permit the setting of breakpoints in the future.

Time-kill curves of daptomycin and Monte Carlo simulation for the treatment of bacteraemia caused by Enterococcus faecium

OBJECTIVES

The aim of this study was to investigate the effect of daptomycin against vancomycin-resistant Enterococcus faecium bacteraemia using computer modelling.

METHODS

Data obtained in vitro from time-kill curves were evaluated by PK/PD modelling and Monte Carlo simulations to determine the logarithmic reduction in the number of colony-forming units (CFU)/mL over 18 days of daptomycin treatment at 6, 8, and 10 mg/kg doses every 24 or 48 h and with variations in creatinine clearance (CL_CR) of 15-29, 30-49, and 50-100 mL/min/1.73 m². Monte Carlo simulations were performed to evaluate the probability of target attainment (PTA) for an area under the unbound drug concentration-time curve/minimum inhibitory concentration (fAUC/MIC) > 36 at the same doses and CL_CR.

RESULTS

Static time-kill model was employed to investigate the antibacterial efficacy of constant daptomycin concentrations. The time-kill curve analysis was performed using mathematical modelling based on a Hill coefficient factor. There was an expressive reduction (> 2 Log CFU/mL) over 18 days of daptomycin treatment in 75th percentile of individuals with CL_CR of 15-100 mL/min/1.73 m²) with daptomycin 6-10 mg/kg/day, except for daptomycin every 48 h. Using fAUC/MIC > 36, PTA was > 90% at MICs ≤ 2 μg/mL.

CONCLUSIONS

Higher daptomycin doses were associated with higher mortality in time-kill curves. The simulations indicated that independent of the CL_CR the therapeutic responses of VRE occur with doses of daptomycin ≥ 6 mg/kg/day and daptomycin every 48 h is insufficient to treat enterococcal bacteraemia.

Optimizing the Dosing Regimens of Daptomycin Based on the Susceptible Dose-Dependent Breakpoint against Vancomycin-Resistant Enterococci Infection

Daptomycin, a lipopeptide antibiotic, is one of the therapeutic options used for the treatment of vancomycin-resistant enterococci (VRE). Recently, the Clinical and Laboratory Standards Institute (CLSI) M100 30th edition has removed the susceptibility (S) breakpoint for Enterococcus faecium and replaced it with a susceptible dose-dependent (SDD) breakpoint of ≤4 μg/mL, with a suggested dosage of 8–12 mg/kg/day. Herein, we aimed to determine the minimum inhibitory concentration (MIC) values of daptomycin against clinical VRE isolates and to study the appropriate daptomycin dosing regimens among critically ill patients based on the new susceptibility CLSI breakpoint. The MIC determination of daptomycin was performed using E-test strips among clinical VRE strains isolated from patients at the Phramongkutklao Hospital. We used Monte Carlo simulation to calculate the probability of target attainment (PTA) and the cumulative fraction of response (CFR) of the ratio of the free area under the curve to MIC (fAUC_0–24/MIC) > 27.4 and fAUC_0–24/MIC > 20 for survival and microbiological response, respectively, at the first day and steady state. Further, we determined that the simulated daptomycin dosing regimen met the minimum concentration (Cmin) requirements for safety of being below 24.3 mg/L. All of the 48 VRE isolates were E. faecium strains, and the percentiles at the 50th and 90th MIC of daptomycin were 1 and 1.5 μg/mL, respectively. At MIC ≤ 2 μg/mL, a daptomycin dosage of 12 mg/kg/day achieved the PTA target of survival and microbiological response at the first 24 h time point and steady state. For a MIC of 4 μg/mL, none of the dosage regimens achieved the PTA target. For CFR, a dosage of 8–12 mg/kg/day could achieve the 90% CFR target at the first day and steady state. All dosing regimens had a low probability of Cmin being greater than 24.3 mg/L. In conclusion, the MIC of VRE against daptomycin is quite low, and loading and maintenance doses with 8 mg/kg/day were determined to be optimal and safe.

Fitness Restoration of a Genetically Tractable Enterococcus faecalis V583 Derivative To Study Decoration-Related Phenotypes of the Enterococcal Polysaccharide Antigen

E. faecalis strain VE14089 was derived from V583 cured of its plasmids. Although VE14089 had no major DNA rearrangements, it presented significant growth and host adaptation differences from the reference strain V583 of our collection. To construct a strain with better fitness, we sequenced the genome of VE14089, identified single nucleotide polymorphisms (SNPs), and repaired the genes that could account for these changes. Using this reference-derivative strain, we provide a novel genetic system to understand the role of the variable region of epa in the enterococcal lifestyle.

Commensal and generally harmless in healthy individuals, Enterococcus faecalis causes opportunistic infections in immunocompromised patients. Plasmid-cured E. faecalis strain VE14089, derived from sequenced reference strain V583, is widely used for functional studies due to its improved genetic amenability. Although strain VE14089 has no major DNA rearrangements, with the exception of an ∼20-kb integrated region of pTEF1 plasmid, the strain presented significant growth differences from the V583 reference strain of our collection (renamed VE14002). In the present study, genome sequencing of strain VE14089 identified additional point mutations. Excision of the integrated pTEF1 plasmid region and sequential restoration of wild-type alleles showing nonsilent mutations were performed to obtain the VE18379 reference-derivative strain. Recovery of the growth ability of the restored VE18379 strain at a level similar to that seen with the reference strain points to GreA and Spx as bacterial fitness determinants. Virulence potential in Galleria mellonella and intestinal colonization in mouse demonstrated host adaptation of the VE18379 strain equivalent to VE14002 host adaptation. We further demonstrated that deletion of the 16.8-kb variable region of the epa locus recapitulates the key role of Epa decoration in host adaptation, providing a genetic system to study the role of specific epa-variable regions in host adaptation independently of other genetic variations.

IMPORTANCEE. faecalis strain VE14089 was derived from V583 cured of its plasmids. Although VE14089 had no major DNA rearrangements, it presented significant growth and host adaptation differences from the reference strain V583 of our collection. To construct a strain with better fitness, we sequenced the genome of VE14089, identified single nucleotide polymorphisms (SNPs), and repaired the genes that could account for these changes. Using this reference-derivative strain, we provide a novel genetic system to understand the role of the variable region of epa in the enterococcal lifestyle.

A rationale for combination ampicillin and daptomycin in renal transplant patients with enterococcal infective endocarditis

Treatment of enterococcal endocarditis in patients with history of renal transplantation is complicated. Treatment failure and/or drug toxicities are not uncommon. Treatment with ampicillin and daptomycin in a renal transplant patient has been rarely reported. Here we report a patient who was successfully treated with this novel combination.

Pharmacodynamics of Daptomycin against Enterococcus faecium and Enterococcus faecalis in the Murine Thigh Infection Model

The Clinical and Laboratory Standards Institute (CLSI) daptomycin MIC susceptibility breakpoint for the treatment of enterococcal infections is ≤4 μg/ml. However, patients receiving daptomycin for the treatment of infections caused by enterococci with MICs of ≤4 μg/ml may experience treatment failures. We assessed the pharmacodynamics of daptomycin against enterococci in a neutropenic murine thigh infection model and determined the exposures necessary for bacteriostasis and a 1-log₁₀-CFU reduction of Enterococcus faecalis and Enterococcus faecium We further characterized daptomycin efficacy at clinically achievable exposures. Six E. faecium and 6 E. faecalis isolates (daptomycin MICs, 0.5 to 32 μg/ml) were studied. Daptomycin was administered at various doses over 24 h to achieve area under the free drug concentration-time curve-to-MIC ratios (fAUC_0-24/MIC) ranging from 1 to 148. Daptomycin regimens that simulate mean human exposures following doses of 6, 8, and 10 mg/kg of body weight/day were also studied. Efficacy was assessed by the differences in the number of log₁₀ CFU per thigh at 24 h. The Hill equation was used to estimate the fAUC_0-24/MIC required to achieve bacteriostasis and a 1-log₁₀-CFU reduction. For E. faecium, a 1-log₁₀-CFU reduction required an fAUC_0-24/MIC of 12.9 (R² = 0.71). For E. faecalis, a 1-log₁₀-CFU reduction was not achieved, while the fAUC_0-24/MIC required for stasis was 7.2 (R² = 0.8). With a human-simulated regimen of 6 mg/kg/day, a 1-log₁₀-CFU reduction was observed in 3/3 E. faecium isolates with MICs of <4 μg/ml and 0/3 E. faecium isolates with MICs of ≥4 μg/ml; however, a 1-log₁₀-CFU reduction was not achieved for any of the 6 E. faecalis isolates. These results, alongside clinical data, prompt a reevaluation of the current breakpoint.

Vancomycin-resistant enterococci with reduced daptomycin susceptibility in Singapore: prevalence and associated factors

Prevalence of vancomycin-resistant enterococci (VRE) and use of daptomycin are increasing in Asia. To determine the prevalence of daptomycin non-susceptible enterococci (DNSE) and understand factors associated with reduced daptomycin susceptibility in VRE, we conducted a case-control study in a 1600-bed adult tertiary hospital in Singapore. All VRE isolates from inpatients in 2012 were tested for daptomycin susceptibility. Patients with VRE isolates of daptomycin minimum inhibitory concentration (MIC) ⩾3 µg/ml were classified as daptomycin-reduced susceptible VRE (DRS-VRE) and those with daptomycin MIC 4 µg/ml (DNSE). About half (135, 55%) had reduced susceptibility to daptomycin (MIC 3-4 µg/ml). None in the DS-VRE group had prior exposure to daptomycin. After adjusting for age, gender, comorbidity, hospitalization duration, surgical history, indwelling device use, and duration of antibiotic exposure in the prior 3 months, >1 movement between wards [odds ratio (OR) 0·35, 95% confidence interval (CI) 0·16-0·74, P = 0·006] and minocycline resistance (OR 0·45, 95% CI 0·25-0·84, P = 0·011) were independently associated with DRS-VRE. Our study suggests that daptomycin exposure, >1 movement between wards, and resistance to minocycline, were associated with reduced daptomycin susceptibility in VRE.

Multiple roles for Enterococcus faecalis glycosyltransferases in biofilm-associated antibiotic resistance, cell envelope integrity, and conjugative transfer

The emergence of multidrug-resistant bacteria and the limited availability of new antibiotics are of increasing clinical concern. A compounding factor is the ability of microorganisms to form biofilms (communities of cells encased in a protective extracellular matrix) that are intrinsically resistant to antibiotics. Enterococcus faecalis is an opportunistic pathogen that readily forms biofilms and also has the propensity to acquire resistance determinants via horizontal gene transfer. There is intense interest in the genetic basis for intrinsic and acquired antibiotic resistance in E. faecalis, since clinical isolates exhibiting resistance to multiple antibiotics are not uncommon. We performed a genetic screen using a library of transposon (Tn) mutants to identify E. faecalis biofilm-associated antibiotic resistance determinants. Five Tn mutants formed wild-type biofilms in the absence of antibiotics but produced decreased biofilm biomass in the presence of antibiotic concentrations that were subinhibitory to the parent strain. Genetic determinants responsible for biofilm-associated antibiotic resistance include components of the quorum-sensing system (fsrA, fsrC, and gelE) and two glycosyltransferase (GTF) genes (epaI and epaOX). We also found that the GTFs play additional roles in E. faecalis resistance to detergent and bile salts, maintenance of cell envelope integrity, determination of cell shape, polysaccharide composition, and conjugative transfer of the pheromone-inducible plasmid pCF10. The epaOX gene is located in a variable extended region of the enterococcal polysaccharide antigen (epa) locus. These data illustrate the importance of GTFs in E. faecalis adaptation to diverse growth conditions and suggest new targets for antimicrobial design.

Comparison of the Effectiveness and Safety of Linezolid and Daptomycin in Vancomycin-Resistant Enterococcal Bloodstream Infection: A National Cohort Study of Veterans Affairs Patients

BACKGROUND

Vancomycin-resistant Enterococcus bloodstream infections (VRE-BSIs) are becoming increasingly common. Linezolid and daptomycin are the primary treatment options for VRE-BSI, but optimal treatment is unclear.

METHODS

This was a national retrospective cohort study comparing linezolid and daptomycin for the treatment of VRE-BSI among Veterans Affairs Medical Center patients admitted during 2004-2013. The primary outcome was treatment failure, defined as a composite of (1) 30-day all-cause mortality; (2) microbiologic failure; and (3) 60-day VRE-BSI recurrence. Poisson regression was conducted to determine if antimicrobial treatment was independently associated with clinical outcomes.

RESULTS

A total of 644 patients were included (linezolid, n = 319; daptomycin, n = 325). Overall, treatment failure was 60.9% (n = 392/644), and 30-day all-cause mortality was 38.2% (n = 246/644). Linezolid was associated with a significantly higher risk of treatment failure compared with daptomycin (risk ratio [RR], 1.37; 95% confidence interval [CI], 1.13-1.67; P = .001). After adjusting for confounding factors in Poisson regression, the relationship between linezolid use and treatment failure persisted (adjusted RR, 1.15; 95% CI, 1.02-1.30; P = .026). Linezolid was also associated with higher 30-day mortality (42.9% vs 33.5%; RR, 1.17; 95% CI, 1.04-1.32; P = .014) and microbiologic failure rates (RR, 1.10; 95% CI, 1.02-1.18; P = .011). No difference in 60-day VRE-BSI recurrence was observed between treatment groups.

CONCLUSIONS

Treatment with linezolid for VRE-BSI resulted in significantly higher treatment failure in comparison to daptomycin. Linezolid treatment was also associated with greater 30-day all-cause mortality and microbiologic failure in this cohort.

Mechanisms of antibiotic resistance in enterococci

Multidrug-resistant (MDR) enterococci are important nosocomial pathogens and a growing clinical challenge. These organisms have developed resistance to virtually all antimicrobials currently used in clinical practice using a diverse number of genetic strategies. Due to this ability to recruit antibiotic resistance determinants, MDR enterococci display a wide repertoire of antibiotic resistance mechanisms including modification of drug targets, inactivation of therapeutic agents, overexpression of efflux pumps and a sophisticated cell envelope adaptive response that promotes survival in the human host and the nosocomial environment. MDR enterococci are well adapted to survive in the gastrointestinal tract and can become the dominant flora under antibiotic pressure, predisposing the severely ill and immunocompromised patient to invasive infections. A thorough understanding of the mechanisms underlying antibiotic resistance in enterococci is the first step for devising strategies to control the spread of these organisms and potentially establish novel therapeutic approaches.

Vancomycin-Resistant Enterococcal Bacteremia Pharmacotherapy

Objective: To review the literature on the pharmacotherapy of bloodstream infections (BSI) caused by vancomycin-resistant enterococci (VRE). Data Sources: A MEDLINE literature search was performed for the period 1946 to May 2014 using the search terms Enterococcus, enterococci, vancomycin-resistant, VRE, bacteremia, and bloodstream infection. References were also identified from selected review articles. Study Selection and Data Extraction: English-language case series, cohort studies, and meta-analyses assessing the options in the pharmacotherapy of VRE BSIs in adult patients were evaluated. Data Synthesis: Studies were identified that utilized linezolid, quinupristin/dalfopristin (Q/D), and daptomycin. In all, 8 comparative retrospective cohort studies, 2 meta-analyses of daptomycin and linezolid, and 3 retrospective comparisons of linezolid and Q/D were included for review. Mortality associated with VRE BSIs was high across studies, and the ability to determine differences in outcomes between agents was confounded by the complex nature of the patients included. Two meta-analyses comparing daptomycin with linezolid for VRE BSIs found modest advantages for linezolid, but these conclusions may be hampered by heterogeneity within the included studies. Conclusions: VRE BSIs remain a difficult-to-treat clinical situation. Differences in toxicity between the agents used to treat it are clear, but therapeutic differences are more difficult to discern. Meta-analyses suggest that a moderate advantage for linezolid over daptomycin may exist, but problems with the nature of studies that they included make definitive conclusions difficult.

Methicillin-Resistant Staphylococcus aureus Prostatic Abscess in a Liver Transplant Recipient

Prostatic abscesses are usually related to gram-negative bacilli. However, methicillin-resistant Staphylococcus aureus (MRSA) has emerged as a substantial cause of prostatic abscesses in recent years. Herein, we report the case of a 31-year-old man with a history of orthotopic liver transplantation 10 years ago who presented with acute onset dysuria and abdominal pain and was diagnosed with a MRSA prostatic abscess. To our knowledge, this is the first case describing a prostatic abscess in a liver transplant recipient and the first reporting MRSA as the causative organism of a prostatic abscess in a solid organ transplant recipient.

Daptomycin for the treatment of bacteraemia due to vancomycin-resistant enterococci

Treatment of severe infections caused by vancomycin-resistant enterococci (VRE) is challenging due to the scarcity of reliable therapeutic alternatives. In this context, daptomycin (DAP), a lipopeptide antibiotic, has emerged as an interesting alternative as it is one of the few compounds that retain in vitro bactericidal activity against VRE isolates, although it has not been approved for this purpose by regulatory agencies. In this review, we will summarise the clinical, animal and in vitro evidence evaluating the efficacy of DAP for the management of deep-seated VRE infections. In addition, we will address important clinical concerns such as the emergence of DAP resistance during therapy and reports of therapeutic failure with DAP monotherapy. Finally, we will discuss possible future strategies (such as the use of higher doses and/or combination therapies) to optimise the use of this antibiotic against VRE.

Activity of daptomycin or linezolid in combination with rifampin or gentamicin against biofilm-forming Enterococcus faecalis or E. faecium in an in vitro pharmacodynamic model using simulated endocardial vegetations and an in vivo survival assay using Galleria mellonella larvae

Enterococci are the third most frequent cause of infective endocarditis. A high-inoculum stationary-phase in vitro pharmacodynamic model with simulated endocardial vegetations was used to simulate the human pharmacokinetics of daptomycin at 6 or 10 mg/kg of body weight/day or linezolid at 600 mg every 12 h (q12h), alone or in combination with gentamicin at 1.3 mg/kg q12h or rifampin at 300 mg q8h or 900 mg q24h. Biofilm-forming, vancomycin-susceptible Enterococcus faecalis and vancomycin-resistant Enterococcus faecium (vancomycin-resistant enterococcus [VRE]) strains were tested. At 24, 48, and 72 h, all daptomycin-containing regimens demonstrated significantly more activity (decline in CFU/g) than any linezolid-containing regimen against biofilm-forming E. faecalis. The addition of gentamicin to daptomycin (at 6 or 10 mg/kg) in the first 24 h significantly improved bactericidal activity. In contrast, the addition of rifampin delayed the bactericidal activity of daptomycin against E. faecalis, and the addition of rifampin antagonized the activities of all regimens against VRE at 24 h. Also, against VRE, the addition of gentamicin to linezolid at 72 h improved activity and was bactericidal. Rifampin significantly antagonized the activity of linezolid against VRE at 72 h. In in vivo Galleria mellonella survival assays, linezolid and daptomycin improved survival. Daptomycin at 10 mg/kg improved survival significantly over that with linezolid against E. faecalis. The addition of gentamicin improved the efficacy of daptomycin against E. faecalis and those of linezolid and daptomycin against VRE. We conclude that in enterococcal infection models, daptomycin has more activity than linezolid alone. Against biofilm-forming E. faecalis, the addition of gentamicin in the first 24 h causes the most rapid decline in CFU/g. Of interest, the addition of rifampin decreased the activity of daptomycin against both E. faecalis and VRE.

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.

The cell wall architecture of Enterococcus faecium: from resistance to pathogenesis

The cell wall of Gram-positive bacteria functions as a surface organelle that continuously interacts with its environment through a plethora of cell wall-associated molecules. Enterococcus faecium is a normal inhabitant of the GI tract of mammals, but has recently become an important etiological agent of hospital-acquired infections in debilitated patients. Insights into the assembly and function of enterococcal cell wall components and their interactions with the host during colonization and infection are essential to explain the worldwide emergence of E. faecium as an important multiantibiotic-resistant nosocomial pathogen. Understanding the biochemistry of cell wall biogenesis and principles of antibiotic resistance at the molecular level may open up new frontiers in research on enterococci, particularly for the development of novel antimicrobial strategies. In this article, we outline the current knowledge on the most important antimicrobial resistance mechanisms that involve peptidoglycan synthesis and the role of cell wall constituents, including lipoteichoic acid, wall teichoic acid, capsular polysaccharides, LPxTG cell wall-anchored surface proteins, WxL-type surface proteins and pili, in the pathogenesis of E. faecium.

Iatrogenic neurology

Iatrogenic disease is one of the most frequent causes of hospital admissions and constitutes a growing public health problem. The most common type of iatrogenic neurologic disease is pharmacologic, and the central and peripheral nervous systems are particularly vulnerable. Despite this, iatrogenic disease is generally overlooked as a differential diagnosis among neurologic patients. The clinical picture of pharmacologically mediated iatrogenic neurologic disease can range from mild to fatal. Common and uncommon forms of drug toxicity are comprehensively addressed in this chapter. While the majority of neurologic adverse effects are listed and referenced in the tables, the most relevant issues are further discussed in the text.

Role of capsular polysaccharide (CPS) in biofilm formation and regulation of CPS production by quorum-sensing in Vibrio vulnificus

Extracellular polysaccharides, such as lipopolysaccharide and loosely associated exopolysaccharides, are essential for Vibrio vulnificus to form biofilms. The role of another major component of the V. vulnificus extracellular matrix, capsular polysaccharide (CPS), which contributes to colony opacity, has been characterized in biofilm formation. A CPS-deficient mutant, whose wbpP gene encoding UDP-GlcNAc C4-epimerase was knocked out, formed significantly more biofilm than wild type, due to increased hydrophobicity of the cell surface, adherence to abiotic surfaces and cell aggregation. To elucidate the direct effect of CPS on biofilm structure, extracted CPS and a CPS-degrading enzyme, α-N-acetylgalactosaminidase, were added in biofilm assays, resulting in reduction and increment of biofilm sizes respectively. Therefore, it is suggested that CPS play a critical role in determining biofilm size by restricting continual growth of mature biofilms. Since CPS is required after maturation, CPS biosynthesis should be controlled in a cell density-dependent manner, e.g. by quorum-sensing (QS) regulation. Analysing transcription of the CPS gene cluster revealed that it was activated by SmcR, a QS master regulator, via binding to the upstream region of the cluster. Therefore, CPS was produced when biofilm cell density reached high enough to turn on QS regulation and limited biofilms to appropriate sizes.

Current use of daptomycin in cardiac surgery and postoperative intensive care

Infections due to multidrug-resistant pathogens have an increasing impact on patients undergoing cardiac surgery. Preoperative infections, such as endocarditis, and postoperative infections, including wound and device infection, influence patient outcomes. Special interest needs to be taken in patients admitted to cardiac surgical intensive care units, as these patients are at high risk for infections, particularly nosocomial pneumonia, catheter-related and wound infections. The increasing numbers of infections due to Gram-positive multidrug-resistant pathogens underline the necessity for newer antibiotics with bactericidal effects and a more favorable profile of side effects. Daptomycin, a lipopeptide antimicrobial agent with bactericide activity against Gram-positive organisms, has been successfully used in the treatment of complicated infections due to Gram-positive multidrug-resistant pathogens, especially regarding endocarditis, wound infections, device and catheter-related infections in intensive care units. In this review, the authors will summarize therapeutic potential of daptomycin in cardiac surgery and postoperative intensive care.

Treatment considerations in vancomycin-resistant enterococcal bacteremia: daptomycin or linezolid? A review

BACKGROUND

Vancomycin resistant enterococcal bloodstream infections are an important cause of morbidity and mortality in hospitalized patients.

AIM OF THE REVIEW

A search of the literature was undertaken to determine the optimal antimicrobial therapy for the management of vancomycin resistant enterococcal bloodstream infections.

METHOD

MEDLINE, EMBASE, and the Cochrane Library (unrestricted to time or language) were searched for studies of vancomycin resistant enterococcal bloodstream infections in adults reporting outcomes of direct comparisons of linezolid versus daptomycin on November 26, 2012. Studies of basic science, reviews, commentaries, pharmacologic, epidemiologic, or pediatric studies, and those studies examining conditions other than enterococcal bacteremia, a single antimicrobial agent or other antimicrobials were excluded.

RESULTS

226 studies were screened for eligibility and yielded eight studies evaluating a total of 807 patients. Inter-rater agreement was 100 %. Qualitative evaluation of the studies was performed using the Newcastle-Ottawa scale. No randomized controlled trials were identified. All studies were retrospective cohorts and non-randomized. 458 (57 %) patients treated with linezolid and 349 (43 %) with daptomycin were analyzed. Variable comorbidities and severity of illness were described in the included studies and reported here for comparison.

CONCLUSION

The optimal treatment of vancomycin resistant enterococcal bloodstream infections is yet to be determined. Well-designed prospective studies are needed to lend more convincing evidence regarding choice of antimicrobial therapy for this important multidrug resistant organism.

Estimation of the subunit stoichiometry of the membrane-associated daptomycin oligomer by FRET

Daptomycin is a lipopeptide antibiotic that kills Gram-positive bacteria by depolarizing their cell membranes. This antibacterial action of daptomycin is correlated with the formation of membrane-associated oligomers. We here examine the number of subunits contained in one oligomer using fluorescence resonance energy transfer (FRET). The results suggest that the oligomer contains approximately 6 to 7 subunits, or possibly twice this number if it spans both membrane monolayers.

Use of daptomycin in the treatment of vancomycin-resistant enterococcal urinary tract infections: a short case series

Background

Vancomycin-resistant enterococci are a leading cause of hospital-acquired urinary tract infection and a growing concern for the clinician. The aim of this study was to evaluate the effectiveness of daptomycin in the treatment of patients with vancomycin-resistant enterococcal urinary tract infection treated in our 200-bed community-based institution.

Methods

Patients with confirmed symptomatic vancomycin-resistant enterococcal urinary tract infection identified by infectious disease consultation between January 1, 2007, and December 8, 2009, vancomycin-resistant enterococci–positive urine culture, and urinary symptoms and/or pyuria on urinalysis, and treated with daptomycin, were included in this case series. Daptomycin was generally administered at a planned dosage regimen of ≥5 mg/kg every 24 hours in patients with normal to moderately impaired kidney function or every 48 hours in patients with severe kidney disease. Microbiologic cure was defined as eradication of vancomycin-resistant enterococci in urine cultures taken after the completion of daptomycin treatment. Clinical cure was defined by symptom resolution, as assessed by the infectious disease clinician caring for the patient.

Results

Included in this case series are 10 patients who received daptomycin for confirmed vancomycin-resistant enterococcal urinary tract infection. Patients had a history of extensive hospital stays. Chart review revealed that all levels of kidney function (3, 2, 3, and 2 patients with kidney disease classified as normal, mild, moderate, and severe/kidney failure, respectively) were represented in the sample and that patients with (n = 5) or without (n = 5) previous urinary tract infection and with (n = 3) or without (n = 7) Foley catheters were included. Treatment with daptomycin achieved clinical cure and vancomycin-resistant enterococcal eradication in all cases in this series.

Conclusion

Treatment with daptomycin was well tolerated and effective in all patients in this series, regardless of renal function, history of urinary tract infection, or Foley catheter use. This study adds to emerging clinical evidence that daptomycin is a valuable treatment for vancomycin-resistant enterococcal urinary tract infection.

Surveillance and management of multidrug-resistant microorganisms

Multidrug-resistant organisms are an established and growing worldwide public health problem and few therapeutic options remain available. The traditional antimicrobials (glycopeptides) for multidrug-resistant Gram-positive infections are declining in efficacy. New drugs that are presently available are linezolid, daptomicin and tigecycline, which have well-defined indications for severe infections, and talavancin, which is under Phase III trial for hospital-acquired pneumonia. Unfortunately the therapies available for multidrug-resistant Gram-negatives, including carbapenem-resistant Pseudomonas aeruginosa, Acinetobacter baumannii and Enterobacteriaceae, are limited to only colistin and tigecycline. Both of these drugs are still not registered for severe infections, such as hospital acquired pneumonia. Consequently, as confirmed by scientific evidence, a multidisciplinary approach is needed. Surveillance, infection control procedures, isolation and antimicrobial stewardship should be implemented to reduce multidrug-resistant organism diffusion.

Susceptibility of enterococci to daptomycin is dependent upon testing methodology

An increase in daptomycin nonsusceptible enterococci (DNSE) was noted in our institution (8.3% 2008 to 34.5% 2011) using MicroScan methods which may overestimate DNSE prevalence. DNSE (N = 150) from the clinical laboratory (2008-2011) underwent susceptibility testing using broth microdilution (BMD), Etest, Sensititire, MicroScan prompt (MSP), and MicroScan turbidity (MST) with only 20% of isolates confirmed as nonsusceptible. Categorical and essential agreement were highest with MSP and MST, but both missed the majority of resistant isolates (70% and 87% missed). Etest MIC values were statistically higher, more likely to be nonsusceptible, had the lowest very major error rate (37%), and the highest falsely nonsusceptible rate (22%). Sensititre MIC values were not statistically different from BMD, but missed 57% of DNSE. PFGE analysis did not define a clonal outbreak. These findings suggest that MicroScan methods overestimate nonsusceptibility, and the lack of correlation between methods raises questions regarding which method is most effective at confirming nonsusceptibility.

In vitro activity of daptomycin against Enterococcus faecalis under various conditions of growth-phases, inoculum and pH

Enterococcus faecalis (E. faecalis) has become a major leading cause of nosocomial endocarditis. Treatment of such infections remains problematic and new therapeutic options are needed. Nine E. faecalis strains were tested: six obtained from patients presenting endocarditis, one with isolated bacteremia, and two reference strains. Antibiotics included daptomycin, alone or in combination, linezolid, tigecycline, rifampicin, gentamicin, teicoplanin, ceftriaxone and amoxicillin. Time-kill studies included colony counts at 1, 4 and 24 h of incubation. Significant bactericidal activity was defined as a decrease of ≥3log₁₀CFU/ml after 24 h of incubation. Antibiotics were tested at a low (10⁶ CFU/ml) and high (10⁹ CFU/ml) inoculum, against exponential- and stationary-phase bacteria. We also performed time kill studies of chemically growth arrested E. faecalis. Various pH conditions were used during the tests. In exponential growth phase and with a low inoculum, daptomycin alone at 60 µg/ml and the combination amoxicillin-gentamicin both achieved a 4-log₁₀ reduction in one hour on all strains. In exponential growth phase with a high inoculum, daptomycin alone was bactericidal at a concentration of 120 µg/ml. All the combinations tested with this drug were indifferent. In stationary phase with a high inoculum daptomycin remained bactericidal but exhibited a pH dependent activity and slower kill rates. All combinations that did not include daptomycin were not bactericidal in conditions of high inoculum, whatever the growth phase. The results indicate that daptomycin is the only antibiotic that may be able of overcoming the effects of growth phase and high inoculum.

Effectiveness and safety of daptomycin in complicated skin and soft-tissue infections and bacteraemia in clinical practice: results of a large non-interventional study

This retrospective analysis of patients from eight countries included in the European Cubicin(®) Outcomes Registry and Experience (EU-CORE(SM)) captures the first post-approval years of clinical experience with daptomycin in its licensed indications. Of the total 1127 patients enrolled in EU-CORE between 2006 and 2008, 373 had a primary complicated skin and soft-tissue infection (cSSTI), most commonly surgical-site infection (48%), and 244 had bacteraemia, 55% of which were catheter-related. The most common pathogens were Staphylococcus aureus in cSSTIs (43%) and coagulase-negative staphylococci in bacteraemia (36%). The most frequently prescribed daptomycin doses were 4 mg/kg and 6 mg/kg for cSSTIs, and 6 mg/kg for bacteraemia. The median duration of inpatient and outpatient treatment, respectively, was 13 days and 8 days for cSSTIs and 8 days and 10 days for bacteraemia. Clinical success was reported for 81% of patients with cSSTIs and 77% with bacteraemia, with 82% success overall for infections caused by S. aureus. A trend towards higher clinical success was noted with higher daptomycin doses in bacteraemia (78% for 6 mg/kg vs. 90% for doses >6 mg/kg). Daptomycin demonstrated a favourable safety profile. Adverse events regardless of relationship to study drug were reported for 11% of patients with cSSTIs and 24% with bacteraemia, most commonly septic shock [7 patients (2%) with cSSTIs and 5 patients (2%) with bacteraemia]. These results demonstrate that daptomycin is effective and well tolerated in the treatment of cSSTIs and bacteraemia caused by Gram-positive bacteria in clinical practice.

Clinical experience with daptomycin in Italy: results from a registry study of the treatment of Gram-positive infections between 2006 and 2009

The purpose of this study was to evaluate post-marketing efficacy and safety data for therapy with daptomycin (DAP) in Italy. Data from patients treated with DAP at 30 centres between January 2006 and July 2009 were analysed according to the protocol of the EU-CORE(SM). In 359 patients, DAP was most commonly prescribed for skin and skin-structure infections (55%), infective endocarditis (13%), and bacteraemia (12.5%). DAP was associated with rapid improvement, and clinical success rates ranging from 77 to 91%, despite almost half of the patient population being ≥65 years of age, 86% having significant underlying disease, and many being affected by drug-resistant pathogens. Staphylococcus aureus represented 35% of all pathogens isolated. Success rates for all staphylococcal and enterococcal infections were >80%, including methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus (MSSA). Clinical outcomes were similar for DAP whether used as first- or second-line therapy. DAP was well tolerated even with prolonged treatment.

Daptomycin for the treatment of infective endocarditis: results from a European registry

OBJECTIVES

Infective endocarditis (IE) is a complex infection associated with high mortality. Daptomycin, a cyclic lipopeptide antibiotic highly active against Gram-positive bacteria, has recently been incorporated into IE treatment guidelines. This retrospective analysis provides insights into the use of daptomycin in IE in the European Cubicin(®) Outcomes Registry Experience (EU-CORE(SM)) between 2006 and 2010.

PATIENTS AND METHODS

Three hundred and seventy-eight (10%) of 3621 enrolled patients received daptomycin for treatment of IE. Two hundred and fifty-nine (69%) had left-sided IE (LIE) and 182 patients (48%) underwent concomitant surgery.

RESULTS

Staphylococcus aureus was the most frequently identified pathogen (n=92; methicillin susceptible, n=50) and daptomycin was used empirically in 134 patients. Among cases of second-line therapy (n=312), the most common reason for switching to daptomycin was failure of the previous regimen (including glycopeptides and penicillins). Daptomycin was administered at 6 mg/kg in 224 patients and at ≥ 8 mg/kg in 72 patients. Clinical success rates were 80% overall, 91% for right-sided IE (RIE) and 76% for LIE, with similar rates seen for infections caused by methicillin-susceptible S. aureus (84%) and methicillin-resistant S. aureus (81%). The clinical success rate in patients treated with ≥ 8 mg/kg daptomycin was 90% [n=72 (RIE, 91%; LIE, 89%)]. No new safety signals were observed.

CONCLUSIONS

In patients with IE registered in EU-CORE, daptomycin was most frequently used as second-line treatment after treatment failure. The majority of patients had LIE and most commonly received daptomycin for the treatment of staphylococcal infections. Clinical success was high in this difficult-to-treat population. The role of doses ≥ 8 mg/kg per day in the empirical treatment of IE deserves further investigation.

Whole-genome analysis of a daptomycin-susceptible enterococcus faecium strain and its daptomycin-resistant variant arising during therapy

Development of daptomycin (DAP) resistance in Enterococcus faecalis has recently been associated with mutations in genes encoding proteins with two main functions: (i) control of the cell envelope stress response to antibiotics and antimicrobial peptides (LiaFSR system) and (ii) cell membrane phospholipid metabolism (glycerophosphoryl diester phosphodiesterase and cardiolipin synthase [cls]). However, the genetic bases for DAP resistance in Enterococcus faecium are unclear. We performed whole-genome comparative analysis of a clinical strain pair, DAP-susceptible E. faecium S447 and its DAP-resistant derivative R446, which was recovered from a single patient during DAP therapy. By comparative whole-genome sequencing, DAP resistance in R446 was associated with changes in 8 genes. Two of these genes encoded proteins involved in phospholipid metabolism: (i) an R218Q substitution in Cls and (ii) an A292G reversion in a putative cyclopropane fatty acid synthase enzyme. The DAP-resistant derivative R446 also exhibited an S333L substitution in the putative histidine kinase YycG, a member of the YycFG system, which, similar to LiaFSR, has been involved in cell envelope homeostasis and DAP resistance in other Gram-positive cocci. Additional changes identified in E. faecium R446 (DAP resistant) included two putative proteins involved in transport (one for carbohydrate and one for sulfate) and three enzymes predicted to play a role in general metabolism. Exchange of the "susceptible" cls allele from S447 for the "resistant" one belonging to R446 did not affect DAP susceptibility. Our results suggest that, apart from the LiaFSR system, the essential YycFG system is likely to be an important mediator of DAP resistance in some E. faecium strains.

Epidemiology of vancomycin-resistant enterococci with reduced susceptibility to daptomycin

A retrospective case-case control study was conducted, including 60 cases with daptomycin-nonsusceptible vancomycin-resistant enterococci (DNS-VRE) matched to cases with daptomycin-susceptible VRE and to uninfected controls (1∶1∶3 ratio). Immunosuppression, presence of comorbid conditions, and prior exposure to antimicrobials were independent predictors of DNS-VRE, although prior daptomycin exposure occurred rarely. In summary, a case-case control study identified independent risk factors for the isolation of DNS-VRE: immunosuppression, multiple comorbid conditions, and prior exposures to cephalosporines and metronidazole.

Mutual inhibition through hybrid oligomer formation of daptomycin and the semisynthetic lipopeptide antibiotic CB-182,462

Daptomycin is a clinically important lipopeptide antibiotic that kills Gram-positive bacteria through membrane depolarization. Its activity requires calcium and the presence of phosphatidylglycerol in the target membrane. Calcium and phosphatidylglycerol also promote the formation of daptomycin oligomers, which have been assumed but not proven to be required for the bactericidal effect. Daptomycin shares substantial structural similarity with another lipopeptide antibiotic, A54145; the two have identical amino acid residues in 5 out of 13 positions and similar ones in 4 more positions. We here examined whether these conserved residues are sufficient for oligomer formation. To this end, we used fluorescence energy transfer and excimer fluorescence to detect hybrid oligomers of daptomycin and CB-182,462, a semisynthetic derivative of A54145. Mixtures of the two compounds indeed produced hybrid oligomers, but at the same time displayed a significantly less than additive antibacterial activity against Bacillus subtilis. The existence of functionally impaired oligomers indicates that oligomer formation is indeed important for antibacterial function. However, it also shows that oligomerization is not sufficient; once formed, the oligomers must take another step in order to acquire antibacterial activity. Thus, the amino acid residues shared between daptomycin and CB-182,462 suffice for formation of the oligomer, but not for its subsequent activation.

Intrinsic and acquired resistance mechanisms in enterococcus

Enterococci have the potential for resistance to virtually all clinically useful antibiotics. Their emergence as important nosocomial pathogens has coincided with increased expression of antimicrobial resistance by members of the genus. The mechanisms underlying antibiotic resistance in enterococci may be intrinsic to the species or acquired through mutation of intrinsic genes or horizontal exchange of genetic material encoding resistance determinants. This paper reviews the antibiotic resistance mechanisms in Enterococcus faecium and Enterococcus faecalis and discusses treatment options.

Glycopeptide resistance in gram-positive cocci: a review

Vancomycin-resistant enterococci (VRE) have emerged as important nosocomial pathogens in the past two decades all over the world and have seriously limited the choices available to clinicians for treating infections caused by these agents. Methicillin-resistant Staphylococcus aureus, perhaps the most notorious among the nosocomial pathogens, was till recently susceptible to vancomycin and the other glycopeptides. Emergence of vancomycin nonsusceptible strains of S. aureus has led to a worrisome scenario where the options available for treating serious infections due to these organisms are very limited and not well evaluated. Vancomycin resistance in clinically significant isolates of coagulase-negative staphylococci is also on the rise in many setups. This paper aims to highlight the genetic basis of vancomycin resistance in Enterococcus species and S. aureus. It also focuses on important considerations in detection of vancomycin resistance in these gram-positive bacteria. The problem of glycopeptide resistance in clinical isolates of coagulase-negative staphylococci and the phenomenon of vancomycin tolerance seen in some strains of Streptococcus pneumoniae has also been discussed. Finally, therapeutic options available and being developed against these pathogens have also found a mention.

Correlation between mutations in liaFSR of Enterococcus faecium and MIC of daptomycin: revisiting daptomycin breakpoints

Mutations in liaFSR, a three-component regulatory system controlling cell-envelope stress response, were recently linked with the emergence of daptomycin (DAP) resistance in enterococci. Our previous work showed that a liaF mutation increased the DAP MIC of a vancomycin-resistant Enterococcus faecalis strain from 1 to 3 μg/ml (the DAP breakpoint is 4 μg/ml), suggesting that mutations in the liaFSR system could be a pivotal initial event in the development of DAP resistance. With the hypothesis that clinical enterococcal isolates with DAP MICs between 3 and 4 μg/ml might harbor mutations in liaFSR, we studied 38 Enterococcus faecium bloodstream isolates, of which 8 had DAP MICs between 3 and 4 μg/ml by Etest in Mueller-Hinton agar. Interestingly, 6 of these 8 isolates had predicted amino acid changes in the LiaFSR system. Moreover, we previously showed that among 6 DAP-resistant E. faecium isolates (MICs of >4 μg/ml), 5 had mutations in liaFSR. In contrast, none of 16 E. faecium isolates with a DAP MIC of ≤2 μg/ml harbored mutations in this system (P < 0.0001). All but one isolate with liaFSR changes exhibited DAP MICs of ≥16 μg/ml by Etest using brain heart infusion agar (BHIA), a medium that better supports enterococcal growth. Our findings provide a strong association between DAP MICs within the upper susceptibility range and mutations in the liaFSR system. Concomitant susceptibility testing on BHIA may be useful for identifying these E. faecium first-step mutants. Our results also suggest that the current DAP breakpoint for E. faecium may need to be reevaluated.

Vancomycin-resistant Enterococcus bacteremia: an evaluation of treatment with linezolid or daptomycin

BACKGROUND

Due to high rates of resistance and a limited number of efficacious antimicrobials for vancomycin-resistant Enterococcus (VRE), appropriate antibiotic selection is vital to treatment success. The purpose of this study was to assess clinical and microbiologic outcomes associated with the use of linezolid or daptomycin in the treatment of VRE bacteremia.

METHODS

A retrospective analysis of adult patients with VRE bacteremia between January 2004 and July 2009 was conducted at a tertiary care hospital in the United States. Clinical and microbiologic outcomes for both therapies were evaluated using multiple criteria.

RESULTS

Of the 361 patients with VRE bacteremia identified, 201 were included in the study (linezolid group, n = 138; daptomycin group, n = 63). More patients in the daptomycin group had hematologic malignancies (33% vs 14%) or received liver transplants (13% vs 4%). There was no difference in clinical or microbiologic cure between the linezolid and daptomycin groups (74% vs 75% and 94% vs 94%, respectively). Recurrence was documented in 3% of linezolid patients vs 12% of daptomycin patients (P = 0.0321). Reinfection was noted in 1% of patients in the linezolid group vs 6% of patients in the daptomycin group (P not significant). The average length of stay (LOS) was 37 days for the linezolid group vs 40 days for the daptomycin group (P not significant). Overall mortality was 20%, occurring in 25/138 linezolid patients vs 15/63 daptomycin patients (P not significant).

CONCLUSIONS

No differences in clinical or microbiologic cure rates, LOS, or mortality were identified between the groups. Various factors may have contributed to the significantly higher recurrence of VRE bacteremia in daptomycin patients. This study suggests that linezolid and daptomycin appear equally efficacious in the treatment of VRE bacteremia.

[Infective endocarditis in the XXI century: epidemiological, therapeutic, and prognosis changes]

Infective endocarditis (IE) is an uncommon and severe disease. Nowadays, in developed countries, IE patients are older, usually have a degenerative heart valve disease, and up to 30% acquire this infection within the health care system. In consequence, staphylococci species are the most frequently isolated microorganisms. Antimicrobial treatment for IE has significantly changed over the last decades. In IE episodes due to Staphylococcus aureus, cloxacillin-resistance makes antimicrobial election more difficult. Other microorganisms, such as enterococci and some species of streptococci, show high rates of resistance to antimicrobial agents established in guidelines. Despite improvements in the diagnosis, and medical and surgical treatment of IE, this disease continues to be associated with high rates of in-hospital mortality. At present, due to epidemiological changes, antimicrobial prophylaxis can avoid few cases of IE. Prevention of nosocomial bacteremia, an early diagnosis of IE, prompt identification of IE patients at a higher risk of mortality, and a multidisciplinary approach of this disease could be valid strategies in order to improve the outcome of these patients.

Genetic basis for in vivo daptomycin resistance in enterococci

BACKGROUND

Daptomycin is a lipopeptide with bactericidal activity that acts on the cell membrane of enterococci and is often used off-label to treat patients infected with vancomycin-resistant enterococci. However, the emergence of resistance to daptomycin during therapy threatens its usefulness.

METHODS

We performed whole-genome sequencing and characterization of the cell envelope of a clinical pair of vancomycin-resistant Enterococcus faecalis isolates from the blood of a patient with fatal bacteremia; one isolate (S613) was from blood drawn before treatment and the other isolate (R712) was from blood drawn after treatment with daptomycin. The minimal inhibitory concentrations (MICs) of these two isolates were 1 and 12 μg per milliliter, respectively. Gene replacements were made to exchange the alleles found in isolate S613 with those in isolate R712.

RESULTS

Isolate R712 had in-frame deletions in three genes. Two genes encoded putative enzymes involved in phospholipid metabolism, GdpD (which denotes glycerophosphoryl diester phosphodiesterase) and Cls (which denotes cardiolipin synthetase), and one gene encoded a putative membrane protein, LiaF (which denotes lipid II cycle-interfering antibiotics protein but whose exact function is not known). LiaF is predicted to be a member of a three-component regulatory system (LiaFSR) involved in the stress-sensing response of the cell envelope to antibiotics. Replacement of the liaF allele of isolate S613 with the liaF allele from isolate R712 quadrupled the MIC of daptomycin, whereas replacement of the gdpD allele had no effect on MIC. Replacement of both the liaF and gdpD alleles of isolate S613 with the liaF and gdpD alleles of isolate R712 raised the daptomycin MIC for isolate S613 to 12 μg per milliliter. As compared with isolate S613, isolate R712--the daptomycin-resistant isolate--had changes in the structure of the cell envelope and alterations in membrane permeability and membrane potential.

CONCLUSIONS

Mutations in genes encoding LiaF and a GdpD-family protein were necessary and sufficient for the development of resistance to daptomycin during the treatment of vancomycin-resistant enterococci. (Funded by the National Institute of Allergy and Infectious Diseases and the National Institutes of Health.).