Efficacy of linezolid in treatment of experimental endocarditis caused by methicillin-resistant Staphylococcus aureus

Clinical characteristics and analysis of prognostic factors in methicillin-resistant Staphylococcus aureus endocarditis: A retrospective multicenter study in Japan

BACKGROUND

Infective endocarditis (IE) caused by MRSA (methicillin-resistant Staphylococcus aureus) is associated with a high mortality rate. This study aimed to elucidate the characteristics of patients with MRSA-IE in Japan and identify the factors associated with prognosis.

METHODS

This retrospective study included patients with a confirmed diagnosis of IE caused by MRSA, between January 2015 and April 2019.

RESULTS

A total of 65 patients from 19 centers were included, with a mean age of 67 years and 26 % were female. Fifty percent of the patients with IE were had nosocomial infections and 25 % had prosthetic valve involvement. The most common comorbidities were hemodialysis (20 %) and diabetes (20 %). Congestive heart failure was present in 86 % of patients (NYHA class I, II: 48 %; III, IV: 38 %). The 30-day and in-hospital mortality rates were 29 % and 46 %, respectively. Multi-organ failure was the primary cause of death, accounting for 43 % of all causes of death. Prognostic factors for in-hospital mortality were age, disseminated intravascular coagulation, daptomycin and/or linezolid as initial antibiotic therapy, and surgery. Surgical treatment was associated with a lower mortality rate (odds ratio [OR], 0.026; 95 % confidence interval [CI], 0.002-0.382; p = 0.008 for 30-day mortality and OR, 0.130; 95 % CI; 0.029-0.584; p = 0.008 for in-hospital mortality).

CONCLUSION

Mortality due to MRSA-IE remains high. Surgical treatment is a significant prognostic predictor of MRSA-IE.

Successful Treatment of Infective Endocarditis With Oral Antibiotics: A Case Report

Infective endocarditis (IE) is a serious and potentially life-threatening infection of the heart valves. It is commonly treated with prolonged courses of intravenous antibiotics, and in some cases, surgical intervention may also be necessary. While the use of oral antibiotics in the treatment of IE is generally limited, there are select cases where they may be considered as an alternative treatment option. Here, we report a case of staphylococcal right-sided IE successfully treated with oral antibiotics (linezolid and rifampicin). Our case highlights the potential for oral antibiotics to be used as step-down therapy for select patients with IE.

A systematic review of optimal pharmacokinetic/pharmacodynamic parameters for beta-lactam therapy in infective endocarditis

BACKGROUND

Beta-lactam antibiotics are the mainstay of therapy for most bacterial causes of infective endocarditis (IE). Traditionally considered as agents with a broad therapeutic index, there is increasing recognition that standard doses may be subtherapeutic or toxic in critically ill patients. Optimizing therapy for efficacy requires a defined pharmacokinetic (PK)/pharmacodynamic (PD) target associated with clinical and microbiological cure.

OBJECTIVES

To elucidate the factors that influence beta-lactam PK and PD variability in IE and to examine optimal PK/PD target parameters for therapy.

METHODS

The review was conducted in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Clinical and laboratory in vivo animal or human studies examining PK and/or PD of beta-lactam antibiotics in IE were eligible. Ovid MEDLINE, Embase and Cochrane Central Registry were searched using defined terms. The Office of Health Assessment and Translation (OHAT) tool was used for assessing risk of bias.

RESULTS

From 2677 abstracts, 62 articles were selected for review and synthesis, comprising: 45 animal studies investigating the broad categories of beta-lactam diffusion into vegetations, PK/PD determinants of outcome, mode of antibiotic delivery and synergistic impact of agents; and 17 human studies totalling 347 participants. Findings supported the importance of time-dependent killing for beta-lactams but heterogeneous data limited the determination of an optimal PK/PD target for IE treatment.

CONCLUSION

Beta-lactam PK and PD in endocarditis are variable and specific to the particular antibiotic-organism combination. Time-dependent killing is important, consistent with non-endocarditis studies, but there is little agreement on optimal drug exposure. Clinical studies examining PK/PD targets in endocarditis are required to further inform drug selection and dosing.

Efficacy of linezolid on Treponema pallidum, the syphilis agent: A preclinical study

BACKGROUND

Penicillin G, the current standard treatment for syphilis, has important drawbacks, but virtually no preclinical or clinical studies have been performed to identify viable alternatives. We tested, both in vitro and in vivo, three marketed antibiotics with adequate pharmacological properties to treat syphilis.

METHODS

We used an in vitro culturing system of T. pallidum to perform drug susceptibility testing and applied quantitative PCR targeting the tp0574 gene to measure bacterial growth. To confirm in vivo efficacy, fifteen rabbits were infected intradermally with T. pallidum at eight sites each and randomly allocated to an experimental treatment (linezolid, moxifloxacin, clofazimine) or a control arm (benzathine penicillin G [BPG], untreated). The primary outcome was treatment efficacy defined as the time to lesion healing measured from the date of treatment start. Secondary outcomes were absence of treponemes or treponemal mRNA in injection sites, absence of seroconversion, and cerebrospinal fluid (CSF) abnormalities and negative rabbit infectivity tests (RIT).

FINDINGS

Linezolid showed in vitro bactericidal activity at concentrations of 0.5 µg/mL or higher. When administered orally to experimentally infected rabbits, it induced healing of early lesions at a time similar to BPG (hazard ratio 3.84; 95% CI 2.05-7.17; p < 0.0001 compared to untreated controls). In linezolid-treated animals, dark-field microscopy and qPCR assessment showed no presence of treponemes after day 3 post-treatment start, serologic test did not convert to positive, CSF had no abnormalities, and RIT was negative. Moxifloxacin and clofazimine failed to inhibit bacterial growth in vitro and could not cure the infection in the rabbit model.

INTERPRETATION

Linezolid, a low-cost oxazolidinone, has in vitro and in vivo activity against T. pallidum, with efficacy similar to BPG in treating treponemal lesions in the animal model. Our findings warrant further research to assess the efficacy of linezolid as an alternative to penicillin G to treat syphilis in human clinical trials.

FUNDING

European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant agreement No. 850450).

Synthesis Approaches to (-)-Cytoxazone, a Novel Cytokine Modulator, and Related Structures

(−)-Cytoxazone, originally isolated from cultures of a Streptomyces species has an oxazolidin-2-one 4,5-disubstituted ring. It is known that this natural product presents a cytokine modulator effect through the signaling pathway of Th2 cells (type 2 cytokines), which are involved in the process of growth and differentiation of cells. From this, the interest in the development of research aimed at the total synthesis of this molecule and its analogs has remained high, which can be confirmed by the large number of publications on the topic, more than 30 to date. This review focuses on the various creative methods for the synthesis of (−)-cytoxazone and its congeners. The assessment of the preparation of this oxazolidinone and related structures serves as a treatise on the efforts made in the synthesis of this important class of compound from its first total synthesis in 1999.

Fosfomycin plus β-Lactams as Synergistic Bactericidal Combinations for Experimental Endocarditis Due to Methicillin-Resistant and Glycopeptide-Intermediate Staphylococcus aureus

The urgent need of effective therapies for methicillin-resistant Staphylococcus aureus (MRSA) infective endocarditis (IE) is a cause of concern. We aimed to ascertain the in vitro and in vivo activity of the older antibiotic fosfomycin combined with different beta-lactams against MRSA and glycopeptide-intermediate-resistant S. aureus (GISA) strains. Time-kill tests with 10 isolates showed that fosfomycin plus imipenem (FOF+IPM) was the most active evaluated combination. In an aortic valve IE model with two strains (MRSA-277H and GISA-ATCC 700788), the following intravenous regimens were compared: fosfomycin (2 g every 8 h [q8h]) plus imipenem (1 g q6h) or ceftriaxone (2 g q12h) (FOF+CRO) and vancomycin at a standard dose (VAN-SD) (1 g q12h) and a high dose (VAN-HD) (1 g q6h). Whereas a significant reduction of MRSA-227H load in the vegetations (veg) was observed with FOF+IPM compared with VAN-SD (0 [interquartile range [IQR], 0 to 1] versus 2 [IQR, 0 to 5.1] log CFU/g veg; P = 0.01), no statistical differences were found with VAN-HD. In addition, FOF+IPM sterilized more vegetations than VAN-SD (11/15 [73%] versus 5/16 [31%]; P = 0.02). The GISA-ATCC 700788 load in the vegetations was significantly lower after FOF+IPM or FOF+CRO treatment than with VAN-SD (2 [IQR, 0 to 2] and 0 [IQR, 0 to 2] versus 6.5 [IQR, 2 to 6.9] log CFU/g veg; P < 0.01). The number of sterilized vegetations after treatment with FOF+CRO was higher than after treatment with VAN-SD or VAN-HD (8/15 [53%] versus 4/20 [20%] or 4/20 [20%]; P = 0.03). To assess the effect of FOF+IPM on penicillin binding protein (PBP) synthesis, molecular studies were performed, with results showing that FOF+IPM treatment significantly decreased PBP1, PBP2 (but not PBP2a), and PBP3 synthesis. These results allow clinicians to consider the use of FOF+IPM or FOF+CRO to treat MRSA or GISA IE.

Comparative efficacies of tedizolid phosphate, vancomycin, and daptomycin in a rabbit model of methicillin-resistant Staphylococcus aureus endocarditis

Tedizolid, the active component of the prodrug tedizolid phosphate, is a novel oxazolidinone that is approximately 4 times more active by weight than linezolid against Staphylococcus aureus in vitro. The in vivo efficacy of tedizolid phosphate (15 mg/kg body weight intravenous [i.v.] twice a day [b.i.d.]) was compared to those of vancomycin (30 mg/kg i.v. b.i.d.) and daptomycin (18 mg/kg i.v. once a day [q.d.]) in a rabbit model of aortic valve endocarditis (AVE) caused by methicillin-resistant S. aureus strain COL (infection inoculum of 10(7) CFU). Median vegetation titers of daptomycin-treated rabbits were significantly lower than those of rabbits treated with tedizolid phosphate (15 mg/kg b.i.d.) (P = 0.016), whereas titers for vancomycin-treated compared to tedizolid-treated rabbits were not different (P = 0.984). The numbers of organisms in spleen and kidney tissues were similar for all treatment groups. A dose-ranging experiment was performed with tedizolid phosphate (2, 4, and 8 mg/kg b.i.d.) compared to vancomycin (30 mg/kg b.i.d.), using a higher infecting inoculum (10(8) CFU) to determine the lowest efficacious dose of tedizolid phosphate. Tedizolid phosphate (2 mg/kg) (equivalent to 60% of the area under the concentration-time curve from 0 to 24 h (AUC0-24) for the human 200-mg dose approved by the U.S. Food and Drug Administration) was not efficacious. Tedizolid phosphate at 4 mg/kg (equivalent to 75% of the AUC0-24 for the human 400-mg dose) and 8 mg/kg produced lower vegetation titers than the control, but neither was as efficacious as vancomycin.

Dose optimisation of antibiotics in children: application of pharmacokinetics/pharmacodynamics in paediatrics

The judicious use of antibiotics to combat infections in children relies upon appropriate selection of an agent, dose and duration to maximise efficacy and to minimise toxicity. Critical to dose optimisation is an understanding of the pharmacokinetics and pharmacodynamics of available drugs. Optimal dosing strategies may take advantage of pharmacokinetic/pharmacodynamic (PK/PD) principles so that antibiotic dosing can be individualised to assure effective bacterial killing in patients who have altered pharmacokinetics or who have infections with less susceptible or resistant organisms. This review will outline the fundamentals of antimicrobial pharmacokinetics/pharmacodynamics through discussion of antibacterial agents most often used in children. We aim to highlight the importance of dose optimisation in paediatrics and describe non-conventional dosing strategies that can take advantage of PK/PD principles at the bedside.

Native valve right sided infective endocarditis

Right-sided infective endocarditis (RSIE) accounts for 5-10% of all cases of infective endocarditis (IE), and is predominantly encountered in the injecting drug user (IDU) population, where HIV and HCV coinfections often coexist. Staphylococcus aureus is the most common pathogen. The pathogenesis of RSIE is still not well understood. RSIE usually presents as a persistent fever with respiratory symptoms whilst signs of systemic embolisation as seen in left-sided IE are notably absent. The prompt diagnosis of RSIE thus requires a high index of suspicion. Transthoracic echocardiography (TTE) can detect the majority of RSIE, whilst transoesophageal echocardiography (TOE) can increase sensitivity. Virulence of the causative organism and vegetation size are the major determinants of prognosis. Most cases of RSIE resolve with appropriate antibiotic administration.

Clinical update on linezolid in the treatment of Gram-positive bacterial infections

Gram-positive pathogens are a significant cause of morbidity and mortality in both community and health care settings. Glycopeptides have traditionally been the antibiotics of choice for multiresistant Gram-positive pathogens but there are problems with their use, including the emergence of glycopeptide-resistant strains, tissue penetration, and achieving and monitoring adequate serum levels. Newer antibiotics such as linezolid, a synthetic oxazolidinone, are available for the treatment of resistant Gram-positive bacteria. Linezolid is active against a wide range of Gram-positive bacteria and has been generally available for the treatment of Gram-positive infections since 2000. There are potential problems with linezolid use, including its bacteriostatic action and the relatively high incidence of reported adverse effects, particularly with long-term use. Long-term use may also be complicated by the development of resistance. However, linezolid has been shown to be clinically useful in the treatment of several serious infections where traditionally bacteriocidal agents have been required and many of its adverse effects are reversible on cessation. It has also been shown to be a cost-effective treatment option in several studies, with its high oral bioavailability allowing an early change from intravenous to oral formulations with consequent earlier patient discharge and lower inpatient costs.

Treatment strategies for infective endocarditis

IMPORTANCE OF THE FIELD

Despite significant advances in medical, surgical, and critical care interventions, infective endocarditis (IE) remains a disease associated with considerable morbidity and mortality. Estimates from the American Heart Association place the incidence of IE in the US at 10,000 - 15,000 new cases each year. This may be due to the changing epidemiology of IE, including increasing antimicrobial resistance, increasing heart surgeries, prosthetic valve implantation, and widespread use of intravenous drugs. Furthermore, a new form of the disease, healthcare-associated IE, which is associated with new therapeutic modalities such as intravenous catheters, hyperalimentation lines, pacemakers, and dialysis shunts, has emerged.

AREAS COVERED IN THIS REVIEW

We present the latest therapeutic and preventive strategies for IE caused by a variety of bacterial and fungal pathogens. The general methods employed included an extensive literature search, confined to the last 10 years, using key words such as 'infective endocarditis', 'culture-negative endocarditis', 'treatment guidelines for IE', and 'prophylaxis for IE'.

WHAT THE READER WILL GAIN

Comprehensive information regarding the changing epidemiology of IE is provided. The latest guidelines with respect to therapy and prophylaxis of IE are reviewed.

TAKE HOME MESSAGE

Successful management of IE depends on maintaining a high index of suspicion for the disease and, when IE is diagnosed, close cooperation of medical and surgical disciplines is required. Further research is needed to better understand and provide optimal therapy for complex situations such as multidrug-resistant and polymicrobial IE.

Ceftobiprole is superior to vancomycin, daptomycin, and linezolid for treatment of experimental endocarditis in rabbits caused by methicillin-resistant Staphylococcus aureus

Beta lactam agents are the most active drugs for the treatment of streptococci and methicillin-susceptible Staphylococcus aureus endocarditis. However, methicillin-resistant S. aureus (MRSA) is resistant to all beta lactam agents licensed to date, and alternative treatments are limited. Ceftobiprole is a novel broad-spectrum cephalosporin that binds with high affinity to PBP 2a, the penicillin binding protein that mediates the methicillin resistance of staphylococci and is active against MRSA. Ceftobiprole was compared to vancomycin, daptomycin, and linezolid in a rabbit model of MRSA aortic valve endocarditis caused by the homogeneously methicillin-resistant laboratory strain COL. Residual organisms in vegetations were significantly fewer in ceftobiprole-treated rabbits than in any other treatment group (P<0.05 for each comparison). In addition, the numbers of organisms in spleens and in kidneys were significantly lower in ceftobiprole-treated rabbits than in linezolid- and vancomycin-treated animals (P<0.05 for each comparison). Anti-MRSA beta lactam agents such as ceftobiprole may represent a significant therapeutic advance over currently available agents for the treatment of MRSA endocarditis.

Treatment failure of methicillin-resistant Staphylococcus aureus endocarditis with linezolid

We report a case of methicillin-resistant Staphylococcus aureus endocarditis treated by vancomycin and cotrimoxazole switched to oral linezolid alone with a complete resolution of the vegetation. Two months after discontinuation of treatment, the patient presented a relapse confirmed by pulsed-field gel electrophoresis involving the same linezolid-susceptible strain and rapidly died.

Treatment strategies for methicillin-resistant Staphylococcus aureus infections in pediatrics

Staphylococcus aureus is an important pathogen that frequently causes clinical disease in children. A wide array of illnesses can be caused by this common pathogen ranging from non-invasive skin infections to severe, life-threatening sepsis. Additionally, as antibacterials have been used to eradicate S. aureus, it has developed resistance to these important therapeutic agents. Methicillin-resistant S. aureus (MRSA) has become an increasing problem in pediatric patients over the past decade. In this review, we discuss the epidemiology, pathogenesis, and treatment options available in treating MRSA infections in children. Specifically, we address the importance of abscess drainage in the treatment of skin and soft tissue infections, the most common clinical manifestation of MRSA infections, and highlight the various agents that are available for treating this common infection. In severe, life-threatening invasive MRSA infections the primary therapeutic option is vancomycin. In cases of MRSA toxic shock syndrome the addition of clindamycin is necessary. In other invasive MRSA infections, such as pneumonia and musculoskeletal infections, the empiric treatment of choice is clindamycin. Finally, newer agents and additional treatment options are discussed.

Pharmacokinetics/pharmacodynamics of antibacterials in the Intensive Care Unit: setting appropriate dosing regimens

Patients admitted to Intensive Care Units (ICUs) are at very high risk of developing severe nosocomial infections. Consequently, antimicrobials are among the most important and commonly prescribed drugs in the management of these patients. Critically ill patients in ICUs include representatives of all age groups with a range of organ dysfunction related to severe acute illness that may complicate long-term illness. The range of organ dysfunction, together with drug interactions and other therapeutic interventions (e.g. haemodynamically active drugs and continuous renal replacement therapies), may strongly impact on antimicrobial pharmacokinetics in critically ill patients. In the last decade, it has become apparent that the intrinsic pharmacokinetic (PK) and pharmacodynamic (PD) properties are the major determinants of in vivo efficacy of antimicrobial agents. PK/PD parameters are essential in facilitating the translation of microbiological activity into clinical situations, ensuring a successful outcome. In this review, we analyse the typical patterns of antimicrobial activity and the corresponding PK/PD parameters, with a special focus on a PK/PD dosing approach of the antimicrobial agent classes commonly utilised in the ICU setting.

Pharmacodynamics of antibiotics to treat multidrug-resistant Gram-positive hospital infections

Hospital infection due to multidrug-resistant Gram-positive bacteria may often represent a life-threatening challenge; thus, to appropriately combat them, clinicians should be confident and systematically apply several important pharmacodynamic concepts. The concept of 'correct antibiotic treatment' should include, in addition to an appropriate choice in terms of antimicrobial susceptibility, an appropriate dosage and administration schedule consistent with the pharmacodynamic principles. In the next few years, it is expected that some of the anti-Gram-positive antimicrobial agents that are currently under investigation will be added to the therapeutic armamentarium. However, optimization of the usage of old drugs still remains a clinical priority and a scientific challenge, whose dignity may be considered of similar importance to that of the assessment of the possible therapeutic role of the new compounds.

New agents for Staphylococcus aureus endocarditis

PURPOSE OF REVIEW

The increasing prevalence of methicillin-resistant Staphylococcus aureus (MRSA) as well as newly discovered S. aureus strains with reduced susceptibility to vancomycin mandates development of new antistaphylococcal agents. This review summarizes currently available and forthcoming antimicrobials for treatment of S. aureus endocarditis.

RECENT FINDINGS

No new antimicrobial has been proven superior to antistaphylococcal penicillins for treatment of methicillin-sensitive S. aureus (MSSA) endocarditis. Vancomycin has become standard treatment for MRSA but poor outcomes have been reported, both with susceptible and intermediately resistant S. aureus strains (VISA). Linezolid has successfully treated individual cases of MRSA endocarditis, but limitations include long-term safety. Daptomycin has recently been proven effective and well tolerated for MSSA and MRSA bacteremia, including right-sided endocarditis. New glycopeptides, including dalbavancin and telavancin, as well as the new cephalosporin ceftobiprole, have not yet been studied for treatment of endocarditis but appear active against MRSA and potentially VISA.

SUMMARY

Antistaphylococcal penicillins remain the treatment of choice for MSSA. Of the currently available newer agents, daptomycin appears to have the most rapid bactericidal activity and provides a much-needed alternative to vancomycin for treatment of MRSA or MSSA bacteremia and right-sided endocarditis.

Pharmacokinetic/pharmacodynamic factors influencing emergence of resistance to linezolid in an in vitro model

Emerging resistance threatens the usefulness of linezolid for the treatment of severe infections caused by multidrug-resistant gram-positive bacteria. Optimal pharmacokinetic (PK)/pharmacodynamic (PD) indices have been described for the antimicrobial efficacy of linezolid (area under the concentration-time curve over 24 h at steady state divided by the MIC, >100; the cumulative percentage of a 24-h period that the drug concentration exceeds the MIC under steady-state PK conditions, >85). The aim of this study was to investigate the influence of these PK/PD indices on the development of resistance to linezolid by using an in vitro PK/PD model. Four dosage regimens were simulated over 72 h (two intermittent bolus regimens of 600 mg every 12 h [q12h] and 120 mg q12h and two continuous-infusion regimens of 120 mg/24 h and 30 mg/24 h) against four reference strains: methicillin-resistant Staphylococcus aureus (MRSA), heteroresistant vancomycin-intermediate S. aureus (hVISA), vancomycin-intermediate S. aureus (VISA), and vancomycin-resistant Enterococcus faecium (VRE). Linezolid concentrations were measured by high-performance liquid chromatography. Changes in susceptibility were characterized by pre- and posttreatment MIC measurements and population analysis profiles (PAPs). The linezolid concentrations that were achieved closely matched those that were targeted. The simulation with 600 mg q12h provided a >3-log10 reduction in the number of CFU/ml for all four strains, as did the 120-mg-q12h regimen for hVISA and VISA and the 30-mg/24-h continuous infusion for VRE and VISA. After 72 h of exposure to the 120-mg/24-h continuous-infusion simulation, the area under the PAP curve for all strains increased substantially (40 to 178%); increases in the MICs for the MRSA and hVISA strains were observed. The results demonstrate that PK/PD considerations are important in optimizing both antibacterial activity and the development of resistance to linezolid. The potential for resistance development appears to be higher when a constant concentration is maintained in the vicinity of the MIC of the bacteria.

Successful treatment of MRSA native valve endocarditis with oral linezolid therapy: a case report

Staphylococcal endocarditis is potentially fatal and is now the most common cause of infective endocarditis with a mortality rate of 25-47% [Hecht SR, Berger M. Right-sided endocarditis in intravenous drug users: prognostic features in 102 episodes. Ann Intern Med 1992;117:560-6]. Its treatment requires maintenance of bactericidal level of antibiotics for prolonged periods to attain a culture-negative state. Although intravenous vancomycin is currently the drug of choice for methicillin resistant Staphylococcus aureus (MRSA) endocarditis, we present a case treated successfully with oral linezolid for 4 weeks due to a lack of venous access.

Treatment of prosthetic valve infective endocarditis due to multi-resistant Gram-positive bacteria with linezolid

OBJECTIVES

Clinical experience with linezolid in the treatment of infective endocarditis either alone or in combination with other agents is limited. We describe our experience in the treatment of two patients with IE due to multi-resistant Gram-positive bacteria.

METHODS

One patient with MRSE and one with VRE endocarditis were treated with regimens containing linezolid. The killing kinetics of linezolid in combination with gentamicin or vancomycin against isolates of Staphylococcus epidermidis and Enterococcus faecalis were analysed in vitro.

RESULTS

Clinical response and eradication of bacteraemia was achieved with linezolid therapy in both patients. Time-kill curve studies showed that linezolid was bacteriostatic against the MRSE and VRE isolates used. Combination with gentamicin or vancomycin did not produce synergy or antagonism but at best only marginal additive effect.

CONCLUSIONS

Although bacteriostatic, linezolid provides an important therapeutic option in IE due to multi-resistant Gram-positive pathogens. It challenges the conventional wisdom that bactericidal synergy is required for the effective treatment of most cases of IE due to Gram-positive organisms.

Linezolid in prophylaxis against experimental aortic valve endocarditis due to Streptococcus oralis or Enterococcus faecalis

There are no experimental studies regarding the prophylactic efficacy of linezolid against infective endocarditis. Nonbacterial thrombotic endocarditis of the aortic valve was induced in rabbits by the insertion of a polyethylene catheter. Twenty-four hours later, animals were randomly assigned to a control group, and groups receiving either ampicillin (two doses of 40 mg/kg of body weight each, given intravenously, 2 h apart) or linezolid (a single per os dose of 75 mg/kg). The first dose of ampicillin and the single dose of linezolid were administered 0.5 and 1 h, respectively, prior to the intravenous inoculation of approximately 10(7) CFU of Streptococcus oralis or Enterococcus faecalis. Linezolid peak levels in rabbit serum were similar to the peak serum levels in humans following a 600-mg oral dose of linezolid. Linezolid prevented endocarditis in 87% of S. oralis-challenged rabbits (P < 0.001 versus controls; P = 0.026 versus ampicillin). In rabbits challenged with E. faecalis, linezolid prevented endocarditis in 73% (P = 0.003 versus controls; P = 0.049 versus ampicillin). Ampicillin prevented endocarditis due to S. oralis or due to E. faecalis in 47% (P = 0.005 versus controls) and in 30% (P = not significant versus controls) of the challenged animals, respectively. In conclusion, linezolid was effective as prophylaxis against endocarditis caused by a strain of S. oralis and to a lesser degree against that caused by a strain of E. faecalis. Its prophylactic efficacy was superior to that of ampicillin.

[Linezolid, the first oxazolidinone antibiotic]

The spread of multiresistant Staphylococcus and Enterococcus strains required the development of new drugs. Linezolid is the first molecule of a new antibiotic family, oxazolidinones, with an original mechanism of action. In this general review, the authors first present its antibacterial activity, its pharmacokinetic properties, its therapeutic uses in serious Gram-positive infections, pneumonia, skin and soft tissue infections, and also in other indications. They then explain the rules for administration and tolerability.

Agents for the Treatment of Multidrug-resistant Gram-positive Endocarditis

Several newer agents with activity against multidrug- resistant gram-positive pathogens are available. These agents have in vitro and clinical data supporting their utility in the treatment of infections caused by pathogens such as methicillin-resistant staphylococci and vancomycin-resistant enterococci. Daptomycin appears to be rapidly bactericidal, and linezolid and quinupristin/dalfopristin also are cidal against staphylococci. Although the agents have several properties that are attractive for use in endocarditis, clinical data are limited. Further investigation with each agent and combination therapy are warranted before definitive recommendations can be made.

In vitro Activity of Linezolid against Staphylococcus aureus: A Population Study

In our study, we aim to determine the existence of microorganisms that are heteroresistant to linezolid among Staphylococcus aureus clinical isolates in our setting between 1996 and 2002; during this period, linezolid was not used in clinical practice. There was no resistant subpopulation to 4 mg/l of linezolid in 99.4% of the strains. On the other hand, 16.46% of the strains exhibited resistant subpopulations to 4 microg/ml of vancomycin. However, the emergence of strains resistant to this drug has been described and the emergence of resistant strains should be monitored.

Trimethoprim-sulfamethoxazole as a viable treatment option for infections caused by methicillin-resistant Staphylococcus aureus

OBJECTIVE

To review available data regarding the efficacy of trimethoprim-sulfamethoxazole (TMP-SMX) for the treatment of infections caused by methicillin-resistant Staphylococcus aureus (MRSA).

DATA SOURCES

A MEDLINE search was performed (January 1966-December 2003) using the search terms Staphylococcus aureus , sulfamethoxazole, trimethoprim, co-trimoxazole, and methicillin resistance. Abstracts from infectious diseases meetings also were reviewed.

DATA SYNTHESIS

The reported rate of TMP-SMX resistance in S. aureus is highly variable. From a mechanistic standpoint, TMP-SMX resistance among MRSA appears to be distinct from multidrug resistance, although some anecdotal reports suggest otherwise. Clonal outbreaks of MRSA resistant to TMP-SMX have been described; of these, the Brazilian clone has more often been resistant to TMP-SMX than the Iberian clone. Rates of TMP-SMX resistance are particularly high in institutions serving large numbers of patients infected by the human immunodeficiency virus, due to increased exposure for Pneumocystis prophylaxis. Limited studies and case reports have found TMP-SMX useful against infections caused by MRSA.

CONCLUSIONS

A large body of anecdotal data, but only one randomized clinical trial, indicates the effectiveness of TMP-SMX as a treatment for MRSA infections. Double-blind, randomized controlled trials are needed to compare the two available oral agents-TMP-SMX and linezolid-against MRSA.

Infective endocarditis: a review of the best treatment options

Despite significant advances in antimicrobial therapy and an enhanced ability to diagnose and treat complications, infective endocarditis is still associated with substantial morbidity and mortality today, and its incidence has not decreased over the past decades. This apparent paradox may be explained by a progressive change in risk factors, leading to an evolution in its epidemiological and clinical features. In fact, new risk factors for endocarditis have emerged, such as intravenous drug abuse, diffusion of heart surgery procedures and prosthetic valve implantation, atherosclerotic valve disease in elderly patients, and nosocomial disease. Recently identified microorganisms (including Bartonella spp., Abiotrophia defectiva, and the HACEK group of bacteria [including Haemophilus spp., Actinobacillus spp., Cardiobacterium hominis, Eikenella corrodens and Kingella kingae]) are sometimes the cause of culture-negative endocarditis, and emerging resistant bacteria (such as methicillin- or vancomycin-resistant Staphylococci and vancomycin-resistant Enterococci) are becoming a new challenge for conventional antibiotic therapy. New therapeutic approaches need to be developed for the treatment of infective endocarditis caused by drug-resistant Gram-positive cocci, and some antimicrobial compounds recently introduced in clinical practice (such as streptogramins and oxazolidinones) may be an effective alternative, but further clinical studies are needed in order to confirm their effectiveness and safety. This review should help redefine the best therapeutic and preventive strategies against infective endocarditis.

Impact of high-inoculum Staphylococcus aureus on the activities of nafcillin, vancomycin, linezolid, and daptomycin, alone and in combination with gentamicin, in an in vitro pharmacodynamic model

We evaluated the impact of high (9.5 log10 CFU/g) and moderate (5.5 log10 CFU/g) inocula of methicillin-susceptible and -resistant Staphylococcus aureus (MSSA and MRSA, respectively) on the activities of nafcillin, linezolid, vancomycin, and daptomycin, alone and in combination with gentamicin in an in vitro pharmacodynamic model with simulated endocardial vegetations over 72 h. Human therapeutic dosing regimens for nafcillin, daptomycin, vancomycin, linezolid, and gentamicin were simulated. At a moderate inoculum, nafcillin (MSSA only), vancomycin, and daptomycin demonstrated equivalent and significant (P < 0.01) bactericidal (99.9% kill) activities (decreases of 3.34 +/- 1.1, 3.28 +/- 0.4, and 3.34 +/- 0.8 log10 CFU/g, respectively). Bactericidal activity was demonstrated at 4 h for nafcillin and daptomycin and at 32 h for vancomycin. Linezolid demonstrated bacteriostatic activity over the course of the study period. At a high inoculum, daptomycin exhibited bactericidal activity against both MSSA and MRSA by 24 h (decrease of 5.51 to 6.31 +/- 0.10 log10 CFU/g). Nafcillin (versus MSSA), vancomycin, and linezolid (MSSA and MRSA) did not achieve bactericidal activity throughout the 72-h experiment. The addition of gentamicin increased the rate of 99.9% kill to 8 h for daptomycin (P < 0.01) and 48 h for nafcillin (MSSA only) (P = 0.01). The addition of gentamicin did not improve the activity of vancomycin or linezolid for either isolate for the 72-h period. Overall, high-inoculum Staphylococcus aureus had a significant impact on the activities of nafcillin and vancomycin. In contrast, daptomycin was affected minimally and linezolid was not affected by inoculum.

Viridans streptococci in endocarditis and neutropenic sepsis: biofilm formation and effects of antibiotics

OBJECTIVES

Viridans group streptococci (VGS) are a frequent cause of bacterial endocarditis or sepsis in patients with neutropenia. Endocarditis in particular, is associated with plaque formation on the endocardium and valve leaflets whereas VGS septicaemia in neutropenic patients is caused by the influx of oral flora bacteria through mucositic lesions. This study examined the in vitro potency for biofilm formation of clinical VGS bloodstream isolates, and the effects of antibiotics on these biofilms.

METHODS

During the years 1998-2000, 40 VGS bloodstream isolates from 18 patients with endocarditis and 22 patients with severe sepsis and neutropenia were collected. The MICs of penicillin, teicoplanin and moxifloxacin were determined using the microdilution broth method according to NCCLS criteria. Biofilms were grown in microtitre plates, dyed with Crystal Violet, and the mean optical density (OD) was used for quantification. Biofilms were incubated with penicillin, teicoplanin and moxifloxacin at various concentrations starting with the MICs for the respective isolates tested.

RESULTS

Isolates from eight out of 18 patients with endocarditis and six out of 22 patients with neutropenia formed biofilms (not significant). For the 14 isolates, the MIC(90)s (range) of penicillin, teicoplanin and moxifloxacin were 0.5 mg/L (0.001-0.5), 0.125 mg/L (0.025-0.125) and 0.5 mg/L (0.05-0.5), respectively. Generally, biofilms persisted although incubated with the antibiotics up to concentrations of 128 x MIC. However, the ODs of biofilms after incubation with an antibiotic were significantly lower than the ODs of biofilms without antibiotic (P<0.05). A significant decrease in the biofilms with increasing antibiotic concentrations was observed for teicoplanin and moxifloxacin, but not for penicillin G.

CONCLUSIONS

VGS isolated from patients with endocarditis and patients with sepsis and neutropenia form biofilms. Biofilms persist even when exposed to antibiotics at concentrations up to 128 x MIC. Nevertheless, teicoplanin and moxifloxacin reduced the density of the biofilms at concentrations >/=16 x MIC. Thus, testing the effects of antibiotics on biofilms may supply useful information in addition to standard in vitro testing, particularly in diseases where biofilm formation is involved in the pathogenesis.

Temporin A soaking in combination with intraperitoneal linezolid prevents vascular graft infection in a subcutaneous rat pouch model of infection with Staphylococcus epidermidis with intermediate resistance to glycopeptides

The efficacy of linezolid and temporin A in the prevention of prosthetic graft infection due to methicillin-resistant Staphylococcus epidermidis with intermediate resistance to glycopeptides was investigated in a subcutaneous rat pouch model. Linezolid and temporin A, alone or combined, greatly reduced the bacterial numbers compared to the effect with control drugs.

Successful treatment of methicillin-resistant Staphylococcus aureus endocarditis with linezolid

We report the successful treatment with linezolid of a methicillin-resistant Staphylococcus aureus (MRSA) endocarditis in a patient with a severe allergic reaction to glycopeptides. Linezolid is a drug with well-recognised activity against S. aureus and proved to be efficacious even in the unusual site of heart valves. This drug could be a good therapeutic choice when glycopeptides treatment is not feasible.

The basis for resistance to beta-lactam antibiotics by penicillin-binding protein 2a of methicillin-resistant Staphylococcus aureus

Penicillin-binding protein 2a (PBP2a) of Staphylococcus aureus is refractory to inhibition by available beta-lactam antibiotics, resulting in resistance to these antibiotics. The strains of S. aureus that have acquired the mecA gene for PBP2a are designated as methicillin-resistant S. aureus (MRSA). The mecA gene was cloned and expressed in Escherichia coli, and PBP2a was purified to homogeneity. The kinetic parameters for interactions of several beta-lactam antibiotics (penicillins, cephalosporins, and a carbapenem) and PBP2a were evaluated. The enzyme manifests resistance to covalent modification by beta-lactam antibiotics at the active site serine residue in two ways. First, the microscopic rate constant for acylation (k2) is attenuated by 3 to 4 orders of magnitude over the corresponding determinations for penicillin-sensitive penicillin-binding proteins. Second, the enzyme shows elevated dissociation constants (Kd) for the non-covalent pre-acylation complexes with the antibiotics, the formation of which ultimately would lead to enzyme acylation. The two factors working in concert effectively prevent enzyme acylation by the antibiotics in vivo, giving rise to drug resistance. Given the opportunity to form the acyl enzyme species in in vitro experiments, circular dichroism measurements revealed that the enzyme undergoes substantial conformational changes in the course of the process that would lead to enzyme acylation. The observed conformational changes are likely to be a hallmark for how this enzyme carries out its catalytic function in cross-linking the bacterial cell wall.

Bactericidal activities of daptomycin, quinupristin-dalfopristin, and linezolid against vancomycin-resistant Staphylococcus aureus in an in vitro pharmacodynamic model with simulated endocardial vegetations

In search of treatment alternatives against vancomycin-resistant S. aureus (VRSA), an in vitro pharmacodynamic model with simulated endocardial vegetations incorporating protein and a high inoculum was used to simulate daptomycin, linezolid, quinupristin-dalfopristin, and vancomycin against the Michigan VRSA strain. Daptomycin and quinupristin-dalfopristin exhibited the greatest bacterial reductions, and all tested agents except vancomycin exhibited bactericidal activity against the VRSA.

Methicillin-resistant Staphylococcus aureus: impact on dermatology practice

Methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) emerged in the 1960s and is now commonly seen in hospitals, clinics and, since the mid-1990s, the community. Risk factors for the acquisition of MRSA include chronic dermatoses, underlying medical illnesses, attending healthcare facilities, use of prescription antibacterials, surgery, intravenous lines, hospitalization in an intensive care unit, and proximity to patients colonized with MRSA. Recent community-associated strains often occur in patients without these risk factors. Staphylococci are readily spread from person to person and readily contaminate the environment. Infection control measures thus involve identifying the infected patients, separating them from other non-infected patients, cleaning of the environment and, most important of all, scrupulous attention to hand hygiene. Alcoholic antiseptic hand rubs offer an alternative to antiseptic hand washes and increase compliance. Treatment of MRSA skin infections is challenging. Topical agents such as mupirocin or fusidic acid can be used, but the organisms often become resistant. Systemic therapy involves non-beta-lactams. Parenteral treatment is generally with glycopeptides such as vancomycin; oral therapy is more complex. Monotherapy with quinolones, rifampin (rifampicin), and fusidic acid often results in the development of resistance and so, if any of these agents are chosen it should be in combination. There are no data on combination therapy, although rifampin-containing combinations are often chosen. Fourth-generation quinolones and linezolid are expensive but promising alternatives.

Efficacy of linezolid alone or in combination with vancomycin for treatment of experimental endocarditis due to methicillin-resistant Staphylococcus aureus

The levels of effectiveness of linezolid, vancomycin, and the combination of linezolid and vancomycin were compared in the rabbit model of endocarditis caused by a clinical methicillin-resistant Staphylococcus aureus (MRSA) isolate. Vancomycin alone was more effective than either linezolid alone or the combination of linezolid and vancomycin for the treatment of endocarditis due to MRSA.

Efficacy of linezolid plus rifampin in an experimental model of methicillin-susceptible Staphylococcus aureus endocarditis

The efficacy of linezolid, alone or in combination with rifampin, against methicillin-susceptible Staphylococcus aureus in rabbits with experimental endocarditis was investigated. Linezolid (50 or 75 mg/kg of body weight), rifampin, and linezolid (25, 50, or 75 mg/kg) plus rifampin produced statistically significant reductions in bacterial counts compared with those in untreated controls. Plasma or valvular vegetation levels of linezolid in the groups treated with the linezolid-rifampin combination were similar to those in the respective linezolid-only treatment groups. At therapeutic levels of linezolid, rifampin resistance was not observed. The results from this experimental model of endocarditis suggest that while rifampin did not provide synergy to the linezolid dosing, it did not antagonize the efficacy of linezolid.