LEADER surveillance program results for 2006: an activity and spectrum analysis of linezolid using clinical isolates from the United States (50 medical centers)

Prevalence of linezolid resistance in Streptococcus pneumoniae isolates: a systematic review and meta-analysis

Aim: This study aimed to understand the current level of linezolid (LNZ) resistance in Streptococcus pneumoniae isolates reported over the past 10 years. Material & methods: An electronic search was conducted for the following keywords: ((Streptococcus pneumoniae [title/abstract]) OR (Pneumococcus [title/abstract]) OR (Pneumococci [title/abstract]) AND (linezolid [title/abstract]) OR (Zyvox [title/abstract])) OR (Zyvoxid [title/abstract])). Result: Out of all the studies, 80 had a cross-sectional design, while 11 followed a cohort approach. The prevalence of LNZ resistance among S. pneumoniae isolates ranged from 0% to 4.86%. Discussion: Urgent, high-powered, randomized, controlled trials with participants from endemic regions are needed to gain a comprehensive understanding of the impact on and significance of LNZ treatment to patients.

Novel fluorobenzothiazole as a dual inhibitor of gyrase B and topoisomerase IV against Gram-positive pathogens

Aim: The development of a novel inhibitor targeting gyrase B and topoisomerase IV offers an opportunity to combat multidrug resistance. Methods: We investigated the activity of RBx 10080758 against Gram-positive bacteria in vitro and in vivo. Results: RBx 10080758 showed a potent 50% inhibitory concentration of 0.13 μM and 0.25 μM against gyrase B and topoisomerase IV, respectively, and exhibited strong whole-cell in vitro activity with MIC ranges of 0.015-0.06 and 0.015-0.03 μg/ml against Staphylococcus aureus and Streptococcus pneumoniae, respectively. In a rat thigh infection model with methicillin-resistant S. aureus, RBx 10080758 at 45 mg/kg exhibited a >3 log10 CFU reduction in thigh muscles. Conclusion: RBx 10080758 displayed potent activity against multiple multidrug-resistant Gram-positive bacteria with a dual-targeting mechanism of action.

Linezolid-resistance Staphylococcus aureus – Prevalence, Emerging Resistance Mechanisms, Challenges and Perspectives

Staphylococcus aureus, an opportunistic pathogen, can root several infections viz skin and tissue infections, bacteraemia, food poisoning, pneumonia, and many other clinical conditions with some variations of virulence factors. In treatment of infections, caused by this Gram-positive pathogen, several antibiotics are being used importantly Methicillin and Vancomycin. This pathogen has high capability of antibiotic resistance development and had evolved new strains such as Methicillin-resistant Staphylococcus aureus (MRSA), and Vancomycin-resistant Staphylococcus aureus (VRSA). Meta-analysis in Ethiopia showed that pooled prevalence of MRSA in environment, food, animal, and human was 54%, 77%, 15%, and 38% respectively (2022). Risk of MRSA isolates from burn ICU was 55 % higher (2018). In Bangladesh, 37.1% isolates from frozen meat chicken (2021) were identified as MRSA. This problem is being dealt with a novel drug called Linezolid which has been proved effective against both MRSA and VRSA. Exacerbating the situation, this pathogen has shown resistance against this unprecedented drug by means of a number of drug resistance mechanisms. Its prevalence has been reporting since the adoption of the drug, but with a minute ratio at one time/place to the very high percentage at another time/place. This inconsistent prevalence must not be ignored, and its surveillance should be augmented as antibiotic treatment is critical for fighting against microbial infections. This review highlights the worldwide reports in which Staphylococcus aureus of either wildtype or Methicillin or Vancomycin resistance that have shown resistance to Linezolid drug for the past 2 decades. At the same time where incidences of Linezolid Resistant Staphylococcus aureus (LRSA) indications are reporting, there is a call for comprehensive strategies to overcome this challenge of antibiotic resistance.

Mechanisms of linezolid resistance in Staphylococcus capitis with the novel mutation C2128T in the 23S rRNA gene in China

PURPOSE

The objective of this study was to investigate the molecular characteristics and potential resistance mechanisms of linezolid-resistant (LZR) Staphylococcus capitis isolates from a tertiary hospital in China.

METHODS

S. capitis isolates were obtained from clinical patient specimens; three of the isolates came from blood cultures and one from the hydrothorax. The agar dilution and E-test methods were used to identify antibiotic resistance. The chloramphenicol-florfenicol resistance (cfr) gene carrier status of the strains was determined by PCR. Whole-genome sequencing (WGS) was used to identify point mutations and L3, L4, and L22 mutations and to study the genetic environment of the cfr gene and the relationships between strains.

RESULTS

The 4 isolates obtained in this study were all linezolid-resistant Staphylococcus strains. A similar of susceptibility profile pattern was observed in all four S. capitis strains, each of which exhibited a multidrug-resistant phenotype. A potentially novel mutation, C2128T, was identified, and the cfr genes of S. capitis strains were all positive. Additionally, the same mutations (C2128T and G2600T) were identified in all 23S rRNA sequences of the isolates, whereas mutations were lacking in the L3, L4, and L22 ribosomal proteins. The genetic environments surrounding cfr were identical in all four isolates. A schematic diagram of the phylogenetic tree showed that they were closely related to AYP1020, CR01, and TW2795, and a total of seven drug resistance genes were identified in these strains.

CONCLUSIONS

The study indicated that the resistance of the Staphylococcus capitis strains to linezolid was caused by multiple mechanisms, and a potential novel mutation, C2128T, that may have an impact on bacterial resistance was identified.

Metal nanoparticles assisted revival of Streptomycin against MDRS Staphylococcus aureus

The ability of microorganisms to generate resistance outcompetes with the generation of new and efficient antibiotics. Therefore, it is critically required to develop novel antibiotic agents and treatments to control bacterial infections. Green synthesized metallic and metal oxide nanoparticles are considered as the potential means to target bacteria as an alternative to antibiotics. Nanoconjugates have also attracted attention because of their increased biological activity as compared to free antibiotics. In the present investigation, silver nanoparticles (AgNPs), zinc oxide nanoparticles (ZnO NPs), copper oxide nanoparticles (CuO NPs), and iron oxide nanoparticles (FeO NPs) have been synthesized by using leaf extract of Ricinus communis. Characterization of nanoparticles was done by using UV-Vis Spectroscopy, Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy, Energy Dispersive X-Ray Analyzer, X-ray Diffraction Analysis, and Dynamic Light Scattering Particle Size Analyzer. Interestingly, Streptomycin when combined with AgNPs, ZnO NPs, CuO NPs, and FeO NPs showed enhanced antibacterial activity against clinical isolates of S. aureus which suggested synergism between the nanoparticles and antibiotics. The highest enhanced antibacterial potential of Streptomycin was observed in conjugation with ZnO NPs (11 ± 0.5 mm) against S. aureus. Minimum inhibitory concentration of conjugates of AgNPs, ZnO NPs, CuO NPs, and FeO NPs with streptomycin against S. aureus was found to be 3.12, 2.5,10, and 12.5 μg/mL respectively. The considerable point of the present investigation is that S. aureus, which was resistant to streptomycin becomes highly susceptible to the same antibiotic when combined with nanoparticles. This particular observation opens up windows to mitigate the current crisis due to antibiotic resistance to combat antimicrobial infections efficiently.

Mobile Oxazolidinone Resistance Genes in Gram-Positive and Gram-Negative Bacteria

Seven mobile oxazolidinone resistance genes, including cfr, cfr(B), cfr(C), cfr(D), cfr(E), optrA, and poxtA, have been identified to date. The cfr genes code for 23S rRNA methylases, which confer a multiresistance phenotype that includes resistance to phenicols, lincosamides, oxazolidinones, pleuromutilins, and streptogramin A compounds. The optrA and poxtA genes code for ABC-F proteins that protect the bacterial ribosomes from the inhibitory effects of oxazolidinones. The optrA gene confers resistance to oxazolidinones and phenicols, while the poxtA gene confers elevated MICs or resistance to oxazolidinones, phenicols, and tetracycline. These oxazolidinone resistance genes are most frequently found on plasmids, but they are also located on transposons, integrative and conjugative elements (ICEs), genomic islands, and prophages. In these mobile genetic elements (MGEs), insertion sequences (IS) most often flanked the cfr, optrA, and poxtA genes and were able to generate translocatable units (TUs) that comprise the oxazolidinone resistance genes and occasionally also other genes. MGEs and TUs play an important role in the dissemination of oxazolidinone resistance genes across strain, species, and genus boundaries. Most frequently, these MGEs also harbor genes that mediate resistance not only to antimicrobial agents of other classes, but also to metals and biocides. Direct selection pressure by the use of antimicrobial agents to which the oxazolidinone resistance genes confer resistance, but also indirect selection pressure by the use of antimicrobial agents, metals, or biocides (the respective resistance genes against which are colocated on cfr-, optrA-, or poxtA-carrying MGEs) may play a role in the coselection and persistence of oxazolidinone resistance genes.

The global prevalence of Daptomycin, Tigecycline, Quinupristin/Dalfopristin, and Linezolid-resistant Staphylococcus aureus and coagulase-negative staphylococci strains: a systematic review and meta-analysis

OBJECTIVE

Methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant coagulase-negative Staphylococcus (MRCoNS) are among the main causes of nosocomial infections, which have caused major problems in recent years due to continuously increasing spread of various antibiotic resistance features. Apparently, vancomycin is still an effective antibiotic for treatment of infections caused by these bacteria but in recent years, additional resistance phenotypes have led to the accelerated introduction of newer agents such as linezolid, tigecycline, daptomycin, and quinupristin/dalfopristin (Q/D). Due to limited data availability on the global rate of resistance to these antibiotics, in the present study, the resistance rates of S. aureus, Methicillin-resistant S. aureus (MRSA), and CoNS to these antibiotics were collected.

METHOD

Several databases including web of science, EMBASE, and Medline (via PubMed), were searched (September 2018) to identify those studies that address MRSA, and CONS resistance to linezolid, tigecycline, daptomycin, and Q/D around the world.

RESULT

Most studies that reported resistant staphylococci were from the United States, Canada, and the European continent, while African and Asian countries reported the least resistance to these antibiotics. Our results showed that linezolid had the best inhibitory effect on S. aureus. Although resistances to this antibiotic have been reported from different countries, however, due to the high volume of the samples and the low number of resistance, in terms of statistical analyzes, the resistance to this antibiotic is zero. Moreover, linezolid, daptomycin and tigecycline effectively (99.9%) inhibit MRSA. Studies have shown that CoNS with 0.3% show the lowest resistance to linezolid and daptomycin, while analyzes introduced tigecycline with 1.6% resistance as the least effective antibiotic for these bacteria. Finally, MRSA and CoNS had a greater resistance to Q/D with 0.7 and 0.6%, respectively and due to its significant side effects and drug-drug interactions; it appears that its use is subject to limitations.

CONCLUSION

The present study shows that resistance to new agents is low in staphylococci and these antibiotics can still be used for treatment of staphylococcal infections in the world.

In Vitro and In Vivo Activities of DS-2969b, a Novel GyrB Inhibitor, and Its Water-Soluble Prodrug, DS11960558, against Methicillin-Resistant Staphylococcus aureus

DS-2969b is a novel GyrB inhibitor under clinical development. In this study, the in vitro activity of DS-2969b and the in vivo activities of DS-2969b and its water-soluble prodrug, DS11960558, against methicillin-resistant Staphylococcus aureus (MRSA) were evaluated. DS-2969b inhibited the supercoiling activity of S. aureus DNA gyrase and the decatenation activity of its topoisomerase IV. DS-2969b showed antibacterial activity against Gram-positive aerobes but not against Gram-negative aerobes, except for Moraxella catarrhalis and Haemophilus influenzae DS-2969b was active against MRSA with an MIC₉₀ of 0.25 μg/ml, which was 8-fold lower than that of linezolid. The presence of a pulmonary surfactant did not affect the MIC of DS-2969b. DS-2969b showed time-dependent slow killing against MRSA. The frequency of spontaneous resistance development was less than 6.2 × 10^-10 in all four S. aureus isolates at 4× MIC of DS-2969b. In a neutropenic MRSA-induced murine muscle infection model, DS-2969b was more efficacious than linezolid by both the subcutaneous and oral routes. DS-2969b and DS11960558 showed efficacy in a neutropenic murine MRSA lung infection model. The pharmacokinetics and pharmacodynamics of DS-2969b and DS11960558 against MRSA were characterized in a neutropenic murine thigh infection model; the percentage of time during the dosing period in which the free drug concentration exceeded the MIC (fT_MIC) correlated best with in vivo efficacy, and the static percent fT_MIC was 43 to 49%. A sufficient fT_MIC was observed in a phase 1 multiple-ascending-dose study of DS-2969b given orally at 400 mg once a day. These results suggest that DS11960558 and DS-2969b have potential for use as intravenous-to-oral step-down therapy for treating MRSA infections with a higher efficacy than linezolid.

In Vitro and In Vivo Activities of a Bi-Aryl Oxazolidinone, RBx 11760, against Gram-Positive Bacteria

RBx 11760, a bi-aryl oxazolidinone, was investigated for antibacterial activity against Gram-positive bacteria. The MIC₉₀s of RBx 11760 and linezolid against Staphylococcus aureus were 2 and 4 mg/liter, against Staphylococcus epidermidis were 0.5 and 2 mg/liter, and against Enterococcus were 1 and 4 mg/liter, respectively. Similarly, against Streptococcus pneumoniae the MIC₉₀s of RBx 11760 and linezolid were 0.5 and 2 mg/liter, respectively. In time-kill studies, RBx 11760, tedizolid, and linezolid exhibited bacteriostatic effect against all tested strains except S. pneumoniae RBx 11760 showed 2-log₁₀ kill at 4× MIC while tedizolid and linezolid showed 2-log₁₀ and 1.4-log₁₀ kill at 16× MIC, respectively, against methicillin-resistant S. aureus (MRSA) H-29. Against S. pneumoniae 5051, RBx 11760 showed bactericidal activity, with 4.6-log₁₀ kill at 4× MIC compared to 2.42-log₁₀ and 1.95-log₁₀ kill for tedizolid and linezolid, respectively, at 16× MIC. RBx 11760 showed postantibiotic effects (PAE) at 3 h at 4 mg/liter against MRSA H-29, and linezolid showed the same effect at 16 mg/liter. RBx 11760 inhibited biofilm production against methicillin-resistant S. epidermidis (MRSE) ATCC 35984 in a concentration-dependent manner. In a foreign-body model, linezolid and rifampin resulted in no advantage over stasis, while the same dose of RBx 11760 demonstrated a significant killing compared to the initial control against S. aureus (P < 0.05) and MRSE (P < 0.01). The difference in killing was statistically significant for the lower dose of RBx 11760 (P < 0.05) versus the higher dose of linezolid (P > 0.05 [not significant]) in a groin abscess model. In neutropenic mouse thigh infection, RBx 11760 showed stasis at 20 mg/kg of body weight, whereas tedizolid showed the same effect at 40 mg/kg. These data support RBx 11760 as a promising investigational candidate.

Evaluation of Antimicrobial Resistance in Staphylococcus aureus Isolates by Years

Objective. Recently, community and hospital-acquired infections with Staphylococcus aureus have increased and raised antibiotic resistant isolates. In this study, we aimed to evaluate the antibiotic resistance profile of S. aureus isolates over several years in various clinical specimens from our hospital. Materials and Methods. S. aureus strains from 2009 to 2014 were isolated from various clinical samples at Yuzuncu Yil University, Dursun Odabas Medical Center, Microbiology Laboratory, and their antibiotic susceptibility test results were retrospectively investigated. The isolates were identified by conventional methods, and antibiotic susceptibility tests were performed by the Phoenix (Becton Dickinson, USA) automated system method according to Clinical and Laboratory Standards Institute (CLSI) standards. Results. A total of 1,116 S. aureus isolates were produced and methicillin-resistant S. aureus (MRSA) to 21% of all S. aureus isolates between 2009 and 2014. According to the results of susceptibility tests of all isolates of S. aureus, they have been identified as sensitive to vancomycin, daptomycin, linezolid, and levofloxacin. While the resistance rates to nitrofurantoin, quinupristin-dalfopristin, and trimethoprim-sulfamethoxazole were determined as 0.3%, 2.4%, and 6%, respectively, resistance rates to penicillin, erythromycin, rifampicin, gentamicin, and clindamycin were determined as 100%, 18%, 14%, 14%, and 11%, respectively. The highest percentage of methicillin resistance was determined as 30% in 2009, and the resistance was determined to have decreased in subsequent years (20%, 16%, 13%, 19%, and 21%) (p < 0.001). Conclusion. Currently, retrospective evaluations of causes of nosocomial infection should be done periodically. We think that any alteration of resistance over the years has to be identified, and all centers must determine their own resistance profiles, in order to guide empirical therapies. Reducing the rate of antibiotic resistance will contribute to reducing the cost of treatment.

Pharmacokinetics of linezolid treatment using intravenous and oral administrations in extremely premature infants

INTRODUCTION

Vancomycin is the usual antibiotic treatment in coagulase-negative staphylococcus sepsis in premature infants but causes renal toxicity. As linezolid is effective in Gram-positive cocci infection, and devoid of renal side-effects, it has been used in Nantes neonatal intensive care units and linezolid plasma concentrations were monitored.

AIM

The aims of this study are to report data on linezolid concentrations in premature infants, describe clinical and bacteriological evolution during treatment, and determine potential side effects.

METHODS

A retrospective observational study of premature infants treated with linezolid in Nantes Hospital from January 2008 through November 2011 was conducted. Linezolid plasma concentrations, possible side effects due to linezolid, and clinical response to linezolid treatment were collected from folder review.

RESULTS

Twenty-four linezolid plasma concentrations were monitored in 16 premature patients, at steady state for continuous intravenous administration or 7 ± 1.5 h after last oral administration. Except for one case, linezolid plasma concentrations were ≥minimal inhibition concentration (MIC) for linezolid for both parenteral and oral administrations. We observed three cases of thrombocytopenia, two of leukopenia, three of neutropenia, and one of severe hyperlactacidemia, resolving after discontinuation of treatment. Clinical signs of infection resolved in 13/16 cases. Bacteria were coagulase-negative Staphylococci in 12/16 cases and were eradicated in 9/12 evaluable cases.

CONCLUSIONS

This study reports an adequate linezolid plasma concentration with regard to the linezolid MIC in extremely premature infants. However, considering adverse events reported, its use should be cautious and may concern only oral administration during the late phase of infection, to limit paradoxical catheter use to treat nosocomial infections. Moreover, safe and efficient anti-Staphylococcus therapies should be identified to treat this vulnerable population.

In vitro susceptibility to methicillin, vancomycin and linezolid of staphylococci isolated from bloodstream infections in eastern Turkey

Staphylococcus species are one of the major causes of bacterial bloodstream infections. Multi-resistant staphylococci infections are major therapeutic problems. This study was aimed to detect methicillin, linezolid and vancomycin susceptibilities of Staphylococcus isolates. A total of 870 Staphylococcus strains isolated from blood cultures of hospitalized patients with BSI. Antimicrobial susceptibilities of methicillin, linezolid and vancomycin were detected according to the Clinical and Laboratory Standards Institute (CLSI). A total of 771 (88.6%) isolates were coagulase-negative staphylococci (CoNS). 700 (80.5%) isolates were methicillin-resistant (MR) and 170 (19.5%) were methicillin-susceptible (MS). All the MS isolates were also susceptible to linezolid. However 15 (1.7%) of MR strains were resistant to linezolid. The minimum inhibitory concentration range for the linezolid-resistant isolates by Etest was 6–32 μg/mL. The difference between linezolid susceptibilities for MS and MR staphylococci was not quite statistically significant (p = 0.052). There was no statistically significant difference between S. aureus and CoNS isolates for linezolid susceptibility. All of the isolates were susceptible to vancomycin. In conclusion, linezolid is currently an efficient option for the treatment of methicillin-resistant staphylococci infections.

Marinopyrrole derivatives as potential antibiotic agents against methicillin-resistant Staphylococcus aureus (III)

The marine natural product, marinopyrrole A (1), was previously shown to have significant antibiotic activity against Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus (MRSA). Although compound (1) exhibits a significant reduction in MRSA activity in the presence of human serum, we have identified key modifications that partially restore activity. We previously reported our discovery of a chloro-derivative of marinopyrrole A (1a) featuring a 2–4 fold improved minimum inhibitory concentration (MIC) against MRSA, significantly less susceptibility to serum inhibition and rapid and concentration-dependent killing of MRSA. Here, we report a novel fluoro-derivative of marinopyrrole A (1e) showing an improved profile of potency, less susceptibility to serum inhibition, as well as rapid and concentration-dependent killing of MRSA.

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

Linezolid, approved for clinical use since 2000, has become an important addition to the anti-Gram-positive infection armamentarium. This oxazolidinone drug has in vitro and in vivo activity against essentially all Gram-positive organisms, including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). The in vitro activity of linezolid was well documented prior to its clinical application, and several ongoing surveillance studies demonstrated consistent and potent results during the subsequent years of clinical use. Emergence of resistance has been limited and associated with invasive procedures, deep organ involvement, presence of foreign material and mainly prolonged therapy. Non-susceptible organisms usually demonstrate alterations in the 23S rRNA target, which remain the main resistance mechanism observed in enterococci; although a few reports have described the detection of cfr-mediated resistance in Enterococcus faecalis. S. aureus isolates non-susceptible to linezolid remain rare in large surveillance studies. Most isolates harbour 23S rRNA mutations; however, cfr-carrying MRSA isolates have been observed in the United States and elsewhere. It is still uncertain whether the occurrences of such isolates are becoming more prevalent. Coagulase-negative isolates (CoNS) resistant to linezolid were uncommon following clinical approval. Surveillance data have indicated that CoNS isolates, mainly Staphylococcus epidermidis, currently account for the majority of Gram-positive organisms displaying elevated MIC results to linezolid. In addition, these isolates frequently demonstrate complex and numerous resistance mechanisms, such as alterations in the ribosomal proteins L3 and/or L4 and/or presence of cfr and/or modifications in 23S rRNA. The knowledge acquired during the past decades on this initially used oxazolidinone has been utilized for developing new candidate agents, such as tedizolid and radezolid, and as linezolid patents soon begin to expire, generic brands will certainly become available. These events will likely establish a new chapter for this successful class of antimicrobial agents.

Mutations within the rplD Gene of Linezolid-Nonsusceptible Streptococcus pneumoniae Strains Isolated in the United States

Three invasive Streptococcus pneumoniae strains nonsusceptible to linezolid were isolated in the United States between 2001 and 2012 from the CDC's Active Bacterial Core surveillance. Linezolid binds ribosomal proteins where structural changes within its target site may confer resistance. Our study identified mutations and deletions near the linezolid binding pocket of two of these strains within the rplD gene, which encodes ribosomal protein L4. Mutations in the 23S rRNA alleles or the rplV gene were not detected.

Summary of linezolid activity and resistance mechanisms detected during the 2012 LEADER surveillance program for the United States

This study summarizes the linezolid susceptibility testing results for 7,429 Gram-positive pathogens from 60 U.S. sites collected during the 2012 sampling year for the LEADER Program. Linezolid showed potent activity when tested against 2,980 Staphylococcus aureus isolates, inhibiting all but 3 at ≤2 μg/ml. Similarly, linezolid showed coverage against 99.5% of enterococci, as well as for all streptococci tested. These results confirm a long record of linezolid activity against U.S. Gram-positive isolates since regulatory approval in 2000.

The emerging problem of linezolid-resistant Staphylococcus

The oxazolidinone antibiotic linezolid has demonstrated potent antimicrobial activity against Gram-positive bacterial pathogens, including methicillin-resistant staphylococci. This article systematically reviews the published literature for reports of linezolid-resistant Staphylococcus (LRS) infections to identify epidemiological, microbiological and clinical features for these infections. Linezolid remains active against >98% of Staphylococcus, with resistance identified in 0.05% of Staphylococcus aureus and 1.4% of coagulase-negative Staphylococcus (CoNS). In all reported cases, patients were treated with linezolid prior to isolation of LRS, with mean times of 20.0 ± 47.0 months for S. aureus and 11.0 ± 8.0 days for CoNS. The most common mechanisms for linezolid resistance were mutation (G2576T) to the 23S rRNA (63.5% of LRSA and 60.2% of LRCoNS) or the presence of a transmissible cfr ribosomal methyltransferase (54.5% of LRSA and 15.9% of LRCoNS). The emergence of linezolid resistance in Staphylococcus poses significant challenges to the clinical treatment of infections caused by these organisms, and in particular CoNS.

Linezolid surveillance results for the United States: LEADER surveillance program 2011

The LEADER surveillance program monitors the in vitro activity of linezolid and comparator agents against Gram-positive bacteria in the United States. In its eighth consecutive year (2011), a total of 60 medical centers from the United States, including seven medical centers specializing in children's health care contributed a total of 7,303 Gram-positive pathogens. The MIC(90) value for Staphylococcus aureus was 2 μg/ml, and for coagulase-negative staphylococci, enterococci, Streptococcus pneumoniae, β-hemolytic streptococci, and viridans group streptococci, the MIC(90) was 1 μg/ml. The "all organism" linezolid-resistant and nonsusceptible rate was only 0.19%.

LEADER surveillance program results for 2010: an activity and spectrum analysis of linezolid using 6801 clinical isolates from the United States (61 medical centers)

The LEADER program monitors the in vitro activity of linezolid and comparator agents across the United States using reference broth microdilution and supportive molecular susceptibility-based investigations. This report summarizes the data from the 2010 program, the seventh consecutive year. A total of 61 medical centers from the USA including 7 medical centers specializing in children's healthcare provided a total of 6801 Gram-positive pathogens. The medical centers represented all 9 US Bureau of Census geographic regions. The organisms tested by reference broth microdilution were 3105 Staphylococcus aureus, 944 coagulase-negative staphylococci (CoNS), 934 Enterococci, 803 Streptococcus pneumoniae, 604 β-haemolytic streptococci, and 411 viridans group and other streptococci. The MIC(90) value for each of the above 6 targeted groups of organisms was 1 μg/mL. The "all organism" linezolid-resistant and nonsusceptible rate was 0.38%, which has been constant at 0.34% (2009) to 0.45% (2006) for the last 4 years. For Staphylococcus aureus, only 0.06% of the isolates were linezolid-resistant (MIC, ≥8 μg/mL); however, 2 additional methicillin-resistant Staphylococcus aureus had a cfr and a MIC of only 4 μg/mL. Resistance to linezolid was detected in 7 enterococci (0.75%) and 14 CoNS isolates (1.48%). This also represents a stable rate of resistance noted since the 2006 LEADER program report. Of note, for the first time in the 7 years of the Leader Program a linezolid-resistant Streptococcus pneumoniae was encountered. Overall, the results of the LEADER program demonstrate that linezolid maintains excellent in vitro activity against target Gram-positive pathogens across the USA. The LEADER program continues to provide valuable reference and molecular-level monitoring of linezolid activity.

In vitro activity and microbiological efficacy of tedizolid (TR-700) against Gram-positive clinical isolates from a phase 2 study of oral tedizolid phosphate (TR-701) in patients with complicated skin and skin structure infections

Tedizolid (TR-700, formerly torezolid) is the active moiety of the prodrug tedizolid phosphate (TR-701), a next-generation oxazolidinone, with high potency against Gram-positive species, including methicillin-resistant Staphylococcus aureus (MRSA). A recently completed randomized, double-blind phase 2 trial evaluated 200, 300, or 400 mg of oral tedizolid phosphate once daily for 5 to 7 days in patients with complicated skin and skin structure infections. This report examines the in vitro activity of tedizolid and Zyvox (linezolid) against Gram-positive pathogens isolated at baseline and describes the microbiological and clinical efficacy of tedizolid. Of 196 isolates tested, 81.6% were S. aureus, and of these, 76% were MRSA. The MIC(50) and MIC(90) of tedizolid against both methicillin-susceptible S. aureus (MSSA) and MRSA were 0.25 μg/ml, compared with a MIC(50) of 1 μg/ml and MIC(90) of 2 μg/ml for linezolid. For coagulase-negative staphylococci (n = 7), viridans group streptococci (n = 15), and beta-hemolytic streptococci (n = 3), the MICs ranged from 0.03 to 0.25 μg/ml for tedizolid and from 0.12 to 1 μg/ml for linezolid. The microbiological eradication rates at the test-of-cure visit (7 to 14 days posttreatment) in the microbiologically evaluable population (n = 133) were similar in all treatment groups, with overall eradication rates of 97.7% for all pathogens, 97.9% for MRSA, and 95.7% for MSSA. The clinical cure rates for MRSA and MSSA infections were 96.9% and 95.7%, respectively, across all dose groups. This study confirms the potent in vitro activity of tedizolid against pathogenic Gram-positive cocci, including MRSA, and its 4-fold-greater potency in comparison with linezolid. All dosages of tedizolid phosphate showed excellent microbiological and clinical efficacy against MRSA and MSSA.

Microbiological and clinical characteristics of vancomycin-resistant Enterococcus faecium bacteraemia in Taiwan: implication of sequence type for prognosis

OBJECTIVES

Vancomycin-resistant enterococci (VRE), particularly vancomycin-resistant Enterococcus faecium (VREfm), have emerged among the leading pathogens causing hospital-acquired infections worldwide. We aimed to examine whether there were newly introduced clones contributing to this increase and to assess the risk factors for mortality in patients with VREfm bacteraemia.

METHODS

Between 2003 and 2010, all medical records of adult patients diagnosed with VREfm bacteraemia at a university hospital in Taiwan were reviewed. Antibiotic susceptibility, genotyping and multilocus sequence typing of the VREfm isolates were performed.

RESULTS

During the study period, the prevalence of non-duplicated blood VRE isolates increased significantly from 3.9% in 2003 to 18.9% in 2010 (P < 0.0001). One-hundred-and-forty-nine patients with VREfm bacteraemia were noted and 102 isolates of VREfm were available for microbiological characterization. All isolates were susceptible to daptomycin and linezolid. Sequence type (ST) 18 and ST414 were the two predominant emerging STs from 2009 to 2010, accounting for 29.7% and 25.0% of all isolates, respectively. Patients who received immunosuppressives, had a high Charlson comorbidity index or experienced septic shock had a significantly higher 14 day mortality rate. Patients who had bacteraemia caused by ST414 isolates and received appropriate antibiotics had a lower 14 day mortality rate.

CONCLUSIONS

The prevalence of the VRE that caused bacteraemia increased from 2003 to 2010. This increase might be attributed to the clonal spread of VREfm belonging to ST18 and ST414. The all-cause 14 day mortality rate was lower in patients with bacteraemia due to VREfm isolates that belonged to ST414.

New therapy options for MRSA with respiratory infection/pneumonia

PURPOSE OF REVIEW

Methicillin-resistant Staphylococcus aureus (MRSA) is a frequent causative agent of nosocomial pneumonia. Because of important clinical consequences of inappropriate treatment, a current review of the potential modifications undergone by S. aureus and adaptation to new treatment options is necessary.

RECENT FINDINGS

Vancomycin has been considered the treatment of choice for pneumonia due to MRSA. However, detection of a progressive increase in the minimum inhibitory concentration for this antibiotic, its limited access to the lung parenchyma, and its considerable adverse effects have called into question its position. Linezolid has been shown to have a better pharmacokinetic and safety profiles. The prior uncertainty regarding the clinical superiority of linezolid appears to have been resolved with the publication of a recent trial. Linezolid achieved a higher clinical and microbiological response rate (the latter was not statistically significant), together with a lower incidence of all types of renal adverse effects in patients with nosocomial pneumonia, compared with vancomycin. Tigecycline, teicoplanin and quinupristin/dalfopristin were inferior to the compared drug in their respective clinical trials. The clinical efficacy of telavancin was similar to that of vancomycin. The renal adverse effects of telavancin have to be clarified. Other drugs are efficacious against MRSA but their profile should be evaluated in nosocomial pneumonia.

SUMMARY

Current therapeutic alternatives for nosocomial pneumonia due to MRSA appear to be limited to vancomycin and linezolid. However, vancomycin pitfalls, together with the apparent clinical superiority of linezolid, appear to restrict its indication. Telavancin could be a good alternative in patients without basal renal failure.

Intraperitoneal vancomycin concentrations during peritoneal dialysis-associated peritonitis: correlation with serum levels

BACKGROUND

For the treatment of peritoneal dialysis-associated peritonitis (PDP), it has been suggested that serum concentrations of vancomycin be kept above 12 mg/L-15 mg/L. However, studies correlating vancomycin concentrations in serum and peritoneal dialysate effluent (PDE) during active infection are sparse. We undertook the present study to investigate this issue and to determine whether achieving the recommended serum level of vancomycin results in therapeutic levels intraperitoneally.

METHODS

We studied patients treated with intraperitoneal (i.p.) vancomycin for non-gram-negative PDP. We gave a single dose (approximately 30 mg/kg) at presentation, and we subsequently measured vancomycin levels in PDE on day 5; we wanted to determine if efflux of vancomycin from serum to PDE during a 4-hour dwell was consistent and resulted in therapeutic levels.

RESULTS

Of the 48 episodes of PDP studied, serum vancomycin concentrations exceeding 12 mg/L were achieved in 98% of patients, but in 11 patients (23%), a PDE vancomycin level below 4 mg/L--the minimal inhibitory concentration (MIC) of many gram-positive organisms--was observed at the end of a 4-hour dwell on day 5. The correlation between the concentrations of vancomycin in serum and PDE (from efflux of antibiotic over 4 hours) was statistically significant, but poor (R(2) = 0.18).

CONCLUSIONS

Our data support the International Society for Peritoneal Dialysis statement that adequate serum vancomycin concentrations can be achieved with intermittent dosing (single dose every 5 days), but cannot guarantee therapeutic PDE levels in the treatment of PDP. Intermittent dosing of vancomycin may not consistently result in PDE concentrations markedly greater than MIC of many important pathogens. Although the clinical significance of this finding remains to be determined, it may be preferable to give smaller but more frequent doses of PDE vancomycin (continuous dosing) for adults with PDP (as is currently recommended for children).

Prevalence, enumeration, and antimicrobial agent resistance of Clostridium difficile in cattle at harvest in the United States

To assess the potential for food contamination with Clostridium difficile from food animals, we conducted a cross-sectional fecal prevalence study in 944 randomly selected cattle harvested at seven commercial meat processing plants, representing four distant regions (median distance of 1,500 km) of the United States. In all, 944 animals were sampled in the summer of 2008. C. difficile was isolated from 1.8% (17 of 944) of cattle, with median fecal shedding concentration of 2.2 log CFU/g (range = 1.6 to 4.8, 95% confidence interval = 1.6, 4.3). Toxigenic C. difficile isolates were recovered from only four (0.4%) cattle. One of these isolates was emerging PCR ribotype 078/toxinotype V. The remaining toxigenic isolates were toxinotype 0, one of which was an isolate with resistance to linezolid, clindamycin, and moxifloxacin (by the E-test). All isolates were susceptible to vancomycin, metronidazole, and tigecycline, but the MICs against linezolid were as high as the highest reported values for human-derived isolates. The source of the linezolid-clindamycin-moxifloxacin resistance in a toxigenic C. difficile isolate from cattle is uncertain. However, since the use of these three antimicrobial agents in cattle is not allowed in North America, it is possible that resistance originated from an environmental source, from other species where those antimicrobial agents are used, or transferred from other intestinal bacteria. This study confirms that commercial cattle can carry epidemiologically relevant C. difficile strains at the time of harvest, but the prevalence at the time they enter the food chain is low.

Proteomic and transcriptomic analysis of linezolid resistance in Streptococcus pneumoniae

Linezolid is an oxazolidinone antibiotic that inhibits the initiation of translation. Although resistance to linezolid is an uncommon event, it has been reported in clinical isolates. The genome sequence of Streptococcus pneumoniae linezolid-resistant mutants recently revealed mutations associated with resistance. A proteomic and transcriptomic screen now reveals a possible increase in the metabolism and transport of carbohydrates in these linezolid-resistant S. pneumoniae mutants. Several glycolytic proteins were shown to be overexpressed in the resistant strains, along with other enzymes and transporters involved in the metabolism of sugars. An increase in energy needs appears to be required to sustain extended levels of resistance to linezolid as the disruption of two ABC transporters putatively involved in the import of carbohydrates leads to a 2-fold sensitization to linezolid. Furthermore, the disruption of the catabolite control protein A, a regulator of the metabolism of sugars whose expression is highly increased in one linezolid-resistant mutant, resulted in a 2-fold increase in linezolid susceptibility. This global scale analysis of gene and protein expression profiling highlights metabolism alterations associated with linezolid resistance in S. pneumoniae.

LEADER Program results for 2009: an activity and spectrum analysis of linezolid using 6,414 clinical isolates from 56 medical centers in the United States

The LEADER Program monitors the in vitro activity of linezolid in sampled U.S. medical centers using reference broth microdilution methods with supporting molecular investigations in a central laboratory design. This report summarizes data obtained in 2009, the 6th consecutive year of this longitudinal study. A total of 6,414 isolates from 56 medical centers in all nine Census regions across the United States participated in 2009. For the six leading species/groups, the following linezolid MIC(90) values were observed: Staphylococcus aureus, 2 μg/ml; coagulase-negative staphylococci (CoNS), 1 μg/ml; Enterococcus spp., 2 μg/ml; Streptococcus pneumoniae, 1 μg/ml; viridans group streptococci, 1 μg/ml; and beta-hemolytic streptococci, 1 μg/ml. Linezolid resistance was only 0.34% overall, with no evidence of significant increase in the LEADER Program since 2006. The predominant linezolid resistant mechanism found was a G2576T mutation in the 23S rRNA. L3/L4 riboprotein mutations were also found. The mobile multidrug-resistant cfr gene was found in four strains (two S. aureus strains and one strain each of S. epidermidis and S. capitis) from four different states, suggesting persistence but a lack of dissemination. Linezolid continues to exhibit excellent activity and spectrum, and this study documents the need for continued monitoring of emerging mechanisms of resistance over a wide geographic area.

Multistep resistance development studies of ceftaroline in gram-positive and -negative bacteria

Ceftaroline, the active component of the prodrug ceftaroline fosamil, is a novel broad-spectrum cephalosporin with bactericidal activity against Gram-positive and -negative isolates. This study evaluated the potential for ceftaroline and comparator antibiotics to select for clones of Streptococcus pneumoniae, Streptococcus pyogenes, Haemophilus influenzae, Moraxella catarrhalis, Klebsiella pneumoniae, Staphylococcus aureus, and Enterococcus faecalis with elevated MICs. S. pneumoniae and S. pyogenes isolates in the present study were highly susceptible to ceftaroline (MIC range, 0.004 to 0.25 μg/ml). No streptococcal strains yielded ceftaroline clones with increased MICs (defined as an increase in MIC of >4-fold) after 50 daily passages. Ceftaroline MICs for H. influenzae and M. catarrhalis were 0.06 to 2 μg/ml for four strains and 8 μg/ml for a β-lactamase-positive, efflux-positive H. influenzae with a mutation in L22. One H. influenzae clone with an increased ceftaroline MIC (quinolone-resistant, β-lactamase-positive) was recovered after 20 days. The ceftaroline MIC for this isolate increased 16-fold, from 0.06 to 1 μg/ml. MICs for S. aureus ranged from 0.25 to 1 μg/ml. No S. aureus isolates tested with ceftaroline had clones with increased MIC (>4-fold) after 50 passages. Two E. faecalis isolates tested had ceftaroline MICs increased from 1 to 8 μg/ml after 38 days and from 4 to 32 μg/ml after 41 days, respectively. The parental ceftaroline MIC for the one K. pneumoniae extended-spectrum β-lactamase-negative isolate tested was 0.5 μg/ml and did not change after 50 daily passages.

[Evaluation of the performance of the MicroScan Pos Breakpoint Combo Panel Type 28 for susceptibility testing of Staphylococcus aureus: low-range minimum inhibitory concentration of vancomycin, cefoxitin screening, and inducible clindamycin resistance detection]

BACKGROUND

Susceptibility testing of Staphylococcus aureus often requires cumbersome supplementary tests. MicroScan Pos Breakpoint Combo Panel Type 28 (PBC28) (Siemens, USA) includes cefoxitin screening to detect methicillin-resistant Staphylococcus aureus (MRSA), inducible clindamycin resistance detection (ICD), and determination of low-range minimum inhibitory concentration of vancomycin (0.5-16 µg/mL). The purpose of this study was to evaluate the performance of PBC28 in comparison with that of Pos Combo Type 1A (PC1A) (Siemens).

METHODS

From December 2009 to March 2010, 500 non-duplicate clinical isolates of S. aureus were tested with PC1A and PBC28. Categorical agreements (CA) between the interpretations of the 2 panels were estimated. The presence of the mecA gene was determined by PCR, and double-disk diffusion test (D-test) was performed on the isolates resistant to erythromycin but susceptible or intermediately resistant to clindamycin. Ninety-six isolates representing various vancomycin minimum inhibitory concentrations (MICs) were tested in parallel with repeat PBC28, broth macrodilution, and epsilometer test (E test).

RESULTS

The CA was 99.3% with a very major error (VME) of 0.2%, major error (ME) of 0.1%, and minor error (mE) of 0.4% in total. PBC28 showed 100% CA for 1 isolate with vancomycin MIC of 4 µg/mL and 35 isolates (7.0%) with MIC of 2 µg/mL. However, only 15, 27, and 35 isolates with vancomycin MIC of 2 µg/mL showed 100% CA in repeat PBC28, broth macrodilution, and E test, respectively. PC1A and PBC28 detected all 314 mecA-positive isolates. Among the 63 isolates tested with the D-test, 58 (92.1%) were positive, and the results were 100% concordant with those of ICD.

CONCLUSIONS

PBC28 can be appropriate susceptibility testing of S. aureus, including MRSA detection and ICD. However, the lower-range vancomycin MIC test was not reproducible enough to reliably differentiate MIC of 2 µg/mL from MIC ≤ 1 µg/mL.

Nosocomial methicillin-resistant Staphylococcus aureus (MRSA) pneumonia: linezolid or vancomycin? - Comparison of pharmacology and clinical efficacy

The incidence of nosocomial pneumonia involving methicillin-resistant Staphylococcus aureus strains (MRSA) is on the rise worldwide. For years, vancomycin has been used as the drug of choice in the treatment of MRSA infections and was recommended as such by clinical guidelines. There is growing evidence that vancomycin, despite low resistance rates is a suboptimal therapeutic option in critically ill patients, particularly in patients with pneumonia. Disadvantages of vancomycin are i) slow bactericide action, ii) poor penetration into pulmonary tissue, iii) the globally slowly increasing vancomycin MICs ("creep") that result in increased clinical failure despite being susceptible according to defined break points and iv) nephrotoxicity. In contrast to other novel antibiotics with MRSA activity, Linezolid is currently approved for the treatment of nosocomial pneumonia in the USA and Europe. Several studies have compared vancomycin with linezolid for nosocomial pneumonia with conflicting results. This review compares both substances regarding pharmacodynamics, resistance, safety and clinical efficacy and discusses preliminary data of the ZEPHyR study. This study compared linezolid versus vancomycin in patients with proven MRSA pneumonia and was the largest trial ever conducted in this population.

Daptomycin nonsusceptible enterococci: an emerging challenge for clinicians

Daptomycin is the only antibiotic with in vitro bactericidal activity against vancomycin-resistant Enterococcus (VRE) that is approved by the Food and Drug Administration (FDA). Data on the potential emergence of daptomycin nonsusceptibility among enterococci remain limited. We systematically reviewed the published literature for reports of isolates of enterococci that were daptomycin nonsusceptible and assessed the clinical significance and outcome of therapy. Based on susceptibility breakpoints approved by the Clinical Laboratory Standards Institute (CLSI), daptomycin has in vitro activity against >90% of enterococcal isolates. Less than 2% of enterococcal isolates were daptomycin nonsusceptible, with minimum inhibitory concentrations (MICs) >4 μg/mL. The prevalence of nonsusceptibility of VRE isolates to daptomycin may be overestimated due to the spread of clonally related isolates in health care settings. Clinicians should be aware of the possibility of the emergence of daptomycin nonsusceptibility and should closely monitor daptomycin MICs of enterococci isolated during treatment.

Comparison of the pharmacokinetic properties of vancomycin, linezolid, tigecyclin, and daptomycin

The rapid antibiotic resistance development has created a major demand for new antimicrobial agents that can combat resistant strains such as methicillin-resistant S. aureus (MRSA). Until a short time ago, the glycopeptide vancomycin was the only therapeutic choice in this situation. However, in recent years some newer agents with different mechanisms of actions have been added to the arsenal, and more are on the horizon. For a successful therapy it is of vital importance that these compounds are used judiciously and dosed appropriately. The present article reviews the pharmacokinetic properties of vancomycin, linezolid, tigecycline and daptomycin. The first major difference between these compounds is their oral bioavailability. Only linezolid can be administered orally, whereas vancomycin, daptomycin and tigecycline are limited to parenteral use. Once in the body, they show very different disposition. Daptomycin has a very small volume of distribution of 7L indicating very little tissue distribution whereas tigecycline has a very large volume of distribution of 350-500 L. Vancomycin and linezolid are in-between with volumes of distribution of approximately 30 and 50 L, close to total body water. However, studies have shown that linezolid shows better tissue penetration than vancomycin. Newer studies using microdialysis, a new technique that allows direct monitoring of unbound tissue levels, support this finding. As far as drug elimination, daptomycin and vancomycin are mainly eliminated into the urine and require dosing adjustments in renally impaired patients, whereas tigecycline is eliminated into the bile and linezolid is metabolized so that in renal patients no dosing adjustments are needed for these compounds. Although the elimination pathways are very different, the resulting half-lives of linezolid, vancomycin, and daptomycin are not greatly different and vary from 4-8 h. Tigecycline, however, has a much longer half-life of up to 1-2 days due to the slow redistribution from tissue binding sites.

Linezolid Resistance in Staphylococci

Linezolid, the first oxazolidinone to be used clinically, is effective in the treatment of infections caused by various Gram-positive pathogens, including multidrug resistant enterococci and methicillin-resistant Staphylococus aureus. It has been used successfully for the treatment of patients with endocarditis and bacteraemia, osteomyelitis, joint infections and tuberculosis and it is often used for treatment of complicated infections when other therapies have failed.

Linezolid resistance in Gram-positive cocci has been encountered clinically as well as in vitro, but it is still a rare phenomenon. The resistance to this antibiotic has been, until now, entirely associated with distinct nucleotide substitutions in domain V of the 23S rRNA genes. The number of mutated rRNA genes depends on the dose and duration of linezolid exposure and has been shown to influence the level of linezolid resistance. Mutations in associated ribosomal proteins also affect linezolid activity. A new phenicol and clindamycin resistance phenotype has recently been found to be caused by an RNA methyltransferase designated Cfr. This gene confers resistance to lincosamides, oxazolidinones, streptogramin A, phenicols and pleuromutilins, decrease the susceptibility of S. aureus to tylosin, to josamycin and spiramycin and thus differs from erm rRNA methylase genes.

Research into new oxazolidinones with improved characteristics is ongoing. Data reported in patent applications demonstrated that some oxazolidinone derivatives, also with improved characteristics with respect to linezolid, are presently under study: at least three of them are in an advanced phase of development.

Analysis of methods commonly used for glycopeptide and oxazolidinone susceptibility testing in Enterococcus faecium isolates

The susceptibility to teicoplanin, vancomycin and linezolid of 30 clinical isolates of Enterococcus faecium was tested by Vitek 2, Phoenix, Etest, broth microdilution and disc diffusion tests. The vanA and vanB resistance genes and the 23S rRNA gene G2576T mutation were detected by PCR and PCR-RFLP, respectively. Resistance rates to teicoplanin ranged from 3 % for Vitek 2 to 57.6 % for the Phoenix test, and those to vancomycin ranged from 56.7 % for Vitek 2 to 86.7 % for the Phoenix test. Only two out of 25 strains carrying the vanA gene were univocally recognized as the VanA phenotype. The only strain with the G2576T mutation did not carry the vanA gene and showed resistance to linezolid by the disc diffusion, Vitek 2 and broth dilution methods (MIC >8 μg ml−1), but was susceptible when tested with the Phoenix test and Etest (MIC ≤4 μg ml−1). Therefore, the resistance to glycopeptides and linezolid was not univocally detected by the susceptibility testing methods used in this study.

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

Nosocomial infections caused by enterococci present a challenge for clinicians because treatment options are often limited due to the widespread occurrence of strains resistant to multiple antibiotics, including vancomycin. Daptomycin is a first-in-class cyclic lipopeptide that has proven efficacy for the treatment of Gram-positive infections. Although methicillin-resistant Staphylococcus aureus has been the most prominent target in the clinical development of daptomycin, this agent has demonstrated potent bactericidal activity in enterococcal infection models and has been used for the treatment of enterococcal infections in humans. In recent years, large-scale susceptibility studies have shown that daptomycin is active against >98% of enterococci tested, irrespective of their susceptibility to other antibacterial agents. This lack of cross-resistance reflects the fact that daptomycin has a mode of action distinct from those of other antibiotics, including glycopeptides. While there are limited data available from randomized controlled trials, extensive clinical experience with daptomycin in enterococcal infections (including bacteraemia, endocarditis, skin and soft tissue infections, bone and joint infections and urinary tract infections) has been reported. This growing body of evidence provides useful insights regarding the efficacy of daptomycin against enterococci in clinical settings.

Clinical efficacy and tolerability of linezolid in pediatric patients: a systematic review

BACKGROUND

Linezolid is marketed for the treatment of severe, vancomycin-resistant infections with gram-positive bacteria in adults. Most information regarding the pharmacokinetic profile, efficacy, and tolerability of linezolid is derived from adult studies.

OBJECTIVE

The aim of this review was to summarize evidence regarding the use of linezolid in infants and children, focusing on the drug's clinical efficacy data and tolerability profile.

METHODS

A literature search was conducted of the Cochrane Library, EMBASE, and MEDLINE databases, from their inception through July 20, 2009, using the following terms: linezolid, newborn, infant, child, pediatrics, adolescent, human, clinical trial, and case report. Articles were excluded if they were redundant or not pertinent. (Articles that did not focus on the use of linezolid in children were considered not pertinent.) Bibliographies of all relevant articles were also evaluated.

RESULTS

Forty-seven publications regarding the use of linezolid in children were included in the review: 5 pharmacokinetic studies, 32 case reports, 6 randomized clinical trials (RCTs), 2 uncontrolled trials, 1 subanalysis of 2 published RCTs, and 1 subanalysis of published data about linezolid's tolerability. Pharmacokinetic data on linezolid use in children were derived from studies that enrolled 447 children. Plasma pharmacokinetics of linezolid in pediatric patients were found to be age dependent. Results from 6 vancomycinor cefadroxil-controlled RCTs (including 1480 children) evaluating linezolid treatment in children reported variable clinical cure rates, ranging from 75.0% to 93.2% in children with skin and skin-structure infections and from 77.5% to 90.0% in children with bacteremia or pneumonia. No significant difference in clinical cure rates between the linezolid group and the comparator group was observed in any study. The most frequently reported adverse events were diarrhea (from 3.1% to 16.8%), nausea and/or vomiting (from 2.9% to 11.9%), and thrombocytopenia (from 1.9% to 4.7%). To date, 3 cases of neuropathy have been described in children.

CONCLUSIONS

The reviewed pediatric studies in skin and skin-structure infections, bacteremia, or pneumonia found that linezolid was associated with high clinical cure rates (75.0%-93.2%) that did not differ significantly from those of vancomycin or cefadroxil. RCTs enrolling children with other types of infection (eg, osteomyelitis, endocarditis), as well as long-term studies, are needed to draw definitive conclusions about linezolid's efficacy and tolerability in pediatric patients. Careful monitoring for adverse events and possible linezolid resistance continues to be essential.

Affinity of ceftaroline and other beta-lactams for penicillin-binding proteins from Staphylococcus aureus and Streptococcus pneumoniae

We compared the affinities of ceftaroline for all penicillin-binding proteins (PBPs) with those of ceftriaxone and cefotaxime in 6 Staphylococcus aureus and 7 Streptococcus pneumoniae isolates with various resistance phenotypes. Ceftaroline MICs were PBP1A, -1B, and -2A > PBP2B, and ceftaroline had >or=4-fold higher 50% inhibitory concentrations (IC(50)s) (0.1 to 4 microg/ml) for PBP2X, -2A, -2B, and -3 than those for the other cephalosporins tested. Among 3 penicillin-resistant S. pneumoniae strains, ceftaroline had a high affinity for PBP2X (IC(50), 0.1 to 1 microg/ml), a primary target for cephalosporin PBP binding activity, and high affinities for PBP2B (IC(50), 0.5 to 4 microg/ml) and PBP1A (IC(50), 0.125 to 0.25 microg/ml) as well, both of which are also known as major targets for PBP binding activity of cephalosporins. Ceftaroline PBP affinities in methicillin-susceptible S. aureus strains were greater than or equal to those of the 3 other beta-lactams tested. Ceftaroline bound to PBP2a in methicillin-resistant S. aureus (IC(50), 0.01 to 1 microg/ml) with up to 256-fold-higher affinity than those of other agents. Ceftaroline demonstrated very good PBP affinity against all S. aureus and S. pneumoniae strains tested, including resistant isolates.

The importance of tissue penetration in achieving successful antimicrobial treatment of nosocomial pneumonia and complicated skin and soft-tissue infections caused by methicillin-resistant Staphylococcus aureus: vancomycin and linezolid

BACKGROUND

The rising prevalence of nosocomial methicillin-resistant Staphylococcus aureus (MRSA) and the recent emergence of community-associated MRSA are major clinical, public health, and economic challenges. MRSA is a leading cause of nosocomial pneumonia and complicated skin and soft-tissue infections (cSSTI). Vancomycin and linezolid are two commonly used antimicrobial agents with activity against Gram-positive pathogens, particularly MRSA, that are used to treat both nosocomial pneumonia and cSSTI. Recently, the therapeutic efficacy of vancomycin in the treatment of hospitalized patients with MRSA infections has been questioned due to the emergence of MRSA strains with reduced susceptibility to vancomycin together with concerns related to inadequate dosing and poor tissue penetration of the drug.

SCOPE

A literature review was conducted to investigate the pharmacokinetics and pulmonary and tissue penetration of vancomycin and linezolid. Using MEDLINE and EMBASE, the most relevant articles in English published over the past 25 years (up to October 2008) were identified and summarized. Studies in human volunteers and adult patients that measured concentrations of antibiotic in serum, epithelial lining fluid (ELF), and tissue were selected for further review.

FINDINGS

For both drugs, pharmacokinetic studies were conducted in diverse patient populations and employed varying techniques to measure tissue concentrations. Vancomycin concentrations in ELF ranged from 5 to 25% of simultaneous plasma levels, while concentrations in whole homogenized lung tissue were slightly higher (24-41%). Distribution of vancomycin into soft tissue was variable. For linezolid, overall mean concentrations in ELF and in soft tissue were generally similar or higher than simultaneous plasma levels, although variability in tissue penetration across studies in healthy volunteers and patients was seen.

LIMITATIONS

The studies included in this review vary greatly in their designs and patient populations; this, together with methodologic difficulties, limits the interpretation of the data.

CONCLUSIONS

In the absence of clinical data correlating ELF concentrations and clinical outcome, the clinical significance of differences in pulmonary penetration of vancomycin and linezolid is unknown. Higher vancomycin serum concentrations may be necessary to achieve appropriate lung concentrations to optimize treatment outcomes. Linezolid demonstrates adequate penetration into lung and other soft issues with sustained concentrations above the minimum inhibitory concentrations for susceptible pathogens, including MRSA, for the majority of the dosing interval. Examination of the pharmacokinetic data adds insights not provided by the clinical trial data and together provides clinicians with a more comprehensive basis for selecting appropriate antimicrobial therapy for the treatment of serious MRSA infections.

Linezolid surveillance program results for 2008 (LEADER Program for 2008)

The LEADER Program was initiated in 2004 and monitors emerging linezolid resistance in sampled US medical centers. This report summarizes the data obtained in 2008, the 5th consecutive year. A total of 57 institutions participated in 2008 representing all 9 US census regions with 100 target organisms per site (6113 isolates, 101.9% compliance to protocol design). The organisms tested by reference broth microdilution methods were Staphylococcus aureus (3156), coagulase-negative staphylococci (CoNS; 856), enterococci (901), Streptococcus pneumoniae (619), and viridans group (223) or beta-hemolytic streptococci (358); also, D-test was used to determine inducible clindamycin resistance in Staphylococcus aureus. Linezolid remained very potent against all sampled species with MIC(90) results ranging from 1 microg/mL (streptococci and CoNS) to 2 microg/mL (Staphylococcus aureus and enterococci). Only 0.36% of sampled strains were nonsusceptible to linezolid, a slight decrease from 0.45% and 0.44% in 2006 and 2007, respectively. The nonsusceptible strains (22) were Staphylococcus aureus (3), CoNS (14), and Enterococcus faecium (5) each with defined target mutations (G2576T in 19 strains; T2504A in 1 strain), mobile cfr element (1 strain Staphylococcus epidermidis with an identical pulsed-field gel electrophoresis pattern to a cfr-positive Staphylococcus epidermidis isolated from the same center in LEADER 2007), or an unknown (1 strain) mechanism. The mobile cfr resistance found in a Staphylococcus aureus strain in 2007 was not observed in 2008. In conclusion, linezolid activity sampled by the 5th year of this LEADER Program showed sustained potency and spectrum (99.64% susceptibility levels). The nonsusceptible strain isolation rates remained stable and the plasmid-mediated ribosomal-based resistance mechanism that emerged in Staphylococcus aureus and Staphylococcus epidermidis strains in 2007 showed no evidence of dissemination or increased prevalence. However, there was evidence of cfr persistence with the S. epidermidis strain. The LEADER Program continues to be an effective and sensitive surveillance tool to detect and monitor novel oxazolidinone resistance phenotypes and genotypes.

Novel ribosomal mutations in Staphylococcus aureus strains identified through selection with the oxazolidinones linezolid and torezolid (TR-700)

TR-700 (torezolid), the active moiety of the novel oxazolidinone phosphate prodrug TR-701, is highly potent against gram-positive pathogens, including strains resistant to linezolid (LZD). Here we investigated the potential of Staphylococcus aureus strains ATCC 29213 (methicillin-susceptible S. aureus [MSSA]) and ATCC 33591 (methicillin-resistant S. aureus [MRSA]) to develop resistance to TR-700. The spontaneous frequencies of mutation of MSSA 29213 and MRSA 33591 resulting in reduced susceptibility to TR-700 at 2 x the MIC were 1.1 x 10(-10) and 1.9 x 10(-10), respectively. These values are approximately 16-fold lower than the corresponding LZD spontaneous mutation frequencies of both strains. Following 30 serial passages in the presence of TR-700, the MIC for MSSA 29213 remained constant (0.5 microg/ml) while increasing eightfold (0.25 to 2.0 microg/ml) for MRSA 33591. Serial passage of MSSA 29213 and MRSA 33591 in LZD resulted in 64- and 32-fold increases in LZD resistance (2 to 128 microg/ml and 1 to 32 microg/ml, respectively). Domain V 23S rRNA gene mutations (Escherichia coli numbering) found in TR-700-selected mutants included T2500A and a novel coupled T2571C/G2576T mutation, while LZD-selected mutants included G2447T, T2500A, and G2576T. We also identified mutations correlating with decreased susceptibility to TR-700 and LZD in the rplC and rplD genes, encoding the 50S ribosomal proteins L3 and L4, respectively. L3 mutations included Gly152Asp, Gly155Arg, Gly155Arg/Met169Leu, and DeltaPhe127-His146. The only L4 mutation detected was Lys68Gln. TR-700 maintained a fourfold or greater potency advantage over LZD against all strains with ribosomal mutations. These data bring to light a variety of novel and less-characterized mutations associated with S. aureus resistance to oxazolidinones and demonstrate the low resistance potential of torezolid.

Polyphyletic emergence of linezolid-resistant staphylococci in the United States

Since the year 2000, linezolid has been used in the United States to treat infections caused by antimicrobial-resistant Gram-positive cocci. At present, linezolid-resistant (Linr) Staphylococcus aureus and Staphylococcus epidermidis strains are rare and the diversity of their genetic backgrounds is unknown. We performed sequence-based strain typing and resistance gene characterization of 46 Linr isolates that were collected from local and national sources between the years 2004 and 2007. Resistance was found to occur in at least three clonal complexes (CCs; lineages) of S. aureus and in at least four subclusters of a predominant, phylogenetically unstable CC of S. epidermidis. New candidate resistance mutations in 23S rRNA and the L4 riboprotein were identified among the S. epidermidis isolates. These findings suggest that linezolid resistance has emerged independently in multiple clones of S. aureus and with a variety of ribosomal mutations in multiple clones of S. epidermidis.