Antimicrobial properties of MX-2401, an expanded-spectrum lipopeptide active in the presence of lung surfactant

MX-2401 is an expanded-spectrum lipopeptide antibiotic selective for Gram-positive bacteria that is a semisynthetic analog of the naturally occurring lipopeptide amphomycin. It was active against Enterococcus spp., including vancomycin-sensitive Enterococcus (VSE), vanA-, vanB-, and vanC-positive vancomycin-resistant Enterococcus (VRE), linezolid- and quinupristin-dalfopristin-resistant isolates (MIC(90) of 4 μg/ml), methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive S. aureus (MSSA) (MIC(90) of 2 μg/ml), coagulase-negative staphylococci, including methicillin-sensitive Staphylococcus epidermidis (MSSE) and methicillin-resistant S. epidermidis (MRSE) (MIC(90) of 2 μg/ml), and Streptococcus spp. including viridans group streptococci, and penicillin-resistant, penicillin-sensitive, penicillin-intermediate and macrolide-resistant isolates of Streptococcus pneumoniae (MIC(90) of 2 μg/ml). MX-2401 demonstrated a dose-dependent postantibiotic effect varying from 1.5 to 2.4 h. Furthermore, MX-2401 was rapidly bactericidal at 4 times the MIC against S. aureus and Enterococcus faecalis, with more than 99.9% reduction in viable bacterial attained at 4 and 24 h, respectively. The MICs of MX-2401 against MRSA, MSSA, VSE, and VRE strains serially exposed for 15 passages to sub- to supra-MICs of MX-2401 remained within three dilutions of the original MIC. In contrast to that of the lipopeptide daptomycin, the antibacterial activity of MX-2401 was not affected in vitro by the presence of lung surfactant, and MX-2401 was active in vivo in the bronchial-alveolar pneumonia mouse model, in which daptomycin failed to show any activity. Moreover, the activity of MX-2401 was not as strongly dependent on the Ca(2+) concentration as is the activity of daptomycin. In conclusion, MX-2401 is a promising new-generation lipopeptide for the treatment of serious infections with Gram-positive bacteria, including hospital-acquired pneumonia.     

In Vivo Pharmacodynamics of New Lipopeptide MX-2401

MX-2401 is a novel lipopeptide (amphomycin analog) with a broad-spectrum bactericidal activity against Gram-positive organisms. We used murine thigh and lung infection models in neutropenic and normal mice to characterize the in vivo pharmacokinetic/pharmacodynamic (PK/PD) activities of MX-2401. The compound (2.5 to 40 mg/kg of body weight) demonstrated linear PK characterized by an area under the concentration-time curve (AUC) of 228 to 3,265 μg·h/ml and half-lives of 5.7 to 8.8 h. MICs ranged from 0.25 to 2 μg/ml. The in vivo postantibiotic effect was prolonged (8.5 h with Staphylococcus aureus and 10.3 to 12.3 with Streptococcus pneumoniae). MX-2401 exhibited dose-dependent in vivo activity against various strains of S. pneumoniae and S. aureus; penicillin and macrolide resistance in the pneumococci and methicillin resistance in the staphylococci had no impact on the antimicrobial activity of the drug. To determine which PK/PD index correlated best with MX-2401 activity, dose fractionation studies over a 72-hour period were performed. The maximum concentration of drug in serum divided by the MIC (C_max/MIC) correlated best with the efficacy for both S. aureus and S. pneumoniae. Static doses required free-drug C_max/MIC values of 0.683 to 1.06. Free-drug 72-h AUC/MIC values for the static dose were in the range of 7.49 to 32.3 and were less than expected. The drug showed modest enhancement in activity in the presence of white blood cells (1.7- to 3.4-fold). The potency of the drug in the lung was only marginally lower than in the thigh (1.3- to 1.9-fold). Based on its PK/PD profile, MX-2401 appears to be a promising new lipopeptide agent for treatment of infections by Gram-positive bacteria, including those induced by antibiotic-resistant pathogens.Concerns about growing antibiotic resistance observed in both hospital and community settings underscore the need to develop new antimicrobial agents active against resistant Gram-positive organisms (2, 3, 14, 21). MX-2401 is a novel semisynthetic lipopeptide based on the amphomycin core (17) with potent in vitro activity against resistant Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant Staphylococcus epidermidis, vancomycin-resistant Enterococcus spp., and penicillin-resistant Streptococcus pneumoniae (11, 15). MX-2401 has been shown to be effective in animal models of peritonitis and thigh and lung infection (18). The intravenous pharmacokinetics (PK) of MX-2401 in rodent species was characterized by a half-life that was significantly longer than that of another lipopeptide antibiotic, daptomycin (17, 19).The goal of the current experiments was to characterize the in vivo pharmacodynamic (PD) characteristics of MX-2401 using experimental thigh and lung infections in neutropenic and normal mice. Studies were performed to investigate (i) the pattern of killing and the presence of postantibiotic effects with MX-2401; (ii) which PK/PD index (the maximum concentration of drug in serum divided by the MIC [C_max-to-MIC ratio], area under the concentration-time curve [AUC-to-MIC ratio], or the time that serum levels exceed the MIC) best predicts the efficacy of MX-2401; (iii) whether the magnitudes of the PK/PD indices required for efficacy are similar in antibiotic-susceptible and -resistant organisms; and (iv) whether MX-2401 activity is impacted by host factors, such as the immune status and infection site.(Part of this work was presented at the 46th Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, CA, 2006.)     

Translocation of silica nanospheres through giant unilamellar vesicles (GUVs) induced by a high frequency electromagnetic field

Membrane model systems capable of mimicking live cell membranes were used for the first time in studying the effects arising from electromagnetic fields (EMFs) of 18 GHz where membrane permeability was observed following exposure. A present lack of understanding of the mechanisms that drive such a rapid change in membrane permeabilization as well as any structural or dynamic changes imparted on biomolecules affected by high-frequency electromagnetic irradiation limits the use of 18 GHz EMFs in biomedical applications. A phospholipid, 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) labelled with a fluorescent marker 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(lissamine rhodamine B sulfonyl) (rhodamine-DOPE) was used in constructing the giant unilamellar vesicles (GUVs). After three cycles of exposure, enhanced membrane permeability was observed by the internalisation of hydrophilic silica nanospheres of 23.5 nm and their clusters. All-atom molecular dynamics simulations of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes exposed to high frequency electric fields of different field strengths showed that within the simulation timeframe only extremely high strength fields were able to cause an increase in the interfacial water dynamics characterized by water dipole realignments. However, a lower strength, high frequency EMF induced changes of the water hydrogen bond network, which may contribute to the mechanisms that facilitate membrane permeabilization in a longer timeframe.

Membrane model systems capable of mimicking live cell membranes were used for the first time in studying the effects arising from electromagnetic fields (EMFs) of 18 GHz where membrane permeability was observed following exposure.     

Correlation of daptomycin bactericidal activity and membrane depolarization in Staphylococcus aureus

The objective of this study was to further elucidate the role of membrane potential in the mechanism of action of daptomycin, a novel lipopeptide antibiotic. Membrane depolarization was measured by both fluorimetric and flow cytometric assays. Adding daptomycin (5 micro g/ml) to Staphylococcus aureus gradually dissipated membrane potential. In both assays, cell viability was reduced by >99% and membrane potential was reduced by >90% within 30 min of adding daptomycin. Cell viability decreased in parallel with changes in membrane potential, demonstrating a temporal correlation between bactericidal activity and membrane depolarization. Decreases in viability and potential also showed a dose-dependent correlation. Depolarization is indicative of ion movement across the cytoplasmic membrane. Fluorescent probes were used to demonstrate Ca(2+)-dependent, daptomycin-triggered potassium release from S. aureus. Potassium release was also correlated with bactericidal activity. This study demonstrates a clear correlation between dissipation of membrane potential and the bactericidal activity of daptomycin. A multistep model for daptomycin's mechanism of action is proposed.     

Daptomycin: a lipopeptide antibiotic for the treatment of serious Gram-positive infections

Infections caused by drug-resistant pathogens are on the rise. Daptomycin, a cyclic lipopeptide with activity against most Gram-positive pathogens, including vancomycin-resistant enterococci and methicillin-resistant Staphylococcus aureus, is a newly US-FDA approved antimicrobial for complicated skin and skin structure infections (cSSSI). Daptomycin has a unique mechanism of action that results in destruction of the membrane potential. The rapid bactericidal activity of daptomycin makes it an attractive antibiotic for serious Gram-positive infections.     

Improved paramagnetic liposomes for MRI visualization of pH triggered release

This work aims at assessing the in vitro potential of paramagnetic pH sensitive liposomes as imaging tools for visualizing drug-delivery and release processes by Magnetic Resonance Imaging (MRI). pH sensitive liposomes (pSLs) were formulated using the fusogenic phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), the membrane stabilizer D-α-tocopherol-hemisuccinate (THS), and were loaded with several paramagnetic complexes including the clinically approved Gadoteridol (marketed as ProHance™). The proposed formulation allows the fast and full release of Gadoteridol at pH 5.5. The leakage of the imaging reporter from the vesicles was associated with a relaxivity enhancement that allowed its visualization by MRI. It was observed that the release mechanism implies the protonation of the THS basic sites that leads to vesicle aggregation, thus enabling the expression of the fusogenic property of POPE. Attempts for improving the MRI properties of pSLs were pursued through the encapsulation of imaging agents with higher relaxivity than Gadoteridol, but it was observed that the release kinetic can be significantly affected by the probe size. Aiming at preparing stealth pSLs, PEG chains were conjugated to the external surface of the vesicles via cleavable disulphide bridges. Such nanomedicines do not release their content at acidic pH as long as the coating polymer is not removed from the surface. The results obtained suggest that the liposomal formulation investigated in this work has the potential for visualizing drug-delivery and release processes by in vivo MRI preclinical studies.     

Ceftaroline Increases Membrane Binding and Enhances the Activity of Daptomycin against Daptomycin-Nonsusceptible Vancomycin-Intermediate Staphylococcus aureus in a Pharmacokinetic/Pharmacodynamic Model

New antimicrobial agents and novel combination therapies are needed to treat serious infections caused by methicillin-resistant Staphylococcus aureus (MRSA) with reduced susceptibility to daptomycin and vancomycin. The purpose of this study was to evaluate the combination of ceftaroline plus daptomycin or vancomycin in an in vitro pharmacokinetic/pharmacodynamic model. Simulations of ceftaroline-fosamil at 600 mg per kg of body weight every 8 h (q8h) (maximum free-drug concentration in serum [fC_max], 15.2 mg/liter; half-life [t_1/2], 2.3 h), daptomycin at 10 mg/kg/day (fC_max, 11.3 mg/liter; t_1/2, 8 h), vancomycin at 2 g q12h (fC_max, 30 mg/liter; t_1/2, 6 h), ceftaroline plus daptomycin, and ceftaroline plus vancomycin were evaluated against a clinical, isogenic MRSA strain pair: D592 (daptomycin susceptible and heterogeneous vancomycin intermediate) and D712 (daptomycin nonsusceptible and vancomycin intermediate) in a one-compartment in vitro pharmacokinetic/pharmacodynamic model over 96 h. Therapeutic enhancement of combinations was defined as ≥2 log₁₀ CFU/ml reduction over the most active single agent. The effect of ceftaroline on the membrane charge, cell wall thickness, susceptibility to killing by the human cathelicidin LL37, and daptomycin binding were evaluated. Therapeutic enhancement was observed with daptomycin plus ceftaroline in both strains and vancomycin plus ceftaroline against D592. Ceftaroline exposure enhanced daptomycin-induced depolarization (81.7% versus 72.3%; P = 0.03) and killing by cathelicidin LL37 (P < 0.01) and reduced cell wall thickness (P < 0.001). Fluorescence-labeled daptomycin was bound over 7-fold more in ceftaroline-exposed cells. Whole-genome sequencing and mutation analysis of these strains indicated that change in daptomycin susceptibility is related to an fmtC (mprF) mutation. The combination of daptomycin plus ceftaroline appears to be potent, with rapid and sustained bactericidal activity against both daptomycin-susceptible and -nonsusceptible strains of MRSA.     

High-Dose Daptomycin Therapy for Left-Sided Infective Endocarditis: a Prospective Study from the International Collaboration on Endocarditis

The use of daptomycin in Gram-positive left-sided infective endocarditis (IE) has significantly increased. The purpose of this study was to assess the influence of high-dose daptomycin on the outcome of left-sided IE due to Gram-positive pathogens. This was a prospective cohort study based on 1,112 cases from the International Collaboration on Endocarditis (ICE)-Plus database and the ICE-Daptomycin Substudy database from 2008 to 2010. Among patients with left-sided IE due to Staphylococcus aureus, coagulase-negative staphylococci, and Enterococcus faecalis, we compared those treated with daptomycin (cohort A) to those treated with standard-of-care (SOC) antibiotics (cohort B). The primary outcome was in-hospital mortality. Time to clearance of bacteremia, 6-month mortality, and adverse events (AEs) ascribable to daptomycin were also assessed. There were 29 and 149 patients included in cohort A and cohort B, respectively. Baseline comorbidities did not differ between the two cohorts, except for a significantly higher prevalence of diabetes and previous episodes of IE among patients treated with daptomycin. The median daptomycin dose was 9.2 mg/kg of body weight/day. Two-thirds of the patients treated with daptomycin had failed a previous antibiotic regimen. In-hospital and 6-month mortalities were similar in the two cohorts. In cohort A, median time to clearance of methicillin-resistant S. aureus (MRSA) bacteremia was 1.0 day, irrespective of daptomycin dose, representing a significantly faster bacteremia clearance compared to SOC (1.0 versus 5.0 days; P < 0.01). Regimens with higher daptomycin doses were not associated with increased incidence of AEs. In conclusion, higher-dose daptomycin may be an effective and safe alternative to SOC in the treatment of left-sided IE due to common Gram-positive pathogens.     

Antimicrobial susceptibility of Gram-positive bacteria isolated from US medical centers: results of the Daptomycin Surveillance Program (2007–2008)

Gram-positive bacterial strains (12?443) consecutively collected during 2007 to 2008 in hospitals located in the United States were tested by reference broth microdilution methods against daptomycin and comparison agents. Methicillin (oxacillin) resistance rates were 55.9% and 74.0% for Staphylococcus aureus and coagulase-negative staphylococci, respectively, and the vancomycin resistance rate among Enterococcus faecalis and Enterococcus faecium were 5.4% and 75.4%, respectively. Daptomycin was very active against all Gram-positive species with MIC₉₀ values of 0.5, 0.25, 0.5, and 2 μg/mL for staphylococci, β-hemolytic streptococci, viridans group streptococci, and enterococci, respectively. Overall, 99.9% of S. aureus, 100.0% of E. faecalis, and 99.5% of E. faecium were susceptible to daptomycin. In addition, daptomycin MIC distributions for S. aureus and enterococci from 2007 to 2008 were very similar to those from 2002 to 2003. In summary, high rates of methicillin-resistant staphylococci and vancomycin-resistant enterococci were observed in US hospitals, but daptomycin remains active against these clinically important Gram-positive organisms with no evidence of potency loss since its approval for clinical use in late 2003.     

Candida albicans Mucin Msb2 Is a Broad-Range Protectant against Antimicrobial Peptides

The human fungal pathogen Candida albicans releases a large glycofragment of the Msb2 surface protein (Msb2*) into the growth environment, which protects against the action of human antimicrobial peptides (AMPs) LL-37 and histatin-5. Quantitation of Msb2*/LL-37 interactions by microscale thermophoresis revealed high-affinity binding (dissociation constant [K_D] = 73 nM), which was lost or greatly diminished by lack of O-glycosylation or by Msb2* denaturation. Msb2* also interacted with human α- and β-defensins and protected C. albicans against these AMPs. In addition, the lipopeptide antibiotic daptomycin was bound and inactivated by Msb2*, which prevented the killing of bacterial pathogens Staphylococcus aureus, Enterococcus faecalis, and Corynebacterium pseudodiphtheriticum. In coculturings or mixed biofilms of S. aureus with C. albicans wild-type but not msb2 mutant strains, the protective effects of Msb2* on the bactericidal action of daptomycin were demonstrated. These results suggest that tight binding of shed Msb2* to AMPs that occurs during bacterial coinfections with C. albicans compromises antibacterial therapy by inactivating a relevant reserve antibiotic.     

Bactericidal action of daptomycin against stationary-phase and nondividing Staphylococcus aureus cells

Most antibiotics with bactericidal activity require that the bacteria be actively dividing to produce rapid killing. However, in many infections, such as endocarditis, prosthetic joint infections, and infected embedded catheters, the bacteria divide slowly or not at all. Daptomycin is a lipopeptide antibiotic with a distinct mechanism of action that targets the cytoplasmic membrane of gram-positive organisms, including Staphylococcus aureus. Daptomycin is rapidly bactericidal against exponentially growing bacteria (a 3-log reduction in 60 min). The objectives of this study were to determine if daptomycin is bactericidal against nondividing S. aureus and to quantify the extent of the bactericidal activity. In high-inoculum methicillin-sensitive S. aureus cultures in stationary phase (10(10) CFU/ml), daptomycin displayed concentration-dependent bactericidal activity, requiring 32 micro/ml to achieve a 3-log reduction. In a study comparing several antibiotics at 100 microg/ml, daptomycin demonstrated faster bactericidal activity than nafcillin, ciprofloxacin, gentamicin, and vancomycin. In experiments where bacterial cell growth was halted by the metabolic inhibitor carbonyl cyanide m-chlorophenylhydrazone or erythromycin, daptomycin (10 microg/ml) achieved the bactericidal end point (a 3-log reduction) within 2 h. In contrast, ciprofloxacin (10 microg/ml) did not produce bactericidal activity. Daptomycin (2 microg/ml) remained bactericidal against cold-arrested S. aureus, which was protected from the actions of ciprofloxacin and nafcillin. The data presented here suggest that, in contrast to that of other classes of antibiotics, the bactericidal activity of daptomycin does not require cell division or active metabolism, most likely as a consequence of its direct action on the bacterial membrane.     

Failures in clinical treatment of Staphylococcus aureus Infection with daptomycin are associated with alterations in surface charge, membrane phospholipid asymmetry, and drug binding

Increasingly frequent reports have described the in vivo loss of daptomycin susceptibility in association with clinical treatment failures. The mechanism(s) of daptomycin resistance is not well understood. We studied an isogenic set of Staphylococcus aureus isolates from the bloodstream of a daptomycin-treated patient with recalcitrant endocarditis in which serial strains exhibited decreasing susceptibility to daptomycin. Since daptomycin is a membrane-targeting lipopeptide, we compared a number of membrane parameters in the initial blood isolate (parental) with those in subsequent daptomycin-resistant strains obtained during treatment. In comparison to the parental strain, resistant isolates demonstrated (i) enhanced membrane fluidity, (ii) increased translocation of the positively charged phospholipid lysyl-phosphotidylglycerol to the outer membrane leaflet, (iii) increased net positive surface charge (P < 0.05 versus the parental strain), (iv) reduced susceptibility to daptomycin-induced depolarization, permeabilization, and autolysis (P < 0.05 versus the parental strain), (v) significantly lower surface binding of daptomycin (P < 0.05 versus the parental strain), and (vi) increased cross-resistance to the cationic antimicrobial host defense peptides human neutrophil peptide 1 (hNP-1) and thrombin-induced platelet microbicidal protein 1 (tPMP-1). These data link distinct changes in membrane structure and function with in vivo development of daptomycin resistance in S. aureus. Moreover, the cross-resistance to hNP-1 and tPMP-1 may also impact the capacity of these daptomycin-resistant organisms to be cleared from sites of infection, particularly endovascular foci.     

Aminoglycoside-Resistant Mutation of Pseudomonas aeruginosa Defective in Cytochrome c552 and Nitrate Reductase

A gentamicin-resistant mutant of Pseudomonas aeruginosa PAO503 was selected after ethyl methane sulfonate mutagenesis. The strain, P. aeruginosa PAO2401 had increased resistance to all aminoglycosides tested but exhibited no change for other antibiotics. The mutation designated aglA (aminoglycoside resistance) was 50% cotransducible with the 8-min ilvB,C marker on the P. aeruginosa chromosome. It showed a marked reduction in cytochrome c₅₅₂ and nitrate reductase (Nar) and a change in terminal oxidase activity. Cytochrome c₅₅₂ is a component of the P. aeruginosa Nar. No changes in succinate and reduced nicotinamide adenine dinucleotide dehydrogenases, ubiquinone content, Mg²⁺/Ca²⁺ membrane adenosine triphosphatase, and energy coupling of electron transport to adenosine 5′-triphosphate synthesis were detected. Transport of gentamicin and dihydrostreptomycin was impaired in PAO2401, but transport of proline, arginine, glutamine, glucose or the polyamine spermidine was not reduced. Ribosomes of PAO2401, and PAO503 bound dihydrostreptomycin equally well, and cell extracts did not inactivate gentamicin or dihydrostreptomycin. Strain PAO2401 is resistant to gentamicin and dihydrostreptomycin because of impaired transport of these compounds. The transport studies indicate a selective coupling of dihydrostreptomycin and gentamicin transport with terminal electron transport. This conclusion was supported by results from another mutant (PAO417-T2) with increased Nar activity, enhanced dihydrostreptomycin and gentamicin transport and a reduction in resistance to these drugs. These results are discussed in relation to a refined model for aminoglycoside transport and briefly relative to plasmid-mediated aminoglycoside resistance.     

Interactions of neutral gold nanoparticles with DPPC and POPC lipid bilayers: simulation and experiment

Molecular dynamics simulations of neutral gold nanoparticles (AuNPs) interacting with dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes were studied using a model system. Spontaneous membrane insertion of AuNPs did not occur on the time scale of atomistic simulations. To overcome the limitations of time scale, we used a harmonic restraining potential to force the AuNPs into the membranes. Free energy calculations indicate that a NP has to cross a free energy barrier of about 134 kJ mol⁻¹ prior to forming a stable contact with the membrane. This energy barrier between lipids and NPs comes from the repulsion between headgroups of lipids and AuNPs. The experimental investigations indicate that, contrary to hydrophobic AuNPs, neutral AuNPs cannot form ion channels across lipid membranes. The adsorption of NPs induces the formation of a highly ordered region in phospholipid bilayers. Our simulation results propose that the cell penetration of small uncoated AuNPs does not involve energy-independent membrane translocation but rather involves the energy-dependent formation of nanoscale membrane holes or energy-dependent endocytosis.

Molecular dynamics simulations of neutral gold nanoparticles (AuNPs) interacting with dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes were studied using a model system.     

Controlling the Size Distribution of Lipid-Coated Bubbles via Fluidity Regulation

Lipid-coated bubbles exhibit oscillation responses capable of enhancing the sensitivity of ultrasound imaging by improving contrast. Further improvements in performance enhancement require control of the size distribution of bubbles to promote correspondence between their resonance frequency and the frequency of the ultrasound. Here we describe a size-controlling technique that can shift the size distribution using a currently available agitation method. This technique is based on regulating the membrane dynamic fluidity of lipid mixtures and provides a general size-controlling variable that could also be applied in other fabrication methods. Three materials (1,2-dihexadecanoyl-sn-glycero-3-phosphocholine, 1,2-dioctadecanoyl-sn-glycero-3-phospho-(1'-rac-glycerol) and polyethylene glycol 40 stearate) with distinct initial fluidities and phase behaviors were used to demonstrate the use of fluidity regulation to control bubble sizes. Bubble size distributions of different formulations were determined by electrical impedance sensing, and bubble volumes and surface areas were calculated. To confirm the relationship between regulated fluidity and mean bubble size, the membrane fluidity of each composition was determined by fluorescence anisotropy, with the results indicating linear relations in the compositions with similar main transition temperatures. Compositions with a higher molar proportion of polyethylene glycol 40 stearate showed higher fluidities and larger bubbles. B-mode ultrasound imaging was performed to investigate the echogenicity and lifetime of the fabricated bubbles, with the results indicating that co-mixing a high-transition-temperature charged lipid (i.e., 1,2-dioctadecanoyl-sn-glycero-3-phospho-(1'-rac-glycerol)) extends the tailoring range of this fluidity regulation technique, allowing refined and continuous changes in mean bubble size (from 0.93 to 2.86 μm in steps of ∼0.5 μm), and also prolongs bubble lifetime. The polydispersity of each composition was also determined to evaluate practicality in particular applications. Our study demonstrates a feasible approach to naturally controling bubble size distribution and provides a practical reference for other fabrication systems and ultrasound imaging applications.     

Antimicrobial activity of daptomycin tested against Gram-positive pathogens collected in Europe,Latin America,and selected countries in the Asia-Pacific Region (2011)

We report the results of the international daptomycin surveillance programs for Europe, Latin America, and selected Asia-Pacific nations. A total of 7948 consecutive Gram-positive organisms of clinical significance were collected in 2011 and susceptibility tested against daptomycin and various comparator agents by Clinical and Laboratory Standards Institute (Clinical and Laboratory Standards Institute. M07-A9. Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically; approved standard: ninth edition Wayne, PA: CLSI. 2012.; Cubicin Package Insert 2012. Cubist Pharmaceuticals, Inc, Lexington, MA. Available at http://www.cubicin.com/pdf/PrescribingInformation.pdf. Accessed January 1, 2012.) broth microdilution methods. The test medium was adjusted to contain physiological levels of calcium (50 mg/L) when testing daptomycin. Daptomycin exhibited potent activity against methicillin-susceptible and -resistant Staphylococcus aureus overall and for each region (MIC_50/90, 0.25–0.5/0.5 μg/mL), with susceptibility rates at 100.0% in Latin America, Australia/New Zealand, and India, and at 99.9% in Europe. The daptomycin MIC_50/90 for coagulase-negative staphylococci was also at 0.25–0.5/0.5 μg/mL, and only 1 isolate was considered nonsusceptible with a MIC value at 2 μg/mL. Daptomycin was also highly active against Enterococcus faecalis (MIC_50/90, 1/1–2 μg/mL) and E. faecium (MIC_50/90, 2/2 μg/mL for both vancomycin-susceptible and -resistant isolates). All enterococcal isolates were susceptible to daptomycin (MIC, ≤4 μg/mL) and tigecycline. Susceptibility to linezolid for E. faecalis was at 100.0%, while for E. faecium regional susceptibility rates were at 100.0% except in Europe (99.0%). Viridans group streptococci (MIC_50/90, 0.25/1 μg/mL) and β-haemolytic streptococci (MIC_50/90, ≤0.06/0.25 μg/mL) continue to be very susceptible to daptomycin. In summary, the results of this investigation document the high potency and wide spectrum of daptomycin when tested against a large resistance-surveillance collection of Gram-positive pathogens and indicate that daptomycin nonsusceptibility remains rare among indicated species after many years of clinical use worldwide.     

In vitro activity of TD-1792, a multivalent glycopeptide-cephalosporin antibiotic, against 377 strains of anaerobic bacteria and 34 strains of Corynebacterium species

TD-1792 is a multivalent glycopeptide-cephalosporin heterodimer antibiotic with potent activity against Gram-positive bacteria. We tested TD-1792 against 377 anaerobes and 34 strains of Corynebacterium species. Against nearly all Gram-positive strains, TD-1792 had an MIC₉₀ of 0.25 μg/ml and was typically 3 to 7 dilutions more active than vancomycin and daptomycin.