Streptogramin Combination Research Articles

Synergy of streptogramin antibiotics occurs independently of their effects on translation.

Streptogramin antibiotics are divided into types A and B, which in combination can act synergistically. We compared the molecular interactions of the streptogramin combinations Synercid (type A, dalfopristin; type B, quinupristin) and NXL 103 (type A, flopristin; type B, linopristin) with the Escherichia coli 70S ribosome by X-ray crystallography. We further analyzed the activity of the streptogramin components individually and in combination. The streptogramin A and B components in Synercid and NXL 103 exhibit synergistic antimicrobial activity against certain pathogenic bacteria. However, in transcription-coupled translation assays, only combinations that include dalfopristin, the streptogramin A component of Synercid, show synergy. Notably, the diethylaminoethylsulfonyl group in dalfopristin reduces its activity but is the basis for synergy in transcription-coupled translation assays before its rapid hydrolysis from the depsipeptide core. Replacement of the diethylaminoethylsulfonyl group in dalfopristin by a nonhydrolyzable group may therefore be beneficial for synergy. The absence of general streptogramin synergy in transcription-coupled translation assays suggests that the synergistic antimicrobial activity of streptogramins can occur independently of the effects of streptogramin on translation.

In vitro pharmacokinetic/pharmacodynamic activity of NXL103 versus clindamycin and linezolid against clinical Staphylococcus aureus and Streptococcus pyogenes isolates

NXL103 (linopristin/flopristin, 30/70) is a novel oral streptogramin combination with activity against a large variety of multidrug-resistant Gram-positive pathogens. The objective of this study was to evaluate the in vitro activity of NXL103 in comparison with oral comparators (clindamycin and linezolid). Six clinical isolates [four meticillin-resistant Staphylococcus aureus (MRSA) and two Streptococcus pyogenes] were exposed for 48h in an in vitro pharmacokinetic/pharmacodynamic (PK/PD) model at a starting inoculum of ca. 106 colony-forming units (CFU)/mL. Antimicrobial simulations included NXL103 500mg every 12h, linezolid 600mg every 12h and clindamycin 450mg every 6h. Bactericidal and static effects were defined as ≥3log10 and <3log10 CFU/mL kill from the starting inoculum, respectively. Experiments were performed in duplicate to ensure reproducibility, and differences between regimens were evaluated by analysis of variance (ANOVA) with Tukey's post-hoc test. NXL103 exhibited lower minimum inhibitory concentrations than comparators, with values ≤0.06mg/L for S. pyogenes and 0.125–0.25mg/L for MRSA isolates. In the PK/PD model, NXL103 demonstrated significantly better activity than linezolid and clindamycin (P<0.05), achieving sustained bactericidal activity within <2h against S. pyogenes strains and between 7.3–32h against MRSA isolates. In contrast, linezolid only exhibited a static effect, whereas clindamycin achieved 3log10 kill at 6h against the unique clindamycin-susceptible S. pyogenes strain evaluated. In conclusion, at therapeutic concentrations NXL103 exhibits promising activity against both MRSA and S. pyogenes strains, including clindamycin-resistant organisms. Further in vitro and in vivo experiments are warranted to explore the therapeutic benefit of NXL103 for the treatment of Gram-positive skin and soft-tissue infections.

Structural basis for streptogramin B resistance in Staphylococcus aureus by virginiamycin B lyase

The streptogramin combination therapy of quinupristin-dalfopristin (Synercid) is used to treat infections caused by bacterial pathogens, such as methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus faecium. However, the effectiveness of this therapy is being compromised because of an increased incidence of streptogramin resistance. One of the clinically observed mechanisms of resistance is enzymatic inactivation of the type B streptogramins, such as quinupristin, by a streptogramin B lyase, i.e., virginiamycin B lyase (Vgb). The enzyme catalyzes the linearization of the cyclic antibiotic via a cleavage that requires a divalent metal ion. Here, we present crystal structures of Vgb from S. aureus in its apoenzyme form and in complex with quinupristin and Mg2+ at 1.65- and 2.8-A resolution, respectively. The fold of the enzyme is that of a seven-bladed beta-propeller, although the sequence reveals no similarity to other known members of this structural family. Quinupristin binds to a large depression on the surface of the enzyme, where it predominantly forms van der Waals interactions. Validated by site-directed mutagenesis studies, a reaction mechanism is proposed in which the initial abstraction of a proton is facilitated by a Mg2+ -linked conjugated system. Analysis of the Vgb-quinupristin structure and comparison with the complex between quinupristin and its natural target, the 50S ribosomal subunit, reveals features that can be exploited for developing streptogramins that are impervious to Vgb-mediated resistance.

Characterisation of in vitro-selected mutants of Ureaplasma parvum resistant to macrolides and related antibiotics

Resistant mutants of Ureaplasma parvum were selected by serial passages of a susceptible strain in subinhibitory concentrations of different macrolides and related antibiotics (erythromycin, azithromycin, josamycin, quinupristin, quinupristin/dalfopristin, pristinamycin and telithromycin). Mechanisms of resistance were characterised by sequencing portions of genes encoding 23S rRNA and ribosomal proteins L4 and L22. Mutants with significantly increased minimum inhibitory concentrations could be selected with all the selector antibiotics, except quinupristin and pristinamycin. Mutants harboured mutations in domain V of the 23S rRNA gene at nucleotides G2056, G2057 or A2058 ( Escherichia coli numbering) and in conserved portions of ribosomal proteins L4 and L22. Most of the mutations were associated with complete loss of macrolide and ketolide activity, whereas streptogramin combinations were less affected.

Spectrum and potency of dalbavancin tested against 3322 Gram-positive cocci isolated in the United States Surveillance Program (2004)

Dalbavancin is an injectable, next generation lipoglycopeptide with an extended serum elimination half-life. Once-weekly dosing has been successful for treatment of skin and skin structure (SSSI) and catheter-related bloodstream infections (CR-BSI). Concurrent with clinical trails, dalbavancin resistance surveillance was initiated in 2003, and results are reported here for the 2004 United States (USA) component. A total of 3322 Gram-positive cocci were tested by reference broth microdilution methods. Organism species tested included: Staphylococcus aureus (2102; 49% oxacillin-resistant), coagulase-negative staphylococci (CoNS; 255, 82% oxacillin-resistant), β-hemolytic streptococci (241), viridans group streptococci (46), and Streptococcus pneumoniae (678). Dalbavancin (MIC 90,0.06–0.12 μg/mL) was comparable in spectrum, but superior in potency to vancomycin (MIC 90,1–2 μg/mL) against staphylococci. Dalbavancin MIC 90 values against the tested streptococci was 0.03 μg/mL. Dalbavancin was more active against tested SSTI pathogens than comparator agents having complete susceptibility rates (100.0%) similar to vancomycin. Vancomycin, (16- to 32-fold), linezolid (8- to 32-fold), daptomycin (4- to 32-fold), and quinupristin/dalfopristin (4- to 32-fold) were less active than dalbavancin. In conclusion, dalbavancin exhibited greater potency than comparison glycopeptides or lipopeptides, streptogramin combinations, and oxazolidinones against Gram-positive pathogens associated with SSSI or CR-BSI. Dalbavancin wild-type MIC distributions remain unchanged compared with prior sampled years (2002–2003) in the USA.

The need for new therapeutic agents: what is in the pipeline?

There is a clinical need for new treatment options for serious Gram-positive infections. Recently introduced agents such as the newer fluoroquinolones and the ketolide telithromycin have limited use as they do not cover methicillin-resistant Staphylococcus aureus (MRSA) or glycopeptide-resistant enterococci (GRE). The clinical use of the streptogramin combination quinupristin/dalfopristin, which has activity against MRSA and vancomycin-resistant Enterococcus faecium, is limited because administration is via a slow infusion of a large volume. The oxazolidinone linezolid is active against MRSA and GRE but resistant organisms and treatment failures have been reported. A number of compounds currently in development show promise, the new glycopeptides oritavancin, dalbavancin and the glycolipodepsipeptide ramoplanin, as well as the new tetracyclines tigecycline and BAY73-7388. However, in some cases, there is concern that resistance may develop quickly to new compounds that are based on existing antimicrobial agents. Therefore daptomycin, a novel lipopeptide with a unique mode of action, is of particular interest. Daptomycin is active against MRSA (including vancomycin-resistant strains) and GRE. Daptomycin displays rapid concentration-dependent killing and is bactericidal even in the stationary phase of growth. Daptomycin-resistant strains are very difficult to generate in vitro. A dosage of 4 mg/kg intravenous once a day has been shown to be efficacious in two evaluator-blinded trials of complicated skin and soft tissue infections with clinical success rates similar for daptomycin and comparators (vancomycin or penicillinase-resistant penicillins). With its activity against key Gram-positive pathogens, including resistant strains, daptomycin has potential as a valuable addition to the available treatment options for serious Gram-positive infections.

Potential utility of a peptide deformylase inhibitor (NVP PDF-713) against oxazolidinone-resistant or streptogramin-resistant Gram-positive organism isolates.

To evaluate the potency of a novel peptide deformylase inhibitor, NVP PDF-713, against Gram-positive organisms having resistances to linezolid or quinupristin/dalfopristin. A total of 45 strains from three genera (six species groups) were tested by reference broth microdilution methods. The mechanism of resistance to the oxazolidinone was determined by sequencing of the gene encoding the ribosomal target. NVP PDF-713 retained activity against linezolid-resistant staphylococci (MIC range 0.25-2 mg/L), Streptococcus oralis (MIC 0.5 mg/L), Enterococcus faecalis (MIC range 2-4 mg/L) and Enterococcus faecium (MIC range 0.5-4 mg/L). Quinupristin/dalfopristin-resistant E. faecium (MIC range 1-2 mg/L) and staphylococci (MIC range 0.12-2 mg/L) were also inhibited by NVP PDF-713. Many (10 of 13 strains) of the linezolid-resistant enterococci were resistant to vancomycin and these clinical strains had a G2576U ribosomal target mutation. NVP PDF-713 appears to be a promising clinical candidate among the peptide deformylase inhibitors for the treatment of infections caused by Gram-positive organisms that possess resistances to oxazolidinones or streptogramin combinations.

In vitro selection and characterization of resistance to macrolides and related antibiotics in Mycoplasma pneumoniae.

Macrolide-resistant mutants of Mycoplasma pneumoniae were selected in vitro from the susceptible reference strain M129, by 23 to 50 serial passages in subinhibitory concentrations of macrolides and related antibiotics, erythromycin A, azithromycin, josamycin, clindamycin, quinupristin, quinupristin-dalfopristin, pristinamycin, and telithromycin. Mutants for which the MICs are increased could be selected with all antibiotics except the streptogramin B quinupristin. Portions of genes encoding 23S rRNA (domains II and V) and ribosomal proteins L4 and L22 of mutants were amplified by PCR, and their nucleotide sequences were compared to those of the susceptible strain M129. No mutation could be detected in domain II of 23S rRNA. Two point mutations in domain V of 23S rRNA, C2611A and A2062G, were selected in the presence of erythromycin A, azithromycin, josamycin, quinupristin-dalfopristin, and telithromycin. Mutants selected in the presence of clindamycin and telithromycin harbored a single amino acid change (H70R or H70L, respectively) in ribosomal protein L4, whereas insertions of one, two, or three adjacent glycines at position 60 (M. pneumoniae numbering) were selected in the presence of both streptogramin combinations. Telithromycin was the sole antibiotic that selected for substitutions (P112R and A114T) and deletions ((111)IPRA(114)) in ribosomal protein L22. Three sequential mutational events in 23S rRNA and in both ribosomal proteins were required to categorize the strain as resistant to the ketolide. Azithromycin and erythromycin A were the only selector antibiotics that remained active (MICs, 0.06 and 1 micro g/ml, respectively) on their mutants selected after 50 passages.

Newer treatment options for skin and soft tissue infections.

In recent years, serious skin and soft tissue infections (SSTIs) caused by multidrug resistant pathogens have become more common. While the majority of SSTIs are caused by Staphylococcus aureus or beta-haemolytic streptococci that are methicillin/oxacillin susceptible, the emergence of methicillin-resistant and vancomycin-resistant community-acquired and nosocomial Gram-positive pathogens has created a need for different therapeutic agents, such as linezolid, quinupristin/dalfopristin, daptomycin, and newer generation carbapenems and fluoroquinolones. This review focuses on agents presently in clinical development for the treatment of SSTIs caused by Gram-positive pathogens such as staphylococci, streptococci and enterococci including methicillin-resistant S. aureus (MRSA) and vancomycin-resistant enterococci (VRE). Newer-generation carbapenems, such as meropenem and ertapenem, are characterised by a broad-spectrum of activity against Gram-positive and -negative aerobes and anaerobes, and are resistant to hydrolysis by many beta-lactamases. Current-generation fluoroquinolones, such as levofloxacin, moxifloxacin and gatifloxacin, have demonstrated better eradication rates for S. aureus than conventional penicillin and cephalosporins. These antimicrobial agents can be used to treat methicillin-susceptible staphylococcal and streptococcal strains. Oxazolidinones, streptogramin combinations and cyclic lipopeptides have novel mechanisms of action and have been studied in several multinational phase III clinical trials in the treatment of complicated and uncomplicated SSTIs. They possess a broad spectrum of activity against multidrug-resistant pathogens, including MRSA and VRE. Linezolid has been shown to be active against a wide variety of community-acquired and nosocomial antimicrobial-resistant pathogens with comparability to vancomycin, as well as resulting in reduced lengths of hospital stay. Cyclic lipopeptides such as daptomycin have a unique mechanism of action by disruption of bacterial membrane electric potentials with less likelihood for development of cross-resistance. Daptomycin has recently been US FDA approved for the treatment of complicated SSTI. However, rapid development of resistance to some of these newer agents has already been reported and this trend magnifies the importance of further need for effective antimicrobial agents. Several investigational agents, such as dalbavancin, oritavancin and tigecycline, are in advanced stages of development and are likely to proceed to licensing in the next few years. With their long half-lives, these agents have an advantage of less frequent dose administration with more rapid bactericidal activity and less likelihood for development of resistance. However, because of their proven activity against highly resistant organisms, these antibacterial agents should be reserved only for life-threatening situations and/or when resistant pathogens are suspected. Rational antimicrobial use coupled with awareness of infection control measures is paramount to avert the emergence of multidrug-resistant organisms.

Oritavancin and tigecycline: investigational antimicrobials for multidrug-resistant bacteria.

The advent of multidrug-resistant gram-positive aerobes such as Staphylococcus aureus, Streptococcus pneumoniae, and the enterococci, which are resistant to beta-lactams, vancomycin, and a host of other commonly used antimicrobials, has complicated our approach to antibiotic therapy. Despite marketing of the first oxazolidinone, linezolid, and the streptogramin combination, quinupristin-dalfopristin, an urgent need exists for more agents to combat these pathogens. Two such agents, the glycopeptide oritavancin (LY333328) and the glycylcycline tigecycline (GAR-936), are in phase III clinical trials. These agents, which require parenteral administration, exhibit substantial in vitro activity against a variety of gram-positive aerobes and anaerobes, including the multidrug-resistant organisms listed previously. Only tigecycline demonstrates useful activity against gram-negative organisms. Combination therapy of these agents with ampicillin or aminoglycosides frequently leads to synergistic in vitro activity against multidrug-resistant staphylococci and streptococci. These agents are also active in a variety of animal models of systemic and localized infections. Few published efficacy and tolerability data are available in humans. If controlled clinical trial data verify these agents' efficacy and tolerability, both drugs should become welcome additions to the available antimicrobials. However, restricting their use to the treatment of infections caused by bacteria resistant to other antimicrobials, especially multidrug-resistant staphylococci and streptococci, may prolong their clinical utility by retarding the development of resistance. Careful surveillance of bacterial sensitivity to these agents should be undertaken to assist clinicians in the decision whether or not to use these agents empirically to treat infections caused by suspected multidrug-resistant gram-positive pathogens.

Quinupristin-dalfopristin resistance in Streptococcus pneumoniae: novel L22 ribosomal protein mutation in two clinical isolates from the SENTRY antimicrobial surveillance program.

Resistance to quinupristin-dalfopristin (Q/D) among gram-positive cocci has been very uncommon. Two clinical isolates among 8,837 (0.02%) Streptococcus pneumoniae isolates were discovered in 2001 to 2002 with Q/D MICs of 4 micro g/ml. Each had a 5-amino-acid tandem duplication (RTAHI) in the L22 ribosomal protein gene (rplV) preventing synergistic ribosomal binding of the streptogramin combination. Similar gene duplication has been reported in Q/D-resistant Staphylococcus aureus.

Multicenter assessment of the linezolid spectrum and activity using the disk diffusion and Etest methods: report of the Zyvox® Antimicrobial Potency Study in Latin America (LA-ZAPS)

Linezolid was the first clinically applied member of the new antimicrobial class called the oxazolidinones. These agents have a powerful spectrum of activity focussed against Gram-positive organisms including strains with documented resistances to other antimicrobial classes. We conducted a multicenter surveillance (Zyvox Antimicrobial Potency Study; ZAPS) trial of qualifying Gram-positive isolates from 24 medical centers in eight countries in Latin America. The activity and spectrum of linezolid was compared to numerous agents including glycopeptides, quinupristin/dalfopristin, beta-lactams and fluoroquinolones when testing 2,640 strains by the standardized disk diffusion method or Etest (AB BIODISK, Solna, Sweden). The linezolid spectrum was complete against staphylococci (median zone diameter, 29 - 32 mm), as was the spectrum of vancomycin and quinupristin/dalfopristin. Among the enterococci, no linezolid resistance was detected, and the susceptibility rate was 93.1 - 96.4%. Only the vancomycin-susceptible Enterococcus faecium strains remained susceptible (92.8%) to quinupristin/dalfopristin. Marked differences in the glycopeptide resistance patterns (van A versus van B) were noted for the 22 isolates of VRE, thus requiring local susceptibility testing to direct therapy. Streptococcus pneumoniae and other species were very susceptible (100.0%) to linezolid, MIC(90) at 0.75 microg/ml. Penicillin non-susceptible rate was 27.7% and erythromycin resistance was at 17.4%. Other streptococci were also completely susceptible to linezolid (MIC(90), 1 microg/ml). These results provide the initial benchmark of potency and spectrum for linezolid in Latin American medical centers. Future comparisons should recognize that the oxazolidinones possess essentially a complete spectrum coverage of the monitored staphylococci, enterococci and streptococcal isolates in 2000-2001. This positions linezolid as the widest spectrum empiric choice against multi-resistant Gram-positive cocci, a spectrum of activity greater than available glycopeptides and the streptogramin combination.

Quinupristin/dalfopristin

Quinupristin/dalfopristin (Q-D) is a synergistic streptogramin combination (30:70 ratio). It has potent in vitro activity against gram-positive pathogens including vancomycin-resistant Enterococcus faecium (VREF), methicillin-resistant Staphylococcus aureus, and drug-resistant Streptococcus pneumoniae, but not Enterococcus faecalis. A minimum inhibitory concentration of 1.0 μg/mL or less, or a zone size greater than 18 mm, indicates susceptibility. Dosing in the United States (7.5 mg/kg) is every 8 hours for serious or life-threatening VREF infections and every 12 hours for complicated methicillin-susceptible Staphylococcus aureus or Streptococcus pyogenes skin or skin structure infections. To minimize side effects, Q-D is given via a central line for a period of at least 60 minutes. Pediatric use is by compassionate or experimental protocol. Q-D is 80 percent excreted in bile and stool. Q-D use with cytochrome P450 3A4 substrate drugs (eg, cyclosporine A) requires drug concentration monitoring. Avoid concomitant corrected QT interval (QTc)-prolonging drugs metabolized by cytochrome P450 3A4. Efficacy in treating VREF is approximately 70 percent, with underlying patient conditions affecting outcome. Adverse effects include pain and inflammation at the infusion site (~40%), hyperbilirubinemia (~25%), arthralgia and myalgia (~7%), nausea and vomiting (~5%), and diarrhea (~3%). Copyright © 2001 by W.B. Saunders Company

Antimicrobial susceptibility of quinupristin/dalfopristin tested against gram-positive cocci from Latin America: results from the global SMART (GSMART) surveillance study.

Gram-positive cocci are important causes of both nosocomial and community-acquired infections, and antimicrobial resistance among these pathogens has become an important problem worldwide. Since resistance among these organisms can vary substantially by geographic location, we conducted a multicenter surveillance study with isolates from five Latin American countries (15 medical centers). Quinupristin/dalfopristin (formerly RP-59500) is a novel streptogramin combination with focused activity against Gram-positive cocci, many exhibiting emerging resistance. The in vitro activity of quinupristin/dalfopristin and 12 other antimicrobial agents were evaluated against 1,948 strains including Staphylococcus aureus (747 strains), coagulase-negative staphylococci (CoNS;446 strains), enterococci (429 strains), and various Streptococcus spp. (326 strains). Oxacillin resistance was observed in 41% of S. aureus (MIC, <e; 2 microg/ml or >e; 13 mm) and 40% of CoNS (MIC, <e; 0.25 microg/ml or >e; 18 mm). Vancomycin, teicoplanin, and quinupristin/dalfopristin (MIC(90), 0.25 - 1 mg/ml) remained effective against all strains, but cross-resistance was high among other tested drugs. The quinupristin/dalfopristin MIC(50) for Streptococcus pneumoniae and other streptococci was only 0.5 mg/ml (13% to 28% were penicillin-resistant; 12% to 22% were macrolide-resistant). Enterococci demonstrated variable inhibition by quinupristin/dalfopristin depending upon identification and the susceptibility testing method used. The demonstrated quinupristin/dalfopristin activity against Enterococcus faecium was confirmed, but potential species identification errors with various commercial systems continue to confuse susceptibility statistics, even though some strains of E. faecium confirmed by PCR-based or other molecular identification techniques did have quinupristin/dalfopristin MICs of >e; 4 microg/ml. Most important, glycopeptide-resistant enterococci are rapidly emerging in Latin America, and quinupristin/dalfopristin appears active against many of these isolates as well as having potency against nearly all staphylococci and streptococci tested at <e; 2 microg/ml or having a zone diameter of >e; 16 mm. Comparisons to GSMART results from other continents show nearly identical quinupristin/dalfopristin activity for each Gram-positive species tested. These results define the role of quinupristin/dalfopristin in Latin American medical centers and provide a benchmark for future in vitro comparisons.

In vitro activity of quinupristin/dalfopristin in comparison with five antibiotics against worldwide clinical isolates of staphylococci

To evaluate the in vitro activity of quinupristin/dalfopristin (Q/D), a streptogramin combination, in comparison with five antibiotics against worldwide clinical isolates of staphylococci. A multicenter in vitro study was performed using the E test during a period of 3 months (April to June) in 1997 on fresh, clinically significant, non repetitive strains of staphylococci from patients hospitalized in 23 different hospitals in 18 countries tested. A total of 2132 staphylococcal isolates including methicillin resistant (MR), methicillin susceptible (MS) S. aureus (1003 MS, 462 MR), S. epidermidis (169 MS, 251 MR), S. haemolyticus (28 MS, 46 MR), S. hominis (28 MS, 16 MR), and coagulase negative staphylococci (86 MS, 43 MR) were analyzed. Q/D was highly active against all species tested. MIC90 (mg/L) ranged from 0.5 to 2 depending on the species. Strains had MIC < or = 1 mg/L in 97.6%. For S. aureus, S. epidermidis, S. hominis and other coagulase-negative staphylococci no differences in MIC90 were observed for MS or MR. One dilution difference was observed for S. haemolyticus, which overall was the less susceptible species. Erythromycin resistance was observed among 57- 87% of MR-strains and was lower among MS-strains (18-56%). Erythromycin resistance had no or little influence on MIC of Q/D. In comparison to vancomycin, Q/D was two to four times more active. The streptogramin combination Q/D showed an excellent in vitro activity against all staphylococcal species tested regardless of the resistance pattern to other drug classes, particularly resistance to methicillin. Q/D was two to four times more active than vancomycin and MIC values varied from 0.5-2 according to the species. The synergy of Q/D was well conserved in macrolide-resistant strains.

Evaluation of quinupristin/dalfopristin (Synercid®) and RPR 106 972 stability in susceptibility testing media

In response to conflicting reports on the chemical stability of quinupristin/dalfopristin, a study was designed to assess the in vitro longevity and effects of media and storage conditions on this streptogramin combination. Broth microdilution trays containing parenteral (quinupristin/dalfopristin) and oral (RPR 106 972) streptogramin combinations as well as pristinomycin components (P-I and P-II) were preincubated at 35°C for 12–72 h before inoculation with control strains ( Streptococcus pneumoniae ATCC 49 619, Haemophilus influenzae ATCC 49 247, Enterococcus faecalis ATCC 29 212, Staphylococcus aureus ATCC 29 213) and five clinical isolates with various drug resistance phenotypes. Overall, the mean quinupristin/dalfopristin activity loss was 24%/12 h, 41%/18 h, 43%/24 h, 69%/48 h and 79%/72 h with no detected loss of potency when measured by E. faecalis until 18 h. RPR 106 972 mean activity loss was 6%/12 h, 19%/18 h, 19%/24 h, 56%/48 h and 71%/72 h with no loss of potency as measured by S. aureus until 48 h. Overall, P-I components had greater stability as compared with P-II for both drug combinations. Bioassays showed similar trends in decreased activity. Bioassay differences among media types were only significant (>3 mm; greater loss of potency) for haemophilus test media for both P-II components at 72 h. The presence of an organism in the medium had no effect on stability assay results. The effect of storage temperature (4, 25°C) on quinupristin/dalfopristin and RPR 106 972 stability was also detrimental to drug potency indicating the requirement for rigid quality assurance for streptogramin diagnostic reagents when determining activity by reference or standardized susceptibility tests.

Frequency of occurrence and antimicrobial susceptibility patterns for pathogens isolated from Latin American patients with a diagnosis of pneumonia: results from the SENTRY antimicrobial surveillance program (1998)

The correct empiric choice of antimicrobial therapy in the treatment of pneumonia in hospitalized patients has established itself as a major therapeutic challenge to clinicians. Selection of an inappropriate antimicrobial agent could lead to increased rates of mortality and morbidity. Characteristics of pathogens responsible for this infection such as species prevalence, overall antimicrobial resistance rates, and mechanisms of detected resistance could serve as an invaluable resource to clinicians in making such therapeutic selections. This report addresses the aforementioned problems/needs by analysis of 712 strains isolated from the lower respiratory tract of patients hospitalized with a diagnosis of pneumonia in 10 Latin American medical centers in the SENTRY Antimicrobial Surveillance Program (1998). The four most frequently isolated pathogens (no/% of total) were: Pseudomonas aeruginosa (191/26.8%), Staphylococcus aureus (171/24.0%), Klebsiella spp. (86/12.1%), and Acinetobacter spp. (75/10.5%); representing nearly 75.0% of all isolates. More than 40 antimicrobial agents (23 reported) were tested against these isolates by reference broth microdilution methodology, and susceptibility profiles were established. The nonfermentative Gram-negative bacteria ( P. aeruginosa and Acinetobacter spp.) exhibited high levels of resistance to the agents tested. Amikacin (77.5% susceptible) was the most active drug tested against P. aeruginosa, followed by piperacillin/tazobactam (73.3% susceptible) > meropenem (72.8%) > imipenem (71.7%) as the only antimicrobials possessing a susceptibility rate of >70.0%. Only the carbapenem class compounds, imipenem (81.3% susceptible) and meropenem (78.3% susceptible) possessed susceptibility rates >50.0% against the Acinetobacter spp. isolates. Based on published interpretive criteria, over 22.0% of the Klebsiella spp. and 12.5% of the Escherichia coli were classified as extended spectrum β-lactamase (ESBL) producers. Of the cephalosporin class compounds tested against the Klebsiella spp. and E. coli isolates, cefepime demonstrated the highest rates of susceptibility (84.9% and 91.7%, respectively). This compound also fared well against the Enterobacter spp. isolates, inhibiting 88.2% of the isolates tested, many of which were resistant to ceftazidime and ceftriaxone. Resistance to oxacillin among the S. aureus isolates was nearly 50.0%, with vancomycin, teicoplanin, and the streptogramin combination quinupristin/dalfopristin inhibiting all isolates. Several clusters of multiply resistant organisms were also observed, and further characterization by ribotyping and pulsed-field gel electrophoresis established possible patient-to-patient spread. The results of this study indicate that rates of resistance among respiratory tract pathogens continue to rise in Latin America, with specific concerns for the high prevalence of nonfermentative Gram-negative bacteria isolated, oxacillin resistance rates in S. aureus, and the epidemic dissemination of multiply-resistant strains in several medical centers. International surveillance programs (SENTRY) should assist in the control of escalating antimicrobial resistance in this geographic area.

Serious streptococcal infections produced by isolates resistant to streptogramins (quinupristin/dalfopristin): Case reports from the SENTRY antimicrobial surveillance program

The emergence and sustained prevalence of Gram-positive organisms resistant to antimicrobials has been of interest for over a decade. Quinupristin/dalfopristin (formerly RP 59500 or Synercid®) is a new injectable streptogramin combination that has been reported to have activity against Gram-positive organisms, even those with documented MLS B resistance. However, the two case reports presented here illustrate three well-documented Streptococcus spp. strains ( S. mitis, S. pneumoniae) to be resistant to quinupristin/dalfopristin (MICs at 3, 8, and 12 μg/ml) following referral as routine isolates in the SENTRY Antimicrobial Surveillance Program. The S. pneumoniae pleural fluid isolate was cross-resistant to erythromycin. Both bacteremic S. mitis strains were resistant to macrolides (erythromycin, azithromycin, clarithromycin), lincosamides (clindamycin), and fluoroquinolones. Patient histories indicated no prior use of MLS class antimicrobials for the S. mitis case, but the patient having the S. pneumoniae isolate did receive prior treatment of erythromycin and clindamycin. All isolates had modestly increased penicillin MICs of 0.12 μg/ml. The mode of resistance to quinupristin/dalfopristin was not evident ( sat A-negative by PCR); and these cases illustrate the existence of streptogramin-resistant isolates before the introduction of this antimicrobial class into human clinical practice.

Quinupristin/dalfopristin-resistant enterococci of the satA (vatD) and satG (vatE) genotypes from different ecological origins in Germany.

The semisynthetic streptogramin combination quinupristin/dalfopristin (Synercid) is a promising alternative for treatment of infections due to multiply resistant gram-positive bacteria including vancomycin-resistant Enterococcus faecium. Resistance is mediated by acetyltransferases SatA (VatD) or SatG (VatE). Recent papers have indicated a possible link between the use of the streptogramin virginiamycin S/M as a feed additive in commercial animal husbandry and a selection of quinupristin/dalfopristin-resistant E. faecium (QDRE). We screened manure samples from two different turkey farms and from six different pig farms (using virginiamycin), samples from a sewage water treatment plant, 24 broiler carcasses, 10 pork samples, and 200 stool samples of nonhospitalized humans for QDRE. Our strain culture collection of hospital E. faecium isolates from the last 2 years was also reviewed for QDRE. All manure and sewage samples were positive for QDRE, as well as 11 from broiler carcasses (46%), 1 from pork (10%), and 28 from human stool specimens (14%). Thirty-six hospital isolates of E. faecium exhibited resistance to quinupristin/dalfopristin. In 141 QDRE of different origin satA (vatD) and satG (vatE) genes were detected (seven isolates from humans with an unknown resistance mechanism). Streptogramin resistance determinants were tansferable in filtermating experiments for 5 of 10 satA (vatD) and 9 of 22 satG (vatE) isolates. Different EcoRI patterns of satG (vatE) plasmids and corresponding hybridizations of the satG (vatE) gene indicated nonhomologous resistance plasmids in isolates of different origin. The results of this study indicate a common gene pool for streptogramin resistance in E. faecium of different ecological origin. A selection of QDRE using the streptogramin virginiamycin S/M as a feed additive and a spread of the resistance via the food chain to humans is probable.

Effects of genes encoding resistance to streptogramins A and B on the activity of quinupristin-dalfopristin against Enterococcus faecium.

Quinupristin-dalfopristin is a streptogramin combination active against multiply resistant Enterococcus faecium. Among 45 E. faecium isolated from patients in various French hospitals, only two strains were intermediate (MIC = 2 microgram/ml) and one, E. faecium HM1032, was resistant (MIC = 16 microgram/ml) to quinupristin-dalfopristin, according to British Society for Antimicrobial Chemotherapy and National Committee for Clinical Laboratory Standards approved breakpoints. The latter strain contained the vgb and satA genes responsible for hydrolysis or acetylation of quinupristin and dalfopristin, respectively, and an ermB gene (also previously referred to as ermAM) encoding a ribosomal methylase. The two intermediate strains had an LS(A) phenotype characterized by resistance to lincomycin (L), increased MICs (>/=8 microgram/ml) of dalfopristin (streptogramin A [S(A)]), and susceptibility to erythromycin and quinupristin. This phenotype was also detected in eight other strains susceptible to quinupristin-dalfopristin. No genes already known and conferring resistance to dalfopristin by acetylation or active efflux were detected in these LS(A) strains. Nineteen other strains resistant to erythromycin but susceptible to the quinupristin-dalfopristin combination displayed elevated MICs of quinupristin after induction (from 16 to >128 microgram/ml) and contained ermB genes. The effects of ermB, vgb, and satA genes on the activity of the streptogramin combination were tested by cloning these genes individually or in various combinations in recipient strains susceptible to quinupristin-dalfopristin, E. faecium HM1070 and Staphylococcus aureus RN4220. The presence of both the satA and vgb genes (regardless of the presence of an ermB gene) was necessary to confer full quinupristin-dalfopristin resistance to the host. The same genetic constructs were introduced into E. faecium BM4107 which displays a LS(A) phenotype. Addition of the satA or vgb gene to this LS(A) background conferred resistance to quinupristin-dalfopristin.