The streptogramin antibiotics: update on their mechanism of action

Total Synthesis Facilitates In Vitro Reconstitution of the C-S Bond-Forming P450 in Griseoviridin Biosynthesis

Griseoviridin is a group A streptogramin natural product from Streptomyces with broad-spectrum antibacterial activity. A hybrid polyketide-nonribosomal peptide, it comprises a 23-membered macrocycle, an embedded oxazole motif, and a macrolactone with a unique ene-thiol linkage. Recent analysis of the griseoviridin biosynthetic gene cluster implicated SgvP, a cytochrome P450 monooxygenase, in late-stage installation of the critical C-S bond. While genetic and crystallographic experiments provided indirect evidence to support this hypothesis, the exact function of SgvP has never been confirmed biochemically. Herein, we report a convergent total synthesis of pre-griseoviridin, the putative substrate of P450 SgvP and precursor to griseoviridin. Our strategy features concise and rapid assembly of two fragments joined via sequential peptide coupling and Stille macrocyclization. Access to pre-griseoviridin then enabled in vitro validation of SgvP as the C-S bond-forming P450 during griseoviridin biosynthesis, culminating in a nine-step chemoenzymatic synthesis of griseoviridin.

Decoding antimicrobial resistance: unraveling molecular mechanisms and targeted strategies

Antimicrobial resistance poses a significant global health threat, necessitating innovative approaches for combatting it. This review explores various mechanisms of antimicrobial resistance observed in various strains of bacteria. We examine various strategies, including antimicrobial peptides (AMPs), novel antimicrobial materials, drug delivery systems, vaccines, antibody therapies, and non-traditional antibiotic treatments. Through a comprehensive literature review, the efficacy and challenges of these strategies are evaluated. Findings reveal the potential of AMPs in combating resistance due to their unique mechanisms and lower propensity for resistance development. Additionally, novel drug delivery systems, such as nanoparticles, show promise in enhancing antibiotic efficacy and overcoming resistance mechanisms. Vaccines and antibody therapies offer preventive measures, although challenges exist in their development. Non-traditional antibiotic treatments, including CRISPR-Cas systems, present alternative approaches to combat resistance. Overall, this review underscores the importance of multifaceted strategies and coordinated global efforts to address antimicrobial resistance effectively.

Targeting Iron - Respiratory Reciprocity Promotes Bacterial Death

Discovering new bacterial signaling pathways offers unique antibiotic strategies. Here, through an unbiased resistance screen of 3,884 gene knockout strains, we uncovered a previously unknown non-lytic bactericidal mechanism that sequentially couples three transporters and downstream transcription to lethally suppress respiration of the highly virulent P. aeruginosa strain PA14 - one of three species on the WHO's 'Priority 1: Critical' list. By targeting outer membrane YaiW, cationic lacritin peptide 'N-104' translocates into the periplasm where it ligates outer loops 4 and 2 of the inner membrane transporters FeoB and PotH, respectively, to suppress both ferrous iron and polyamine uptake. This broadly shuts down transcription of many biofilm-associated genes, including ferrous iron-dependent TauD and ExbB1. The mechanism is innate to the surface of the eye and is enhanced by synergistic coupling with thrombin peptide GKY20. This is the first example of an inhibitor of multiple bacterial transporters.

Molecular mechanisms of antibiotic resistance revisited

Antibiotic resistance is a global health emergency, with resistance detected to all antibiotics currently in clinical use and only a few novel drugs in the pipeline. Understanding the molecular mechanisms that bacteria use to resist the action of antimicrobials is critical to recognize global patterns of resistance and to improve the use of current drugs, as well as for the design of new drugs less susceptible to resistance development and novel strategies to combat resistance. In this Review, we explore recent advances in understanding how resistance genes contribute to the biology of the host, new structural details of relevant molecular events underpinning resistance, the identification of new resistance gene families and the interactions between different resistance mechanisms. Finally, we discuss how we can use this information to develop the next generation of antimicrobial therapies.

The relationship between target-class and the physicochemical properties of antibacterial drugs

The discovery of novel mechanism of action (MOA) antibacterials has been associated with the concept that antibacterial drugs occupy a differentiated region of physicochemical space compared to human-targeted drugs. With, in broad terms, antibacterials having higher molecular weight, lower logP and higher polar surface area (PSA). By analysing the physicochemical properties of about 1700 approved drugs listed in the ChEMBL database, we show, that antibacterials for whose targets are riboproteins (i.e., composed of a complex of RNA and protein) fall outside the conventional human 'drug-like' chemical space; whereas antibacterials that modulate bacterial protein targets, generally comply with the 'rule-of-five' guidelines for classical oral human drugs. Our analysis suggests a strong target-class association for antibacterials-either protein-targeted or riboprotein-targeted. There is much discussion in the literature on the failure of screening approaches to deliver novel antibacterial lead series, and linkage of this poor success rate for antibacterials with the chemical space properties of screening collections. Our analysis suggests that consideration of target-class may be an underappreciated factor in antibacterial lead discovery, and that in fact bacterial protein-targets may well have similar binding site characteristics to human protein targets, and questions the assumption that larger, more polar compounds are a key part of successful future antibacterial discovery.

Susceptibility to quinupristin-dalfopristin and linezolid in 839 clinical isolates of gram-positive cocci from Norway

A total of 839 clinical isolates of Gram-positive cocci from Norway including Staphylococcus aureus (n = 214), coagulase negative Staphylococcus spp. (n = 100), Streptococcus pyogenes (n = 99), Streptococcus agalactiae (n = 80), Streptococcus pneumoniae (n = 127), Streptococcus spp. viridans group (n = 70), Enterococcus faecalis (n = 75), and Enterococcus faecium (n = 74), were tested by E-test for susceptibility to a range of antimicrobials including the novel antibiotics quinupristin-dalfopristin and linezolid. Subgroups of oxacillin resistant S. aureus and coagulase negative Staphylococcus spp., penicillin non-susceptible S. pneumoniae and vancomycin resistant Enterococcus spp. were specifically included as they are the intended targets for these new drugs. All isolates were susceptible to linezolid (MIC5o and MIC9o 0.25-2.0 mg/l, MIC range 0.12-2 mg/l). Staphylococcal and streptococcal isolates were also susceptible to quinupristin-dalfopristin except for some intermediately susceptible viridans group isolates (MIC54, and MIC90 0.25-2 mg/l, MIC range 0.125-2 mg/l). Enterococcus faecium (MIC90 = 4.0 mg/l) and Enterococcus faecalis (MIC50 = 8.0 mg/l, MIC90 > or = 32 mg/l) were less susceptible to this substance. There was no linkage between reduced susceptibility to linezolid or quinupristin-dalfopristin and resistance to other classes of antimicrobials. The study demonstrated a high prevalence of in vitro susceptibility to linezolid and quinupristin-dalfopristin, which is necessary for their use in the treatment of infections with resistant Gram-positive pathogens. The results were used to evaluate the appropriateness of breakpoints and to define a baseline for monitoring possible future emergence of resistance to quinupristin-dalfopristin and linezolid in Norway.

Will longer antimicrobial patents improve global public health?

The problem of antimicrobial resistance has led some infectious disease experts and their professional societies to propose the use of transferable intellectual property rights (wildcard patents) and patent term extensions as methods to encourage antimicrobial research and development. We evaluate recent approvals of new antimicrobial classes and find that the number of new introductions is higher than previously suggested. More importantly, creating new patent rights is shown to be an inefficient and possibly counterproductive response to antimicrobial resistance. Wildcard patents would operate as a more than US$40 billion annual tax on heart disease, hypertension, chronic obstructive pulmonary disease, asthma, and depression to inefficiently cross-subsidise antimicrobial research and development. Patent term extensions would likewise cost more than $5 billion per year, hinder access, and allocate resources inefficiently. Alternative uses for these funds are proposed, with greater potential positive effects on global public health. Additional public funding of antimicrobial research could be a more effective use of these funds. Conservation efforts to encourage the prudent use of antimicrobial drugs should be directly reimbursed. Patent owners should be compensated for both conservation efforts and valuable innovation.

Susceptibility to daptomycin, quinupristin-dalfopristin and linezolid and some other antibiotics in clinical isolates of methicillin resistant and methicillin sensitive S.aureus from the Oslo area

Our study compared the susceptibility of 136 clinical isolates of Staphylococcus aureus and 119 multidrug-resistant Staphylococcus aureus (MRSA) isolates from Oslo to a range of antibiotics, including the novel antibiotics quinupristin-dalfopristin, linezolid and daptomycin. All isolates were susceptible to daptomycin, linezolid and quinupristin-dalfopristin, although a subgroup was less susceptible to the latter. There was no linkage between reduced susceptibility to daptomycin, linezolid or quinupristin-dalfopristin and resistance to other classes of antimicrobials. In addition, MRSA strains from 2004 have become more sensitive to fucidin and rifampicin. The results can be used to evaluate the appropriateness of breakpoints and to define a baseline for monitoring possible future emergence of resistance to daptomycin, quinupristin-dalfopristin and linezolid in Staphylococcus aureus in Norway.

Quinupristin-dalfopristin nonsusceptibility in pneumococci from sickle cell disease patients

Sickle cell disease (SCD) is a risk factor for fatal pneumococcal infection. Nonsusceptibilty to quinupristin-dalfopristin (Q-D) was absent from 105 non-SCD-associated pneumococcal isolates but was present in 33/148 (22%) SCD-associated isolates. One-third of the isolates harbored a known resistance mechanism. Q-D is not optimal for use for the treatment of pneumococcal infection in SCD patients.

Pharmacoeconomics of treatment with the newer anti-Gram-positive agents

The unmet medical need of emerging resistance among Gram-positive pathogens, such as methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci and penicillin-resistant Streptococcus pneumoniae, has driven industry towards the identification and development of novel anti-Gram-positive agents. Among the newer agents are improved quinolones, a lipopeptide, an oxazolidinone and novel glycopeptides. Scientific distinctions between these drugs, which impact on the placement, usage and, ultimately, the pharmacoeconomics of several of these new agents, may lead to further consideration despite poor initial observations of minimal improvement. Key differences in the characteristics of these drugs (i.e., spectrum, activity, resistance emergence, efficacy, target, safety) provide a basis for an emerging pharmacoeconomic-based distinction between these newer anti-Gram-positive agents.

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.

Examining unmet needs in infectious disease

In the past 30 years, more than 30 new aetiological agents of infectious disease have been identified. Some of these are responsible for entirely novel and life-threatening disorders, such as AIDS, Ebola fever, hantavirus pulmonary syndrome and Nipah virus encephalitis. During the same period, some longstanding infectious diseases (such as tuberculosis) have became resurgent, as a result of a combination of complacency, increased travel and social dislocation, and also increasing drug resistance. This review looks at some of the key unmet needs in this therapeutic area and discusses strategies to address them.

New inhibitors of bacterial protein synthesis from a combinatorial library of macrocycles

A mixture-based combinatorial library of 14-membered macrocycles was synthesized to target ribosomal RNA and uncover a new class of antibacterial agents. High-throughput screening identified a macrocyclic mixture that inhibited cell-free-coupled transcription/translation in Escherichia coli-derived extracts, with an IC(50) value in the 25-50 microM range. In a follow-up library of 64 single macrocycles, 8 gave IC(50) values ranging from 12 to 50 microM in the cell-free protein synthesis inhibition assay. Some of the macrocycles were screened in a translation inhibition assay, and IC(50) values generally paralleled those obtained in the uncoupled transcription/translation assay. Additional analogues were prepared in a preliminary structure-activity relationship study, and more potent macrocycles were identified with low micromolar activity (IC(50) values = 2-3 microM). Some of these macrocycles displayed antibacterial activity against lipopolysaccharide mutant E. coli bacterial cells (IC(50) values = 12-50 microM).

Molecular analysis of streptogramin resistance in enterococci

The new semi-synthetic streptogramin antibiotic combination quinupristin/dalfopristin (Synercid) is a promising alternative for a treatment of infections with multiple resistant gram-positive pathogens, e.g. glycopeptide- and multi-resistant Enterococcus faecium. Streptogramins consist of two unrelated compounds, a streptogramin A and B, which act synergistically when given in combination. Mechanisms conferring resistance against both components are essential for resistance against the combination in E. faecium. In this species resistance to streptogramin A compounds is mediated via related acetyltransferases VatD and VatE. Resistance against streptogramins B is either encoded by the widespread ermB gene cluster conferring resistance to macrolide-lincosamide-streptogramin B antibiotics or via expression of the vgbA gene, which encodes a staphylococcal-type lactonase. E. faecalis is intrinsically resistant to streptogramins. Due to a wide use of streptogramins (virginiamycins S/M) in commercial animal farming a reservoir of streptogramin-resistant E. faecium isolates had already been selected. Determinants for streptogramin resistance are localized on plasmids that can be transferred into an E. faecium recipient both in vitro in filter-matings and in vivo in the digestive tracts of rats. Hybridization and sequencing experiments revealed a linkage of resistance determinants for streptogramins A and B on definite plasmid fragments.

Resistance to quinupristin-dalfopristin due to mutation of L22 ribosomal protein in Staphylococcus aureus

The mechanism of resistance to the streptogramin antibiotics quinupristin and dalfopristin was studied in a Staphylococcus aureus clinical isolate selected under quinupristin-dalfopristin therapy, in four derivatives of S. aureus RN4220 selected in vitro, and in a mutant selected in a model of rabbit aortic endocarditis. For all strains the MICs of erythromycin, quinupristin, and quinupristin-dalfopristin were higher than those for the parental strains but the MICs of dalfopristin and lincomycin were similar. Portions of genes for domains II and V of 23S rRNA and the genes for ribosomal proteins L4 and L22 were amplified and sequenced. All mutants contained insertions or deletions in a protruding beta hairpin that is part of the conserved C terminus of the L22 protein and that interacts with 23S rRNA. Susceptible S. aureus RN4220 was transformed with plasmid DNA encoding the L22 alteration, resulting in transformants that were erythromycin and quinupristin resistant. Synergistic ribosomal binding of streptogramins A and B, studied by analyzing the fluorescence kinetics of pristinamycin I(A)-ribosome complexes, was abolished in the mutant strain, providing an explanation for quinupristin-dalfopristin resistance.

Treatment options for vancomycin-resistant enterococcal infections

Serious infection with vancomycin-resistant enterococci (VRE) usually occurs in patients with significantly compromised host defences and serious co-morbidities, and this magnifies the importance of effective antimicrobial treatment. Assessments of antibacterial efficacy against VRE have been hampered by the lack of a comparator treatment arm(s), complex treatment requirements including surgery, and advanced illness-severity associated with a high crude mortality. Treatment options include available agents which don't have a specific VRE approval (chloramphenicol, doxycycline, high-dose ampicillin or ampicillin/sulbactam), and nitrofurantoin (for lower urinary tract infection). The role of antimicrobial combinations that have shown in vitro or animal-model in vivo efficacy has yet to be established. Two novel antimicrobial agents (quinupristin/ dalfopristin and linezolid) have emerged as approved therapeutic options for vancomycin-resistant Enterococcus faecium on the basis of in vitro susceptibility and clinical efficacy from multicentre, pharmaceutical company-sponsored clinical trials. Quinupristin/dalfopristin is a streptogramin, which impairs bacterial protein synthesis at both early peptide chain elongation and late peptide chain extrusion steps. It has bacteriostatic activity against vancomycin-resistant E. faecium [minimum concentration to inhibit growth of 90% of isolates (MIC(90)) = 2 microg/ml] but is not active against Enterococcus faecalis (MIC(90 )= 16 microg/ml). In a noncomparative, nonblind, emergency-use programme in patients who were infected with Gram-positive isolates resistant or refractory to conventional therapy or who were intolerant of conventional therapy, quinupristin/dalfopristin was administered at 7.5 mg/kg every 8 hours. The clinical response rate in the bacteriologically evaluable subset was 70.5%, and a 65.8% overall response (favourable clinical and bacteriological outcome) was observed. Resistance to quinupristin/dalfopristin on therapy was observed in 6/338 (1.8%) of VRE strains. Myalgia/arthralgia was the most frequent treatment-limiting adverse effect. In vitro studies which combine quinupristin/dalfopristin with ampicillin or doxycyline have shown enhanced killing effects against VRE; however, the clinical use of combined therapy remains unestablished. Linezolid, an oxazolidinone compound that acts by inhibiting the bacterial pre-translational initiation complex formation, has bacteriostatic activity against both vancomycin resistant E. faecium (MIC(90) = 2 to 4 microg/ml) and E. faecalis (MIC(90) = 2 to 4 microg/ml). This agent was studied in a similar emergency use protocol for multi-resistant Gram-positive infections. 55 of 133 evaluable patients were infected with VRE. Cure rates for the most common sites were complicated skin and soft tissue 87.5% (7/8), primary bacteraemia 90.9% (10/11), peritonitis 91.7% (11/12), other abdominal/pelvic infections 91.7% (11/12), and catheter-related bacteraemia 100% (9/9). There was an all-site response rate of 92.6% (50/54). In a separate blinded, randomised, multicentre trial for VRE infection at a variety of sites, intravenous low dose linezolid (200mg every 12 hours) was compared to high dose therapy (600 mg every 12 hours) with optional conversion to oral administration. A positive dose response (although statistically nonsignificant) was seen with a 67% (39/58) and 52% (24/46) cure rate in the high- and low-dose groups, respectively. Adverse effects of linezolid therapy have been predominantly gastrointestinal (nausea, vomiting, diarrhoea), headache and taste alteration. Reports of thrombocytopenia appear to be limited to patients receiving somewhat longer courses of treatment (>14 to 21 days). Linezolid resistance (MIC > or = 8 microg/ml) has been reported in a small number of E. faecium strains which appears to be secondary to a base-pair mutation in the genome encoding for the bacterial 23S ribosome binding site. At present a comparative study between the two approved agents for VRE (quinupristin/dalfopristin and linezolid) has not been performed. Several investigational agents are currently in phase II or III trials for VRE infection. This category includes daptomycin (an acidic lipopeptide), oritavancin (LY-333328; a glycopeptide), and tigilcycline (GAR-936; a novel analogue of minocycline). Finally, strategies to suppress or eradicate the VRE intestinal reservoir have been reported for the combination of oral doxycyline plus bacitracin and oral ramoplanin (a novel glycolipodepsipeptide). If successful, a likely application of such an approach is the reduction of VRE infection during high risk periods in high risk patient groups such as the post-chemotherapy neutropenic nadir or early post-solid abdominal organ transplantation.

Molecular detection of antimicrobial resistance

The determination of antimicrobial susceptibility of a clinical isolate, especially with increasing resistance, is often crucial for the optimal antimicrobial therapy of infected patients. Nucleic acid-based assays for the detection of resistance may offer advantages over phenotypic assays. Examples are the detection of the methicillin resistance-encoding mecA gene in staphylococci, rifampin resistance in Mycobacterium tuberculosis, and the spread of resistance determinants across the globe. However, molecular assays for the detection of resistance have a number of limitations. New resistance mechanisms may be missed, and in some cases the number of different genes makes generating an assay too costly to compete with phenotypic assays. In addition, proper quality control for molecular assays poses a problem for many laboratories, and this results in questionable results at best. The development of new molecular techniques, e.g., PCR using molecular beacons and DNA chips, expands the possibilities for monitoring resistance. Although molecular techniques for the detection of antimicrobial resistance clearly are winning a place in routine diagnostics, phenotypic assays are still the method of choice for most resistance determinations. In this review, we describe the applications of molecular techniques for the detection of antimicrobial resistance and the current state of the art.

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

OBJECTIVE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Linkage of determinants for streptogramin A, macrolide-lincosamide-streptogramin B, and chloramphenicol resistance on a conjugative plasmid in Enterococcus faecium and dissemination of this cluster among streptogramin-resistant enterococci

A new streptogramin A resistance gene, satG (= vatE), has been recently identified in Enterococcus faecium UW1965 (Werner and Witte 1999. Antimicrob. Agents Chemother. 43: 1813-1814). Further sequence analysis of this plasmid revealed that vatE is in a cluster together with other resistance genes. The identified ORFs were nearly identical with the already known genes ermB and cat. The ermB fragment exhibited more than 99% identity with a resistance region from the streptococcal plasmid pIP501, whereas the cat fragment also contained a truncated rep gene homologue with more than 99% identity to sequences in small staphylococcal plasmids. The cat-rep and the ermB-vatE segments were linked by an IS1216V insertion sequence widely distributed among enterococci. PCR analysis of additional 76 streptogramin-resistant isolates possessing vatE and ermB revealed a linkage of both genes in 45 isolates (59%); 15 of them with a gene arrangement, cat-repU-IS1216V-ermB-vatE, identical to the reference strain UW1965. An identical linkage of IS1216V-ermB-vatE was found among isolates from poultry manure, poultry meat, stool samples of humans, and hospital patients indicating a possible spread of the resistance gene cluster via the food chain to humans.

Synergy and duality in peptide antibiotic mechanisms

The molecular mechanisms by which peptide antibiotics disrupt bacterial DNA synthesis, protein biosynthesis, cell wall biosynthesis, and membrane integrity are diverse, yet historically have been understood to follow a theme of one antibiotic, one inhibitory mechanism. In the past year, mechanistic and structural studies have shown a rich diversity in peptide antibiotic mechanism. Novel secondary targeting mechanisms for peptide antibiotics have recently been discovered, and the mechanisms of peptide antibiotics involved in synergistic relationships with antibiotics and proteins have been more clearly defined. In apparent response to selective pressures, antibiotic-producing organisms have elegantly integrated multiple functions and cooperative interactions into peptide antibiotic design for the purpose of improving antimicrobial success.

Quinupristin/dalfopristin: a review of its use in the management of serious gram-positive infections

Quinupristin/dalfopristin is the first parenteral streptogramin antibacterial agent, and is a 30:70 (w/w) ratio of 2 semisynthetic pristinamycin derivatives. The combination has inhibitory activity against a broad range of gram-positive bacteria including methicillin-resistant staphylococci, vancomycin-resistant Enterococcus faecium (VREF), drug-resistant Streptococcus pneumoniae, other streptococci, Clostridium perfringens and Peptostreptococcus spp. The combination also has good activity against selected gram-negative respiratory tract pathogens including Moraxella catarrhalis, Legioniella pneumophila and Mycoplasma pneumoniae. Quinupristin/dalfopristin has poor activity against E. faecalis. The combination is bactericidal against staphylococci and streptococci, although constitutive erythromycin resistance can affect its activity. As for many other agents, quinupristin/dalfopristin is generally bacteriostatic against E. faecium. In patients with methicillin-resistant S. aureus (MRSA) or VREF infections participating in prospective emergency-use trials, quinupristin/dalfopristin 7.5 mg/kg every 8 or 12 hours achieved clinical or bacteriological success in > or =64% of patients. Emergence of resistance to quinupristin/dalfopristin was uncommon (4% of patients) in those with VREF infections. Quinupristin/dalfopristin 7.5 mg/kg 8- or 12-hourly also achieved similar clinical success rates to comparator agents in patients with presumed gram-positive complicated skin and skin structure infections or nosocomial pneumonia (administered in combination with aztreoman) in 3 large multicentre randomised trials. Systemic adverse events associated with quinupristin/dalfopristin include gastrointestinal events (nausea, vomiting and diarrhoea), rash and pruritus. Myalgias and arthralgias also occur at an overall incidence of 1.3%, although higher rates (2.5 to 31%) have been reported in patients with multiple comorbidities. Venous events are common if the drug is administered via a peripheral line; however, several management options (e.g. use of central venous access, increased infusion volume) may help to minimise their occurrence. Hyperbilirubinaemia has been documented in 3.1% of quinupristin/dalfopristin recipients versus 1.3% of recipients of comparator agents. Quinupristin/dalfopristin inhibits cytochrome P450 3A4 and therefore has the potential to increase the plasma concentrations of substrates of this enzyme.

CONCLUSIONS

Quinupristin/dalfopristin, the first parenteral streptogramin, offers a unique spectrum of activity against multidrug-resistant gram-positive bacteria. In serious gram-positive infections for which there are other treatment options available, the spectrum of activity and efficacy of quinupristin/ dalfopristin should be weighed against its tolerability and drug interaction profile. However, in VREF or unresponsive MRSA infections, where few proven treatment options exist, quinupristin/dalfopristin should be considered as a treatment of choice for these seriously ill patients.