The bacterial envelope as a target for novel anti-MRSA antibiotics

Lipopeptides as anti-infectives: a practical perspective

Lipopeptide antibiotics represent an old class of antibiotics that were discovered over 50 years ago, which includes the old polymyxins but also new entries, such as the recently approved daptomycin. They generally consist of a hydrophilic cyclic peptide portion attached to a fatty acid chain which facilitates insertion into the lipid bilayer of bacterial membranes. This review presents an overview of this class of antibiotics, focusing on their therapeutic applications and putting particular emphasis on chemical modifications introduced to improve their activity.

Vancomycin analogs: Seeking improved binding of d-Ala-d-Ala and d-Ala-d-Lac peptides by side-chain and backbone modifications

In order to seek vancomycin analogs with improved performance against VanA and VanB resistant bacterial strains, extensive computational investigations have been performed to examine the effects of side-chain and backbone modifications. Changes in binding affinities for tripeptide cell-wall precursor mimics, Ac(2)-l-Lys-d-Ala-d-Ala (3) and Ac(2)-l-Lys-d-Ala-d-Lac (4), with vancomycin analogs were computed with Monte Carlo/free energy perturbation (MC/FEP) calculations. Replacements of the 3-hydroxyl group in residue 7 with small alkyl or alkoxy groups, which improve contacts with the methyl side chain of the ligands'd-Ala residue, are predicted to be the most promising to enhance binding for both ligands. The previously reported amine backbone modification as in 5 is shown to complement the hydrophobic modifications for binding monoacetylated tripeptides. In addition, replacement of the hydroxyl groups in residues 5 and 7 by fluorine is computed to have negligible impact on binding the tripeptides, though it may be pharmacologically advantageous.

Genetics of antimicrobial resistance in Staphylococcus aureus

Strains of Staphylococcus aureus that are resistant to multiple antimicrobial compounds, including most available classes of antibiotics and some antiseptics, are a major threat to patient care owing to their stubborn intransigence to chemotherapy and disinfection. This reality has stimulated extensive efforts to understand the genetic nature of the determinants encoding antimicrobial resistance, together with the mechanisms by which these determinants evolve over time and are spread within bacterial populations. Such studies have benefited from the application of molecular genetics and in recent years, the sequencing of over a dozen complete staphylococcal genomes. It is now evident that the evolution of multiresistance is driven by the acquisition of discrete preformed antimicrobial resistance genes that are exchanged between organisms via horizontal gene transfer. Nonetheless, chromosomal mutation is the catalyst of novel resistance determinants and is likely to have an enhanced influence with the ongoing introduction of synthetic antibiotics.

Enzymatic investigation of the Staphylococcus aureus type I signal peptidase SpsB - implications for the search for novel antibiotics

Staphylococcus aureus has one essential type I signal peptidase (SPase), SpsB, which has emerged as a potential target in the search for antibiotics with a new mode of action. In this framework, the biochemical properties of SpsB are described and compared with other previously characterized SPases. Two different substrates have been used to assess the in vitro processing activity of SpsB: (a) a native preprotein substrate immunodominant staphylococcal antigen A and (b) an intramolecularly quenched fluorogenic synthetic peptide based on the sequence of the SceD preprotein of Staphylococcus epidermidis for fluorescence resonance energy transfer-based analysis. Activity testing at different pH showed that the enzyme has an optimum pH of approximately 8. The pH-rate profile revealed apparent pK(a) values of 6.6 and 8.7. Similar to the other SPases, SpsB undergoes self-cleavage and, although the catalytic serine is retained in the self-cleavage product, a very low residual enzymatic activity remained. In contrast, a truncated derivative of SpsB, which was nine amino acids longer at the N-terminus compared to the self-cleavage product, retained activity. The specificity constants (k(cat)/K(m)) of the full-length and the truncated derivative were 1.85 +/- 0.13 x 10(3) m(-1).s(-1) and 59.4 +/- 6.4 m(-1).s(-1), respectively, as determined using the fluorogenic synthetic peptide substrate. These observations highlight the importance of the amino acids in the transmembrane segment and also those preceding the catalytic serine in the sequence of SpsB. Interestingly, we also found that the activity of the truncated SpsB increased in the presence of a non-ionic detergent.

Total synthesis of N-acetylglucosamine-1,6-anhydro-N-acetylmuramylpentapeptide and evaluation of its turnover by AmpD from Escherichia coli

The bacterial cell wall is recycled extensively during the course of cell growth. The first recycling event involves the catalytic action of the lytic transglycosylase enzymes, which produce an uncommon 1,6-anhydropyranose moiety during separation of the muramyl residues from the peptidoglycan, the major constituent of the cell wall. This product, an N-acetyl-beta-D-glucosamine-(1-->4)-1,6-anhydro-N-acetyl-beta-D-muramylpeptide, is either internalized to initiate the recycling process or diffuses into the milieu to cause stimulation of the pro-inflammatory responses by the host. We report the total syntheses of N-acetyl-beta-D-glucosamine-(1-->4)-1,6-anhydro-N-acetyl-beta-D-muramyl-L-Ala-gamma-D-Glu-meso-DAP-D-Ala-D-Ala (compound 1, the product of lytic transglycosylase action on the cell wall of gram-negative bacteria) and N-acetyl-beta-D-glucosamine-(1-->4)-1,6-anhydro-N-acetyl-beta-D-muramyl-L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Ala (compound 2, from lytic transglycosylase action on the cell wall of gram-positive bacteria). The syntheses were accomplished in 15 linear steps. Compound 1 is shown to be a substrate of the AmpD enzyme of the gram-negative bacterium Escherichia coli, an enzyme that removes the peptide from the disaccharide scaffold in the early cytoplasmic phase of cell wall turnover.

Ceftaroline: a novel broad-spectrum cephalosporin with activity against meticillin-resistant Staphylococcus aureus

Ceftaroline is a broad-spectrum cephalosporin currently under clinical investigation for the treatment of complicated skin and skin-structure infections (cSSSI), including those caused by meticillin-resistant Staphylococcus aureus (MRSA), and community-acquired pneumonia (CAP). Ceftaroline has the ability to bind to penicillin-binding protein (PBP)2a, an MRSA-specific PBP that has low affinity for most other beta-lactam antibacterials. The high binding affinity of ceftaroline to PBP2a (median inhibitory concentration 0.90 microg/mL) correlates well with its low minimum inhibitory concentration for MRSA. Ceftaroline is active in vitro against Gram-positive cocci, including MRSA, meticillin-resistant Staphylococcus epidermidis, penicillin-resistant Streptococcus pneumoniae and vancomycin-resistant Enterococcus faecalis (not E. faecium). The broad-spectrum activity of ceftaroline includes many Gram-negative pathogens but does not extend to extended-spectrum beta-lactamase-producing or AmpC-derepressed Enterobacteriaceae or most nonfermentative Gram-negative bacilli. Ceftaroline demonstrates limited activity against anaerobes such as Bacteroides fragilis and non-fragilis Bacteroides spp. Limited data show that ceftaroline has a low propensity to select for resistant subpopulations. Ceftaroline fosamil (prodrug) is rapidly converted by plasma phosphatases to active ceftaroline. For multiple intravenous doses of 600 mg given over 1 h every 12 hours for 14 days, the maximum plasma concentration was 19.0 microg/mL and 21.0 microg/mL for first and last dose, respectively. Ceftaroline has a volume of distribution of 0.37 L/kg (28.3 L), low protein binding (<20%) and a serum half-life of 2.6 hours. No drug accumulation occurs with multiple doses and elimination occurs primarily through renal excretion (49.6%). Based on Monte Carlo simulations, dosage adjustment is recommended for patients with moderate renal impairment (creatinine clearance 30-50 mL/min); no adjustment is needed for mild renal impairment. Currently, limited clinical trial data are available for ceftaroline. A phase II study randomized 100 patients with cSSSI to intravenous ceftaroline 600 mg every 12 hours or intravenous vancomycin 1 g every 12 hours with or without intravenous aztreonam 1 g every 8 hours (standard therapy) for 7-14 days. Clinical cure rates were 96.7% for ceftaroline compared with 88.9% for standard therapy. Adverse events were similar between groups and generally mild in nature. In a phase III trial, 702 patients with cSSSI were randomized to ceftaroline 600 mg or vancomycin 1 g plus aztreonam 1 g, each administered intravenously every 12 hours for 5-14 days. Ceftaroline was noninferior to vancomycin plus aztreonam in treating cSSSI caused by both Gram-positive and -negative pathogens. Adverse event rates were similar between groups. Ceftaroline is well tolerated, which is consistent with the good safety and tolerability profile of the cephalosporin class. In summary, ceftaroline is a promising treatment for cSSSI and CAP, and has potential to be used as monotherapy for polymicrobial infections because of its broad-spectrum activity. Further clinical studies are needed to determine the efficacy and safety of ceftaroline, and to define its role in patient care.

De novo designed synthetic mimics of antimicrobial peptides

Antimicrobial peptides are small cationic amphiphiles that play an important role in the innate immune system. Given their broad specificity, they appear to be ideal therapeutic agents. As a result, over the last decade, there has been considerable interest in developing them as intravenously administered antibiotics. However, it has proven difficult to accomplish this goal with peptide-based structures. Although it has been possible to solve some relatively simple problems such as susceptibility to proteolysis, more severe problems have included the expense of the materials, toxicity, limited efficacy, and limited tissue distribution. In an effort to overcome these problems, we developed small synthetic oligomers designed to adopt amphiphilic conformations and exhibit potent antimicrobial activity while being nontoxic to host cells. One class of these synthetic mimics of antimicrobial peptides (SMAMPs) is being developed as intravenous antibiotics.

Activities of antistaphylococcal antibiotics towards the extracellular and intraphagocytic forms of Staphylococcus aureus isolates from a patient with persistent bacteraemia and endocarditis

Decreased susceptibility of Staphylococcus aureus to antistaphylococcal agents may be associated with inability to eradicate intracellular forms, which could explain therapeutic failures. This hypothesis was tested using clinical isolates obtained from a patient with persistent staphylococcal bacteraemia under therapy. Four isogenic isolates (three from tissue, one from blood) with increased MICs for vancomycin (1-4 mg/L) and for daptomycin (1-4 mg/L) were collected after an initial 16-day treatment with vancomycin-rifampicin-gentamicin, followed by 13-20 days of treatment with daptomycin-rifampicin-gentamicin. Isolates were tested for MICs and for: (i) vancomycin (BODIPY-FL-vancomycin) and daptomycin binding; (ii) cell wall turnover (loss of N-acetyl-d-[1-(14)C]glucosamine in 30 min after 1 h of labelling); and (iii) Triton X-100-induced autolysis. Extracellular (broth) and intracellular (THP-1 macrophages) activities of rifampicin, linezolid and fusidic acid at C(max), and of vancomycin, daptomycin, quinupristin-dalfopristin and oritavancin over a wide range of extracellular concentrations (with pharmacological modelling to determine E(max)), were measured at 24 h. Increases in vancomycin MICs correlated with increased drug binding, and decreased cell wall turnover and detergent-induced autolysis. Increases in daptomycin MICs correlated with decreased daptomycin binding. Intracellular activity was weak (E(max) <1 log(10) CFU decrease) for vancomycin against all isolates, and for daptomycin against isolates with MICs >1 mg/L. Among all antibiotics tested, only quinupristin-dalfopristin and oritavancin provided close to bactericidal intracellular activities (1.6-2.5 log(10) CFU decreases at C(max)). Determination of the intracellular susceptibility of S. aureus, combined with improved methods of diagnosis, could be useful when dealing with persistent staphylococcal infections and could improve therapy.

Lytic transglycosylase MltB of Escherichia coli and its role in recycling of peptidoglycan strands of bacterial cell wall

The cell wall is an indispensable structure for the survival of bacteria and a target for antibiotics. Peptidoglycan is the major constituent of the cell wall, which is comprised of backbone repeats of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). A peptide stem is appended to the NAM unit, which in turn experiences cross-linking with a peptide from another peptidoglycan in the final steps of cell wall assembly. In the normal course of bacterial growth, as much as 60% of the parental cell wall is recycled, a process that is not fully understood. A polymeric cell wall is fragmented by the family of lytic transglycosylases, and certain key fragments are transported to the cytoplasm for recycling. The genes for the six known lytic transglycosylases of Escherichia coli were cloned, and the enzymes were purified in this study. It is shown that MltB is the only lytic transglycosylase to turn over a synthetic peptidoglycan fragment of two NAG-NAM repeats; hence this enzyme is likely to be the lytic transglycosylase responsible for processing of shorter peptidoglycan strands. Lytic transglycosylases have been proposed to go through an oxocarbenium species that would trap the 6-hydroxyl moiety of the glucosamine residue of muramic acid to generate the so-called 1,6-anhydromuramyl moiety. It is documented herein by characterization of the products of turnover that this process takes place to the total exclusion of the entrapment of a water molecule by the reactive intermediary oxocarbenium species. Furthermore, turnover of the E. coli sacculus (whole cell wall) by MltB was characterized. It is documented that each MltB molecule is able to process the cell wall 14000 times in the course of a single doubling time for E. coli.