Characterization of vancomycin resistance in Enterococcus faecium and Enterococcus faecalis

Development of an in vitro Model of Human Gut Microbiota for Screening the Reciprocal Interactions With Antibiotics, Drugs, and Xenobiotics

Altering the gut microbiota can negatively affect human health. Efforts may be sustained to predict the intended or unintended effects of molecules not naturally produced or expected to be present within the organism on the gut microbiota. Here, culture-dependent and DNA-based approaches were combined to UHPLC-MS/MS analyses in order to investigate the reciprocal interactions between a constructed Human Gut Microbiota Model (HGMM) and molecules including antibiotics, drugs, and xenobiotics. Our HGMM was composed of strains from the five phyla commonly described in human gut microbiota and belonging to Firmicutes, Bacteroidetes, Proteobacteria, Fusobacteria, and Actinobacteria. Relevantly, the bacterial diversity was conserved in our constructed human gut model through subcultures. Uneven richness distribution was revealed and the sensitivity of the HGMM was mainly affected by antibiotic exposure rather than by drugs or xenobiotics. Interestingly, the constructed model and the individual cultured strains respond with the same sensitivity to the different molecules. UHPLC-MS/MS analyses revealed the disappearance of some native molecules in the supernatants of the HGMM as well as in those of the individual strains. These results suggest that biotransformation of molecules occurred in the presence of our gut microbiota model and the coupled approaches performed on the individual cultures may emphasize new bacterial strains active in these metabolic processes. From this study, the new HGMM appears as a simple, fast, stable, and inexpensive model for screening the reciprocal interactions between the intestinal microbiota and molecules of interest.

Regulation of bla system in ST59-related oxacillin-susceptible mecA-positive Staphylococcus aureus

OBJECTIVES

Oxacillin-susceptible mecA-positive Staphylococcus aureus (OS-MRSA) is clinically significant and isolated globally but the mechanism of its occurrence remains indistinct. We sought to assess the mechanism of regulating oxacillin susceptibility in OS-MRSA isolates by evaluating the evolutionary dynamics of OS-MRSA and the discrepancies of mecA-regulating genes in OS-MRSA and oxacillin-resistant MRSA (OR-MRSA).

METHODS

Nine OS-MRSA isolates and 77 OR-MRSA isolates were sequenced using next-generation sequencing (NGS) platforms. Two representative OS-MRSA isolates (ET-13, ET-16) were induced to be oxacillin resistant and sequenced also. OS-MRSA ET-16 and its counterpart isolate with induced oxacillin resistance, ET-16I, and their mutants were used to confirm the role of the bla system in regulating methicillin susceptibility. Oxacillin MICs were determined using Etests. Expression of mecA and blaR1 was quantified by quantitative RT-PCR.

RESULTS

A deletion in blaR1 in most OS-MRSA isolates (7/9; 77.78%) was found using NGS data, and overexpression of OR-blaR1 in OS-MRSA isolate ET-16 restored its oxacillin resistance. OS-MRSA could be induced to be oxacillin resistant, while growth was suppressed in the induced isolates. Plasmid containing the bla locus was lost in most induced isolates during the induction process and complementation of blaR1-blaI from OS-MRSA ET-16 to the induced isolate ET-16I converted its oxacillin susceptibility.

CONCLUSIONS

Deletion in blaR1 resulted in oxacillin susceptibility in OS-MRSA, and loss of the bla regulator in OS-MRSA restored oxacillin resistance. The bla system played a crucial role in regulating oxacillin susceptibility in OS-MRSA isolates.

Regulation of Resistance in Vancomycin-Resistant Enterococci: The VanRS Two-Component System

Vancomycin-resistant enterococci (VRE) are a serious threat to human health, with few treatment options being available. New therapeutics are urgently needed to relieve the health and economic burdens presented by VRE. A potential target for new therapeutics is the VanRS two-component system, which regulates the expression of vancomycin resistance in VRE. VanS is a sensor histidine kinase that detects vancomycin and in turn activates VanR; VanR is a response regulator that, when activated, directs expression of vancomycin-resistance genes. This review of VanRS examines how the expression of vancomycin resistance is regulated, and provides an update on one of the field’s most pressing questions: How does VanS sense vancomycin?

ddcP, pstB, and excess D-lactate impact synergism between vancomycin and chlorhexidine against Enterococcus faecium 1,231,410

Vancomycin-resistant enterococci (VRE) are important nosocomial pathogens that cause life-threatening infections. To control hospital-associated infections, skin antisepsis and bathing utilizing chlorhexidine is recommended for VRE patients in acute care hospitals. Previously, we reported that exposure to inhibitory chlorhexidine levels induced the expression of vancomycin resistance genes in VanA-type Enterococcus faecium. However, vancomycin susceptibility actually increased for VanA-type E. faecium in the presence of chlorhexidine. Hence, a synergistic effect of the two antimicrobials was observed. In this study, we used multiple approaches to investigate the mechanism of synergism between chlorhexidine and vancomycin in the VanA-type VRE strain E. faecium 1,231,410. We generated clean deletions of 7 of 11 pbp, transpeptidase, and carboxypeptidase genes in this strain (ponA, pbpF, pbpZ, pbpA, ddcP, ldt_fm, and vanY). Deletion of ddcP, encoding a membrane-bound carboxypeptidase, altered the synergism phenotype. Furthermore, using in vitro evolution, we isolated a spontaneous synergy escaper mutant and utilized whole genome sequencing to determine that a mutation in pstB, encoding an ATPase of phosphate-specific transporters, also altered synergism. Finally, addition of excess D-lactate, but not D-alanine, enhanced synergism to reduce vancomycin MIC levels. Overall, our work identified factors that alter chlorhexidine and vancomycin synergism in a model VanA-type VRE strain.

Current status of oxacillin-susceptible mecA-positive Staphylococcus aureus infection in Shanghai, China: A multicenter study

BACKGROUND

Oxacillin-susceptible mecA-positive Staphylococcus aureus (OS-MRSA) represents an important issue, as its oxacillin susceptibility has contributed to misidentification by conventional susceptibility tests and consequently potential therapeutic failure, but limited data on the current status of OS-MRSA infection in Chinese hospitals are available.

METHODS

This multicenter study performed a battery of susceptibility tests and diagnostic tests for 956 S. aureus isolates from 10 hospitals, including automated susceptibility testing on VITEK 2, broth microdilution, disk diffusion, and detection of PBB2a, mecA gene and mecC gene. For all identified OS-MRSA, multi-locus sequence typing (MLST), together with spa typing, SCCmec typing and PVL detecting, was carried out.

RESULTS

OS-MRSA, most of which were from pediatric inpatients, represented 1.8% (17/956) of total isolates. Of these 17 OS-MRSA, 10 were ST59, followed by ST965 (3/17), and 11 carried SCCmec type IV, while 5 carried SCCmec type V, but only one was Panton-Valentine leucocidin (PVL)-positive, also, 16 had one or two point mutations within mecA promoter. OS-MRSA had inducible oxacillin resistance and significantly lower MDR (Multi-Drug Resistant) rate. We observed that the VITEK 2 system exhibited some deficiency in OS-MRSA detection, whereas cefoxitin disk diffusion was shown to be a reliable and cost-saving alternative and should be supplemented in detecting S. aureus with borderline oxacillin susceptible MICs.

CONCLUSION

This study has characterized phenotypically and molecularly OS-MRSA in China, and provided insights into more effective management of OS-MRSA.

Prevalence and Genetic Diversity of Enterococcus faecalis Isolates from Mineral Water and Spring Water in China

Enterococcus faecalis is an important opportunistic pathogen which is frequently detected in mineral water and spring water for human consumption and causes human urinary tract infections, endocarditis and neonatal sepsis. The aim of this study was to determine the prevalence, virulence genes, antimicrobial resistance and genetic diversity of E. faecalis from mineral water and spring water in China. Of 314 water samples collected from January 2013 to January 2014, 48 samples (15.3%) were contaminated E. faecalis. The highest contamination rate occurred in activated carbon filtered water of spring water (34.5%), followed by source water of spring water (32.3%) and source water of mineral water (6.4%). The virulence gene test of 58 E. faecalis isolates showed that the detection rates of asa1, ace, cylA, gelE and hyl were 79.3, 39.7, 0, 100, 0%, respectively. All 58 E. faecalis isolates were not resistant to 12 kinds of antibiotics (penicillin, ampicillin, linezolid, quinupristin/dalfopristin, vancomycin, gentamicin, streptomycin, ciprofloxacin, levofloxacin, norfloxacin, nitrofurantoin, and tetracycline). Enterobacterial repetitive intergenic consensus-PCR classified 58 isolates and three reference strains into nine clusters with a similarity of 75%. This study is the first to investigate the prevalence of E. faecalis in mineral water and spring water in China. The results of this study suggested that spring water could be potential vehicles for transmission of E. faecalis.

Ternary nylon-3 copolymers as host-defense peptide mimics: beyond hydrophobic and cationic subunits

Host-defense peptides (HDPs) are produced by eukaryotes to defend against bacterial infection, and diverse synthetic polymers have recently been explored as mimics of these natural peptides. HDPs are rich in both hydrophobic and cationic amino acid residues, and most HDP-mimetic polymers have therefore contained binary combinations of hydrophobic and cationic subunits. However, HDP-mimetic polymers rarely duplicate the hydrophobic surface and cationic charge density found among HDPs ( Hu , K. ; et al. Macromolecules 2013 , 46 , 1908 ); the charge and hydrophobicity are generally higher among the polymers. Statistical analysis of HDP sequences ( Wang , G. ; et al. Nucleic Acids Res. 2009 , 37 , D933 ) has revealed that serine (polar but uncharged) is a very common HDP constituent and that glycine is more prevalent among HDPs than among proteins in general. These observations prompted us to prepare and evaluate ternary nylon-3 copolymers that contain a modestly polar but uncharged subunit, either serine-like or glycine-like, along with a hydrophobic subunit and a cationic subunit. Starting from binary hydrophobic-cationic copolymers that were previously shown to be highly active against bacteria but also highly hemolytic, we found that replacing a small proportion of the hydrophobic subunit with either of the polar, uncharged subunits can diminish the hemolytic activity with minimal impact on the antibacterial activity. These results indicate that the incorporation of polar, uncharged subunits may be generally useful for optimizing the biological activity profiles of antimicrobial polymers. In the context of HDP evolution, our findings suggest that there is a selective advantage to retaining polar, uncharged residues in natural antimicrobial peptides.

Two interdependent mechanisms of antimicrobial activity allow for efficient killing in nylon-3-based polymeric mimics of innate immunity peptides

Novel synthetic mimics of antimicrobial peptides have been developed to exhibit structural properties and antimicrobial activity similar to those of natural antimicrobial peptides (AMPs) of the innate immune system. These molecules have a number of potential advantages over conventional antibiotics, including reduced bacterial resistance, cost-effective preparation, and customizable designs. In this study, we investigate a family of nylon-3 polymer-based antimicrobials. By combining vesicle dye leakage, bacterial permeation, and bactericidal assays with small-angle X-ray scattering (SAXS), we find that these polymers are capable of two interdependent mechanisms of action: permeation of bacterial membranes and binding to intracellular targets such as DNA, with the latter necessarily dependent on the former. We systemically examine polymer-induced membrane deformation modes across a range of lipid compositions that mimic both bacteria and mammalian cell membranes. The results show that the polymers' ability to generate negative Gaussian curvature (NGC), a topological requirement for membrane permeation and cellular entry, in model Escherichia coli membranes correlates with their ability to permeate membranes without complete membrane disruption and kill E. coli cells. Our findings suggest that these polymers operate with a concentration-dependent mechanism of action: at low concentrations permeation and DNA binding occur without membrane disruption, while at high concentrations complete disruption of the membrane occurs. This article is part of a Special Issue entitled: Interfacially Active Peptides and Proteins. Guest Editors: William C. Wimley and Kalina Hristova.

Evidence for phenylalanine zipper-mediated dimerization in the X-ray crystal structure of a magainin 2 analogue

High-resolution structure elucidation has been challenging for the large group of host-defense peptides that form helices on or within membranes but do not manifest a strong folding propensity in aqueous solution. Here we report the crystal structure of an analogue of the widely studied host-defense peptide magainin 2. Magainin 2 (S8A, G13A, G18A) is a designed variant that displays enhanced antibacterial activity relative to the natural peptide. The crystal structure of magainin 2 (S8A, G13A, G18A), obtained for the racemic form, features a dimerization mode that has previously been proposed to play a role in the antibacterial activity of magainin 2 and related peptides.

Effects of Cyclic vs. Acyclic Hydrophobic Subunits on the Chemical Structure and Biological Properties of Nylon-3 Co-Polymers

Nylon-3 co-polymers containing both hydrophobic and cationic subunits can mimic the activity profile of host-defense peptides, if subunit identity and proportion are carefully selected. These sequence- and stereo-random co-polymers inhibit bacterial growth at relatively low concentrations, apparently via disruption of bacterial membranes, but they are relatively non-disruptive toward eukaryotic cell membranes (low hemolytic activity). In all previous examples, the hydrophobic subunits have contained cycloalkyl groups that incorporate the backbone Cα-Cβ bond. Here we have explored the effects of using analogous acyclic hydrophobic subunits. The results indicate that the replacing cyclic with acyclic hydrophobic subunits has a modest influence on biological properties. This influence appears to arise from differences in subunit flexibility.

Interplay among subunit identity, subunit proportion, chain length, and stereochemistry in the activity profile of sequence-random peptide mixtures

Fmoc-based solid-phase synthesis methodology was used to prepare peptide mixtures containing one type of hydrophobic residue and one type of cationic residue. Each mixture was random in terms of sequence but highly controlled in terms of length. Analysis of the antibacterial and hemolytic properties of these mixtures revealed that selective antibacterial activity can be achieved with heterochiral binary mixtures but not homochiral binary mixture, if the proper amino acid residues are used.

Functionally Diverse Nylon-3 Copolymers from Readily Accessible β-Lactams

A new family of β-lactams is described that enables anionic ring-opening polymerization (AROP) to prepare nylon-3 materials bearing diverse appended functionality, including carboxylic acid, thiol, hydroxyl and secondary amine groups. Nylon-3 copolymers generated with the new β-lactams are shown to display distinctive self-assembly behavior and biological properties.

C-terminal functionalization of nylon-3 polymers: effects of C-terminal groups on antibacterial and hemolytic activities

Nylon-3 polymers contain β-amino-acid-derived subunits and can be viewed as higher homologues of poly(α-amino acids). This structural relationship raises the possibility that nylon-3 polymers offer a platform for development of new materials with a variety of biological activities, a prospect that has recently begun to receive experimental support. Nylon-3 homo- and copolymers can be prepared via anionic ring-opening polymerization of β-lactams, and use of an N-acyl-β-lactam as coinitiator in the polymerization reaction allows placement of a specific functional group, borne by the N-acyl-β-lactam, at the N-terminus of each polymer chain. Controlling the unit at the C-termini of nylon-3 polymer chains, however, has been problematic. Here we describe a strategy for specifying C-terminal functionality that is based on the polymerization mechanism. After the anionic ring-opening polymerization is complete, we introduce a new β-lactam, approximately 1 equiv relative to the expected number of polymer chains. Because the polymer chains bear a reactive imide group at their C-termini, this new β-lactam should become attached at this position. If the terminating β-lactam bears a distinctive functional group, that functionality should be affixed to most or all C-termini in the reaction mixture. We use the new technique to compare the impact of N- and C-terminal placement of a critical hydrophobic fragment on the biological activity profile of nylon-3 copolymers. The synthetic advance described here should prove to be generally useful for tailoring the properties of nylon-3 materials.

Structure-activity relationships among random nylon-3 copolymers that mimic antibacterial host-defense peptides

Host-defense peptides are natural antibiotics produced by multicellular organisms to ward off bacterial infection. Since the discovery of these molecules in the 1980s, a great deal of effort has been devoted to elucidating their mechanisms of action and to developing analogues with improved properties for possible therapeutic use. The vast majority of this effort has focused on materials composed of a single type of molecule, most commonly a peptide with a specific sequence of alpha-amino acid residues. We have recently shown that sequence-random nylon-3 copolymers can mimic favorable properties of host-defense peptides, and here we document structure-activity relationships in this polymer family. Although the polymers are heterogeneous in terms of subunit order and stereochemistry, these materials display structure-activity relationships comparable to those that have been documented among host-defense peptides and analogous synthetic peptides. Previously such relationships have been interpreted in terms of a specific and regular folding pattern (usually an alpha-helix), but our findings show that these correlations between covalent structure and biological activity do not require the adoption of a specific or regular conformation. In some cases our observations suggest alternative interpretations of results obtained with discrete peptides.

Successful therapy of experimental endocarditis caused by vancomycin-resistant Staphylococcus aureus with a combination of vancomycin and beta-lactam antibiotics

VRS1 is the first isolated strain of vancomycin-resistant Staphylococcus aureus (VRSA) found to carry the vanA gene complex previously described in Enterococcus. Under vancomycin pressure, VRS1 makes aberrant cell walls consisting of stem tetrapeptide and depsipeptide that lack the terminal D-Ala-D-Ala residues targeted by vancomycin. Previous data have suggested that this aberrant cell wall is not cross-linked by PBP2a, the enzyme responsible for cell wall transpeptidation in the presence of beta-lactam antibiotics. We examined the efficacy of treating VRS1 with a combination of vancomycin and beta-lactam antibiotics in vitro and in vivo. We found that the MIC of oxacillin for VRS1 decreased from >256 microg/ml to <1 microg/ml in the presence of vancomycin. Using the rabbit model of endocarditis, we treated VRS1-infected rabbits with nafcillin alone, vancomycin alone, or a combination of nafcillin and vancomycin. Treatment with nafcillin in combination with vancomycin cleared bloodstream infections within 24 h and sterilized 12/13 spleens (92%), as well as 8/13 kidneys (62%), following 3 days of treatment. Mean aortic valve vegetation counts were reduced 3.48 log(10) CFU/g with the combination therapy (compared to untreated controls) and were significantly lower than with either vancomycin or nafcillin given alone. VRS1 was extremely virulent in this model, as no untreated rabbits survived the 3-day trial. Treatment of clinical infections due to VRSA with the combination of vancomycin and beta-lactams may be an option, based on these results.

Biocidal activity of polystyrenes that are cationic by virtue of protonation

[structure: see text] Poly(1) kills bacteria (Gram-positive and -negative) and lyses human erythrocytes; this biocidal profile is similar to that of the peptide toxin mellitin. Poly(1) has antibacterial activity comparable to that of a potent derivative of the host defense peptide magainin II, but lacks magainin's selectivity for bacteria over erythrocytes. An analogous N-quaternized polymer, poly(3), is less biocidal than poly(1), suggesting that reversible N-protonation leads to greater biocidal activity than does irreversible N-quaternization.

Methods used for the isolation, enumeration, characterisation and identification of Enterococcus spp

This paper reviews the methodology applied for the identification and characterisation of enterococci and covers phenotypic, genotypic and phylogenetic techniques. Although conventional phenotypic typing schemes are useful for rapid and simple identification of enterococcal species for routine applications, other methods like standardised sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE), multilocus enzyme electrophoresis (MLEE), antimicrobial susceptibility testing, serotyping, pyrolysis mass spectrometry (pyMS) and vibrational spectroscopic methods allow a more in-depth characterisation of enterococci. Many of the recently described enterococcal species exhibit deviations from hitherto so-called classical enterococci with regard to their phenotypical properties. Therefore, genotypic methods have to be used to clarify their possible assignment to the genus Enterococcus. In this review, special emphasis is given on recently developed polymerase chain reaction (PCR)-based typing methods such as random amplified polymorphic DNA (RAPD), amplified fragment length polymorphism (AFLP), specific and random amplification (SARA) and modifications of PCR-ribotyping as well as pulsed-field gel electrophoresis (PFGE) and partial sequence analysis. The use of PCR and probes for genus and species identification of enterococci is also considered like the application of sequence data of conserved DNA regions (e.g., ribosomal ribonucleic acid (rRNA) genes) in the case of species identification.

Induction of VanA vancomycin resistance genes in Enterococcus faecalis: use of a promoter fusion to evaluate glycopeptide and nonglycopeptide induction signals

To characterize induction of VanA resistance a plasmid was constructed in which the gene for firefly luciferase lucA was placed under the control of the promoter for the VanA resistance genes, the vanH promoter. This system afforded convenient quantitative measurement of induction of the VanA genes. Glycopeptide antibiotics and antibiotics representing 19 different mechanisms of action were evaluated for their ability to induce. Antibiotics that acted as inducers were all inhibitors of late steps of peptidoglycan synthesis. These included moenomycin, bacitracin, tunicamycin, ramoplanin and glycopeptides, but not penicillin or other beta-lactam antibiotics. Glycopeptide antibiotics were the most potent inducers. Both glycopeptides with little or no antimicrobial activity and semisynthetic glycopeptides active against VanA resistant enterococci were inducers. Overall, results suggest that an induction response may involve both an internal signal, such as precursor accumulation, and the glycopeptide molecule itself as a signal. The system may be useful as a screen for new antimicrobial agents.

Chlorobiphenyl-desleucyl-vancomycin inhibits the transglycosylation process required for peptidoglycan synthesis in bacteria in the absence of dipeptide binding

Novel glycopeptide analogs are known that have activity on vancomycin resistant enterococci despite the fact that the primary site for drug interaction, D-ala-D-ala, is replaced with D-ala-D-lactate. The mechanism of action of these compounds may involve dimerization and/or membrane binding, thus enhancing interaction with D-ala-D-lactate, or a direct interaction with the transglycosylase enzymes involved in peptidoglycan polymerization. We evaluated the ability of vancomycin (V), desleucyl-vancomycin (desleucyl-V), chlorobiphenyl-vancomycin (CBP-V), and chlorobiphenyl-desleucyl-vancomycin (CBP-desleucyl-V) to inhibit (a) peptidoglycan synthesis in vitro using UDP-muramyl-pentapeptide and UDP-muramyl-tetrapeptide substrates and (b) growth and peptidoglycan synthesis in vancomycin resistant enterococci. Compared to V or CBP-V, CBP-desleucyl-V retained equivalent potency in these assays, whereas desleucyl-V was inactive. In addition, CBP-desleucyl-V caused accumulation of N-acetylglucosamine-beta-1, 4-MurNAc-pentapeptide-pyrophosphoryl-undecaprenol (lipid II). These data show that CBP-desleucyl-V inhibits peptidoglycan synthesis at the transglycosylation stage in the absence of binding to dipeptide.

Isolation and molecular characterization of vancomycin-resistant Enterococcus faecium in Malaysia

Nineteen strains of vancomycin-resistant Enterococcus faecium isolated from 10 of 75 (13.3%) tenderloin beef samples were examined for resistance to selected antibiotics, presence of plasmids, and genetic diversity by random amplification of polymorphic DNA analysis. All strains showed multiple resistant to the antibiotics tested. Multiple antibiotic indexing of the vancomycin-resistant E. faecium strains showed that all (100%) originated from high risk contamination environments where antibiotics were often used. Plasmids ranging in size from 1.5 to 36 megadalton were detected in 15 of 19 (79%) strains. Thus, three plasmid profiles and eight antibiotypes were observed among the E. faecium strains. A high degree of polymorphism was obtained by combining the results of the two primers used; with the 19 E. faecium strains being differentiated into 19 RAPD-types. These preliminary results suggest that RAPD-PCR has application for epidemiologic studies and that resistance patterns and plasmid profiling could be used as an adjunct to RAPD for the typing of E. faecium in the study area.

In vitro selection of glycopeptide-resistant variants of Enterococci

In order to study the possible phenotypic and genotypic changes related to glycopeptide pressure on enterococci, a study was undertaken using stepwise in vitro exposure to achieve the following objectives: (i) to evaluate the development of resistance and cross-resistance between vancomycin and teicoplanin; (ii) to determine the stability of the acquired level of resistance; (iii) to determine the phenotypic and genotypic changes related to glycopeptide pressure; and (iv) to assess the spectrum of antibiotic-susceptibility of all strains. Our results showed that no variants resistant to glycopeptides could be selected after in vitro glycopeptide exposure experiments. However some strains showed increased MIC values: 8 mg/l to vancomycin in eight strains selected by vancomycin itself, while teicoplanin produced intermediate values to vancomycin in only three strains. The phenotypes were stable in vitro after numerous passages in antibiotic-free medium and three out of nine strains with a changed MIC level, showed 40, 42 and 43 kDa proteins in cell membrane preparations. The profile of antibiotic resistance was comparable in all isogenic strains tested with the exception of three selected strains that became susceptible to penicillin G. The pressure produced by glycopeptides, particularly vancomycin has contributed to an increased level of MIC that can influence the acquisition and/or full expression of this resistance.

Resistance issues and treatment implications: pneumococcus, Staphylococcus aureus, and gram-negative rods

During the last decade there has been an unexpectedly rapid evolution of antimicrobial resistance in the respiratory pathogens for community- and hospital-acquired pneumonia. In order to choose the most optimal therapy for their patients, it is essential that physicians be aware of the prevalence and mechanisms of resistance and their implications on the effectiveness of the various antimicrobials.

vanA gene cluster in a vancomycin-resistant clinical isolate of Bacillus circulans

We report on the cloning and sequencing of the vanA gene cluster present in the glycopeptide-resistant clinical isolate Bacillus circulans VR0709 (R. Fontana, M. Ligozzi, C. Pedrotti, E. M. Padovani, and G. Cornaglia, Eur. J. Clin. Microbiol. Infect. Dis. 16:473-474, 1997). The presence of a vanA-related gene in VR0709 was demonstrated in a PCR assay which permitted the specific amplification of an internal segment of vanA. Southern blotting suggested that the vanA gene was located in the chromosome in a 7. 6-kb EcoRI fragment. DNA sequence analysis revealed the presence of all seven genes of the vanA cluster (vanR, vanS, vanH, vanA, vanX, vanY, and vanZ). The degree of identity between homologous proteins encoded by Tn1546 and the chromosome of B. circulans VR0709 ranged from 87 to 95%. Neither PCR nor Southern blotting with specific primers and probes, respectively, showed the presence of open reading frames (ORFs) 1 and 2 which encode the transposase and the resolvase of Tn1546, respectively, the transposon found to carry the vanA gene cluster in enterococci. Determination of the sequences of the flanking regions of the van gene cluster of B. circulans revealed perfect inverted repeats of 10 bp which delineated a 9.2-kb region containing the van gene cluster and an ORF which encoded a putative protein (178 residues) which displayed a low level of identity (28%) to the resolvase of Tn1546. These results suggest that glycopeptide resistance in B. circulans VR0709 is associated with the acquisition of a vanA gene cluster which shows a high degree of homology with that of enterococci. In B. circulans, however, the cluster is not carried by Tn1546 and is borne by the chromosome.

Molecular interactions of a semisynthetic glycopeptide antibiotic with D-alanyl-D-alanine and D-alanyl-D-lactate residues

LY191145 is an N-alkylated glycopeptide antibiotic (the p-chlorobenzyl derivative of LY264826) with activity against vancomycin-susceptible and -resistant bacteria. Similar to vancomycin, LY191145 inhibited polymerization of peptidoglycan when muramyl pentapeptide served as a substrate but not when muramyl tetrapeptide was used, signifying a substrate-dependent mechanism of inhibition. Examination of ligand binding affinities for LY191145 and the effects of this agent on R39 D,D-carboxypeptidase action showed that, similar to vancomycin, LY191145 had an 800-fold greater affinity for N,N'-diacetyl-L-Lys-D-Ala-D-Ala than for N,N'-diacetyl-L-Lys-D-Ala-D-Lac. The antibacterial activity of LY191145 was antagonized by N,N'-diacetyl-L-Lys-D-Ala-D-Ala, but the molar excess required for complete suppression exceeded that needed to suppress inhibition by vancomycin. LY191145 is strongly dimerized and the p-chlorobenzyl side chain facilitates interactions with bacterial membranes. These findings are consistent with a mechanism of inhibition where interactions between antibiotic and D-Ala-D-Ala or D-Ala-D-Lac residues depend on intramolecular effects occurring at the subcellular target site.

Inhibition of peptidoglycan biosynthesis in vancomycin-susceptible and -resistant bacteria by a semisynthetic glycopeptide antibiotic

LY191145 is a p-chlorobenzyl derivative of LY264826 (A82846B) with activity against both vancomycin-susceptible and -resistant enterococci. Incorporation of L-[14C]lysine into peptidoglycan of intact vancomycin-susceptible and -resistant Enterococcus faecium was inhibited by LY191145 (50% inhibitory concentrations of 1 and 5 microgram/ml, respectively). Inhibition was accompanied by accumulation of UDP-muramyl-peptide precursors in the cytoplasm. This agent inhibited late-stage steps in peptidoglycan biosynthesis in permeabilized E. faecium when either the UDP-muramyl-pentapeptide precursor from vancomycin-susceptible E. faecium or the UDP-muramyl-pentadepsipeptide precursor from vancomycin-resistant E. faecium was used as a substrate. Inhibition of late-stage steps led to accumulation of an N-acetyl-[14C]glucosamine-labeled lipid intermediate indicative of inhibition of the transglycosylation step. Inhibition of peptidoglycan polymerization without affecting cross-linking in a particulate membrane-plus-wall-fragment assay from Aerococcus viridans was consistent with this explanation. The fact that inhibition of peptidoglycan biosynthesis by LY191145 was not readily antagonized by an excess of free acyl-D-alanyl-D-alanine or acyl-D-alanyl-D-lactate ligands indicates that the manner in which this compound inhibits transglycosylation may not be identical to that of vancomycin.

Bacterial resistance to vancomycin: five genes and one missing hydrogen bond tell the story

A plasmid-borne transposon encodes enzymes and regulator proteins that confer resistance of enterococcal bacteria to the antibiotic vancomycin. Purification and characterization of individual proteins encoded by this operon has helped to elucidate the molecular basis of vancomycin resistance. This new understanding provides opportunities for intervention to reverse resistance.

In vitro conjugative transfer of VanA vancomycin resistance between Enterococci and Listeriae of different species

In a study designed to gain data on the in vitro transferability of vancomycin resistance from enterococci of the VanA phenotype to listeriae of different species, three clinical Enterococcus isolates-Enterococcus faecium LS10, Enterococcus faecalis LS4, and Enterococcus faecalis A3208, all harboring a plasmid that strongly hybridized with a vanA probe-were used as donors in transfer experiments. Strains of five Listeria species were used as recipients. From Enterococcus faecium LS10, glycopeptide resistance was transferred to Listeria monocytogenes, Listeria ivanovii, and Listeria welshimeri recipients, whereas no transfer occurred to Listeria seeligeri or Listeria innocua strains. From the two Enterococcus faecalis isolates, no transfer occurred to any Listeria recipient. MICs of both vancomycin and teicoplanin were > or = 256 mg/l for all transconjugants tested. Furthermore, all transconjugants harbored a plasmid that strongly hybridized with the vanA probe, with vanA consistently located in an EcoRI fragment of about 4 kb. Exposure of Listeria transconjugants to vancomycin resulted in synthesis of a membrane protein similar in size (39 kDa) to a vancomycin-induced membrane protein of Enterococcus faecium LS10. In retransfer experiments with Listeria transconjugants used as donors, glycopeptide resistance was transferred to all Listeria recipients tested, including strains of Listeria innocua and Listeria seeligeri, which were unable to receive the resistance from Enterococcus faecium LS10. The frequency of vanA transfer to listerial recipients was greater in retransfer experiments than in the primary matings. These findings suggest that the vanA resistance determinant might spread to the established pathogen Listeria monocytogenes, both directly from a resistant enterococcus and through strains of nonpathogenic Listeria species acting as intermediate resistance vehicles.

Current perspectives on glycopeptide resistance

In the last 5 years, clinical isolates of gram-positive bacteria with intrinsic or acquired resistance to glycopeptide antibiotics have been encountered increasingly. In many of these isolates, resistance arises from an alteration of the antibiotic target site, with the terminal D-alanyl-D-alanine moiety of peptidoglycan precursors being replaced by groups that do not bind glycopeptides. Although the criteria for defining resistance have been revised frequently, the reliable detection of low-level glycopeptide resistance remains problematic and is influenced by the method chosen. Glycopeptide-resistant enterococci have emerged as a particular problem in hospitals, where in addition to sporadic cases, clusters of infections with evidence of interpatient spread have occurred. Studies using molecular typing methods have implicated colonization of patients, staff carriage, and environmental contamination in the dissemination of these bacteria. Choice of antimicrobial therapy for infections caused by glycopeptide-resistant bacteria may be complicated by resistance to other antibiotics. Severe therapeutic difficulties are being encountered among patients infected with enterococci, with some infections being untreatable with currently available antibiotics.

Induction of vancomycin resistance in Enterococcus faecium by non-glycopeptide antibiotics

Bacitracin and other antibiotics that inhibit late stages in peptidoglycan biosynthesis induce vancomycin resistance in a high-level, inducibly vancomycin-resistant strain of Enterococcus faecium. Exposure to bacitracin led to synthesis of the lactate-containing UDP-MurNAc-pentadepsipeptide precursor required for vancomycin resistance. These findings indicate that inhibition of peptidoglycan biosynthesis can lead to induction of vancomycin resistance and raise the possibility that multiple signals may serve to induce resistance.

Inducible and constitutive expression of resistance to glycopeptides and vancomycin dependence in glycopeptide-resistant Enterococcus avium

A clinical isolate of Enterococcus avium, Ea1, which exhibited inducible, low-level resistance to vancomycin and teicoplanin, and two mutants selected from this strain, Ea3 and Ea31, were studied. Ea3 was vancomycin dependent and derived from Ea1, while Ea31 was not vancomycin dependent, was constitutively resistant, and was derived from Ea3. Hybridization studies revealed that vanA was present in Ea1 and suggested that it was located on a high-molecular-weight plasmid. In the absence of induction, Ea1 synthesized only the natural UDP-MurNAc-pentapeptide precursor, and after induction it synthesized an additional precursor identified as UDP-MurNAc-tetrapeptide-D-lactate. The latter was the only precursor found in Ea3 and Ea31, even after precursor accumulation. From these results, we infer that (i) the low level of resistance to glycopeptides in strain Ea1 may be in part due to the residual synthesis of the normal precursor and (ii) the vancomycin dependence of mutant Ea3 could be due to the fact that this strain does not produce any peptidoglycan precursor in the absence of induction.

Teicoplanin. A pharmacoeconomic evaluation of its use in the treatment of gram-positive infections

Teicoplanin, a glycopeptide antibiotic, is active against Gram-positive organisms, including methicillin-resistant staphylococci. It has demonstrated similar efficacy to vancomycin in the treatment of Gram-positive infections in febrile patients with neutropenia; fewer comparative data are available in patients with other infection types. Compared with vancomycin, teicoplanin is associated with less nephrotoxicity, appears to cause fewer anaphylactoid reactions, requires less monitoring and is more convenient to administer (once daily by intravenous bolus or intramuscular injection vs 2 to 4 times daily by intravenous infusion). Two European cost-minimisation studies have demonstrated that while the acquisition cost per dose of teicoplanin was approximately twice that of vancomycin, the cost of 2 weeks' therapy with either agent was similar (difference of 1 to 2%). However, in order to fully explore potential differences between these agents, a full economic analysis which considers all treatment-related costs is needed. Home therapy of Gram-positive infections, a setting in which teicoplanin may be preferred over vancomycin because of its tolerability profile and ease of administration, is particularly worthy of future economic study. Thus, there are a number of areas needing further study before the optimum formulary positioning of teicoplanin can be definitely stated. Nevertheless, present evidence suggests that teicoplanin is likely to have pharmacoeconomic advantages over vancomycin in at least some situations.

An old antibiotic for a new multiple-resistant Enterococcus faecium?

Enterococci have become an important cause of nosocomial infections and may demonstrate high-level resistance to multiple antibiotics. We present the case of a 68-year-old man with a history of small cell lung cancer, who developed bacteremia due to a strain of Enterococcus faecium. The isolate was resistant to multiple antibiotics including vancomycin, ampicillin, aminoglycosides, quinolones, and macrolides. The patient was successfully treated with doxycycline and removal of an infected central venous catheter.

Emergence of high-level resistance to glycopeptides in Enterococcus gallinarum and Enterococcus casseliflavus

Enterococcus gallinarum BM4231 and Enterococcus casseliflavus BM4232, isolated from the feces of a patient under oral therapy with vancomycin, were resistant to high levels of vancomycin (MICs of > 256 micrograms/ml) and teicoplanin (MICs of 128 and 64 micrograms/ml, respectively). This phenotype is new for these bacterial species that are naturally resistant to low levels of vancomycin and appears to be due to in vivo acquisition of plasmid pIP218 carrying the vanA gene cluster.

Teicoplanin. A reappraisal of its antimicrobial activity, pharmacokinetic properties and therapeutic efficacy

Since an earlier review in the Journal substantial additional data have accumulated, further clarifying the in vitro activity, pharmacokinetic profile, clinical efficacy and tolerability of teicoplanin. Recent therapeutic trials confirm the efficacy of teicoplanin in the treatment of microbiologically confirmed Gram-positive infections, including septicaemia, endocarditis, and infections of skin and soft tissue, bone and joints, and the lower respiratory tract. As teicoplanin can be administered once daily intramuscularly as well as intravenously, it has potential for outpatient treatment of severe Gram-positive infections. Teicoplanin is appropriate as treatment of patients with fever and neutropenia, but there is still controversy over the timing for introduction of glycopeptide antibiotics into therapeutic regimens. Teicoplanin is generally reserved for secondary therapy of patients with documented bacteraemia who fail to respond to initial empirical antibiotic regimens, but probably should be part of the initial empirical regimen in the setting of a high incidence of methicillin-resistant staphylococci. Teicoplanin has a lower propensity than vancomycin to impair renal function when either drug is combined with an aminoglycoside, causes fewer anaphylactoid reactions, and appears to be of comparable efficacy. Thus, teicoplanin may be preferred to vancomycin in the treatment of Gram-positive infections, and where a glycopeptide antibiotic is deemed a necessary inclusion in a regimen for empirical treatment in patients with fever and neutropenia.

A gene conferring resistance to vancomycin but not teicoplanin in isolates of Enterococcus faecalis and Enterococcus faecium demonstrates homology with vanB, vanA, and vanC genes of enterococci

We report the sequence of a 630-bp fragment of a gene associated with resistance to high levels of vancomycin in a clinical isolate of Enterococcus faecalis which retained susceptibility to teicoplanin. This gene was similar to the recently sequenced vanB and partially homologous with vanA, but it showed less-marked similarity to vanC. A DNA probe, derived from this polymerase chain reaction-amplified gene fragment, hybridized specifically with genomic DNA from Enterococcus faecium and E. faecalis isolates which were vancomycin resistant (MICs ranged from 8 to 512 micrograms/ml) but susceptible to teicoplanin. Curing of vancomycin resistance was associated with loss of DNA hybridization with the gene probe. Transfer of DNA which hybridized with the probe accompanied transfer of vancomycin resistance to a susceptible recipient strain. Neither curing nor transfer of vancomycin resistance was consistently related to loss or acquisition, respectively, of plasmid DNA.

Epidemiologic analysis and genotypic characterization of a nosocomial outbreak of vancomycin-resistant enterococci

We are reporting on a nosocomial outbreak of 213 cases of vancomycin-resistant enterococcus infection involving 2,812 enterococcal isolates from patients over a period of 36 months. In 1990, the Enterococcus faecium vancomycin susceptibility rate was found to be 85.7% (36 of 42 cases), and an incidence of 10.9% (42 of 383) was noted. The 1991 data showed E. faecium with a vancomycin susceptibility rate of 61.8% (110 of 178) and an incidence of 26.0% (178 of 684). Subsequently, in 1992, the incidence of E. faecium increased to 34.0% (599 of 1,745), with a decreased vancomycin susceptibility rate of 25.8% (155 of 599). The E. faecalis vancomycin susceptibility rate remained near 97% (1,768 of 1,823) over the 36-month period. Of 115 vancomycin-resistant enterococcus (VRE) clinical isolates identified by the MicroScan MIC Combo-6 panels (Baxter Healthcare, Sacramento, Calif.), the agar dilution method indicated the resistance rate to be 92.3% (106 of 115) (high level), 3.5% (4 of 115) midlevel, and 3.5% (4 of 115) (low level). Genotypic characterization of 32 different VRE isolates by field-inversion gel electrophoresis demonstrated 19 dissimilar restriction endonuclease patterns, with 9 patterns associated with VRE quinolone resistance. Statistical analysis of case-control data for 32 patients with VRE infections indicated a positive association with intrabdominal surgical procedures (odds ratio, 24.12), multidrug therapy (odds ratio, 37.80), preexposure to vancomycin (odds ratio, 20.21), and death (odds ratio, 17.50).

Cytotoxic effect of hemolytic culture supernatant from Enterococcus faecalis on mouse polymorphonuclear neutrophils and macrophages

We reconfirmed that the LD50s of hemolytic Enterococcus faecalis strains were significantly less than those of nonhemolytic E. faecalis strains in normal mice. Hemolysin produced by E. faecalis lysed human, horse, rabbit, and mouse erythrocytes, but not cow and sheep erythrocytes. Sphingomyelin comprises a part of the lipid composition of the erythrocyte membrane of all mammalian species tested. But phosphatidylcholine exists only in human, horse, rabbit, and mouse. These two lipids inhibited lysis of horse erythrocytes by hemolytic E. faecalis. Phosphatidylcholine is probably the binding component on the membrane of erythrocytes for E. faecalis hemolysin. The hemolytic culture supernatant lysed not only erythrocytes but also mouse polymorphonuclear neutrophils (PMNs) and macrophages.

Environmental strains of Enterococcus faecium with inducible high-level resistance to glycopeptides

High-level resistance to glycopeptides in Enterococcus faecium is associated with an inducible 39-kDa cytoplasmic membrane protein. The present paper shows that such glycopeptide-resistant E. faecium strains can not only be isolated in a definite clinical setting but also from waste water of sewage treatment plants. Nearer characterization of these and of clinical isolates by resistance pattern, biotyping, and genotyping (DNA-fingerprinting with pulsed-field gel electrophoresis) has shown that different glycopeptide-resistant E. faecium strains have been isolated from clinical sources and from waste water.