Molecular characterization and multilaboratory evaluation of Enterococcus faecalis ATCC 51299 for quality control of screening tests for vancomycin and high-level aminoglycoside resistance in enterococci

Bactericidal synergism between phage endolysin Ply2660 and cathelicidin LL-37 against vancomycin-resistant Enterococcus faecalis biofilms

Antibiotic resistance and the ability to form biofilms of Enterococcus faecalis have compromised the choice of therapeutic options, which triggered the search for new therapeutic strategies, such as the use of phage endolysins and antimicrobial peptides. However, few studies have addressed the synergistic relationship between these two promising options. Here, we investigated the combination of the phage endolysin Ply2660 and the antimicrobial peptide LL-37 to target drug-resistant biofilm-producing E. faecalis. In vitro bactericidal assays were used to demonstrate the efficacy of the Ply2660-LL-37 combination against E. faecalis. Larger reductions in viable cell counts were observed when Ply2660 and LL-37 were applied together than after individual treatment with either substance. Transmission electron microscopy revealed that the Ply2660-LL-37 combination could lead to severe cell lysis of E. faecalis. The mode of action of the Ply2660-LL-37 combination against E. faecalis was that Ply2660 degrades cell wall peptidoglycan, and subsequently, LL-37 destroys the cytoplasmic membrane. Furthermore, Ply2660 and LL-37 act synergistically to inhibit the biofilm formation of E. faecalis. The Ply2660-LL-37 combination also showed a synergistic effect for the treatment of established biofilm, as biofilm killing with this combination was superior to each substance alone. In a murine peritoneal septicemia model, the Ply2660-LL-37 combination distinctly suppressed the dissemination of E. faecalis isolates and attenuated organ injury, being more effective than each treatment alone. Altogether, our findings indicate that the combination of a phage endolysin and an antimicrobial peptide may be a potential antimicrobial strategy for combating E. faecalis.

Pheromone Activity after Stimulation with Ampicillin in a Plasmid-Free Enterococcus faecalis Strain

Enterococci exhibit clumping under the selective pressure of antibiotics. The aim of this study was to analyze the effect of supernatants from a plasmid-free clone (C29) of Enterococcus faecalis subjected to 0.25×, 0.5×, and 0.75× of the minimal inhibitory concentration (MIC) of ampicillin on the expression of an aggregation substance (AS) by a donor plasmid clone (1390R). A clumping assay was performed. The relative expression of prgB (gene that encodes AS) was determined and semiquantified in 1390R, and iad1 expression was determined and semiquantified in C29. AS expression was analyzed in the stimulated 1390R cells by confocal microscopy, flow cytometry, and ELISA. Adherence was also measured. Maximal clumping was observed with the pheromone medium 0.25×. Only the 1390R strain stimulated with the C29 supernatant without ampicillin and with 0.25× was able to express prgB. No expression of prgB was observed at 0.5× and 0.75×. The difference in relative expression (RE) of 1390R without ampicillin and with 0.25× was 0.5-fold. AS expression in 1390R showed the greatest increase upon stimulation with 0.25×. When 1390R was stimulated with 0.5× and 0.75×, AS expression was also observed but was significantly lower. Ampicillin stimulated C29 switch-off pheromone expression in recipient cells, which in turn switched off AS expression in donor cells. We observed that although prgB was switched off after 0.5× stimulation in C29, the supernatants induced expression in certain 1390R strains. In conclusion, ampicillin was able to modulate pheromone expression in free plasmid clones which, in turn, modulated AS expression in plasmid donor cells. The fact that PrgB gene expression was switched off after the ampicillin stimulus at 0.5× MIC, whereas AS proteins were present on the surface of the bacteria, suggested that a mechanism of rescue associated with mechanism pheromone sensing may be involved.

Antimicrobial Susceptibility of Fresh Produce-Associated Enterobacteriaceae and Enterococci in Oman

Fresh produce bacteria may have phenotypic and/or genotypic antimicrobial resistance traits that may lead to various consequences on the environment and human health. This study evaluated the susceptibility of fresh produce bacteria (banana, cabbage, capsicum, carrots, cucumber, dates, lettuce, mango, papaya, pomegranate, radish, tomato and watermelon) to chlorhexidine and the antibiotic resistance of enterococci. Eighty-eight Enterobacteriaceae bacteria and 31 enterococci were screened for their susceptibility to chlorhexidine using the broth microdilution method. Susceptibility of enterococci to various antibiotics was determined using agar dilution, colorimetric, and Kirby-Bauer disc diffusion methods. Enterococci were more susceptible to chlorhexidine than Enterobacteriaceae indicated by chlorhexidine minimum inhibitory concentration (MIC) of 1 to 8 µg/mL for the former and 1 to 64 µg/mL for the latter. The IntI 1, qacEΔ1, qacE and qacG genes were distributed weakly in three, two, two, and three Enterobacteriaceae isolates, respectively. Enterococci had resistance to chloramphenicol (3%), tetracycline (19%), erythromycin (68%), ciprofloxacin (55%), and vancomycin (10%) while 19% of them were multi-drug resistant. In conclusion, this research detected a low to moderate level of antibiotic resistance in enterococci. Some Enterobacteriaceae bacteria had reduced chlorhexidine MICs that were not 10x less than the recommended concentration (100–200 µg/mL) in food production areas which might challenge the success of the disinfection processes or have clinical implications if the involved bacteria are pathogens. The prevalence of antimicrobial-resistant bacteria in fresh produce should be monitored in the future.

Mobile genetic elements beyond the VanB-resistance dissemination among hospital-associated enterococci and other Gram-positive bacteria

An increasing resistance to vancomycin among clinically relevant enterococci, such as Enterococcus faecalis and Enterococcus faecium is a cause of a great concern, as it seriously limits treatment options. The vanB operon is one of most common determinants of this type of resistance. Genes constituting the operon are located in conjugative transposons, such as Tn1549-type transposons or, more rarely, in ICEEfaV583-type structures. Such elements show differences in structure and size, and reside in various sites of bacterial chromosome or, in the case of Tn1549-type transposons, are also occasionally associated with plasmids of divergent replicon types. While conjugative transposition contributes to the acquisition of Tn1549-type transposons from anaerobic gut commensals by enterococci, chromosomal recombination and conjugal transfer of plasmids appear to represent main mechanisms responsible for horizontal dissemination of vanB determinants among hospital E. faecalis and E. faecium. This review focuses on diversity of genetic elements harbouring vanB determinants in hospital-associated strains of E. faecium and E. faecalis, the mechanisms beyond vanB spread in populations of these bacteria, and provides an overview of the vanB-MGE distribution among other enterococci and Gram-positive bacteria as potential reservoirs of vanB genes.

Antimicrobial assay of combination surfactant irrigant regimen on vancomycin-resistant Enterococcus faecalis. An in vitro direct contact test

BACKGROUND

Vancomycin-resistant enterococci (VRE) are on the rise globally in primary intraradicular infections and resistant to most intracanal irrigants and medicaments. The aim of this study was to evaluate the efficacy of irrigants and identify a cost-effective regimen to eradicate VRE.

MATERIALS AND METHODS

In this in vitro study irrigants were categorized as primary and surfactant groups with individual concentrations consisting of 10 samples each. Primary irrigants; sodium hypochlorite (NaOCl), chlorhexidine (CHX), and iodine potassium iodide (IKI) were prepared in concentrations of 5%, 2.5%, 2%, and 1%. Surfactants cetrimide (CTR) and sodium dodecyl sulfate (SDS) were prepared in concentrations of 0.25%, 0.5%, 1%, and 2%. Biopure MTAD was chosen as the control group. ATCC 51299 (VRE) was evaluated for antimicrobial susceptibility to the above irrigants by direct contact test for 5 min. The effect of each test irrigant was determined by calculating the percentage kill of viable bacteria by spectrophotometer. Statistical analysis was done by means of a one-way ANOVA and Mann-Whitney U-test (P < 0.05 consider significant).

RESULTS

About 2.5% and 5% CHX were significant over mixture of tetracycline, acid and detergent (MTAD) (P < 0.05). 5% CHX could achieve 100% elimination while 2.5% CHX and 5% IKI had 99.90%. 2% CHX and 2.5% IKI had 99% effective kill percentage. All concentrations of NaOCl were ineffective (90%) as compared to MTAD (95%). CTR (0.5%, 1% and 2%) and SDS (2%) were significant (P < 0.05) over MTAD. Combination surfactant regimens of 2% CHX +0.5% CTR and 2% CHX +1% SDS achieved 99.90% eradication potential and were significant (P < 0.05) over MTAD.

CONCLUSION

Surfactant regimens were highly effective and superior to MTAD. CTR and SDS by their organic solvent property enhanced the antibacterial action of CHX.

A New Series of Pyrrole-Based Chalcones: Synthesis and Evaluation of Antimicrobial Activity, Cytotoxicity, and Genotoxicity

In an effort to develop new potent antimicrobial and anticancer agents, new pyrrole-based chalcones were designed and synthesized via the base-catalyzed Claisen-Schmidt condensation of 2-acetyl-1-methylpyrrole with 5-(aryl)furfural derivatives. The compounds were evaluated for their in vitro antimicrobial effects on pathogenic bacteria and Candida species using microdilution and ATP luminescence microbial cell viability assays. MTT assay was performed to determine the cytotoxic effects of the compounds on A549 human lung adenocarcinoma, HepG2 human hepatocellular carcinoma, C6 rat glioma, and NIH/3T3 mouse embryonic fibroblast cell lines. 1-(1-Methyl-1H-pyrrol-2-yl)-3-(5-(4-chlorophenyl)furan-2-yl)prop-2-en-1-one (7) and 1-(1-methyl-1H-pyrrol-2-yl)-3-(5-(2,5-dichlorophenyl)furan-2-yl)prop-2-en-1-one (9) were found to be the most potent antifungal agents against Candida krusei and therefore these compounds were chosen for flow cytometry analysis and Ames MPF assay. ATP bioluminescence assay indicated that the antifungal activity of compounds 7 and 9 against C. krusei was significantly higher than that of other compounds and the reference drug (ketoconazole), whereas flow cytometry analysis revealed that the percentage of dead cells treated with compound 7 was more than that treated with compound 9 and ketoconazole. According to Ames MPF assay, compounds 7 and 9 were found to be non-genotoxic against TA98 and TA100 with/without metabolic activation. MTT assay indicated that 1-(1-methyl-1H-pyrrol-2-yl)-3-(5-(2-nitrophenyl)furan-2-yl)prop-2-en-1-one (3) showed more selective anticancer activity than cisplatin against the HepG2 cell line. On the other hand, 1-(1-methyl-1H-pyrrol-2-yl)-3-(5-(4-nitrophenyl)furan-2-yl)prop-2-en-1-one (1) was found to be more effective and selective on the A549 cell line than cisplatin.

Genomic Sequence of a Clinical Vancomycin-Resistant Reference Strain, Enterococcus faecalis ATCC 51299

In this paper, we present a draft genome sequence of a quality control reference strain, Enterococcus faecalis ATCC 51299 (multilocus sequencing type [MLST] ST6), which is sensitive to teicoplanin but resistant to vancomycin. It is used in an agar screening test for streptomycin, gentamicin, and vancomycin resistance and the resistance marker vanB.

Biohybrid polymer-antimicrobial peptide medium against Enterococcus faecalis

Antimicrobial peptides (AMPs) are conserved evolutionary components of the innate immune system that are being tested as alternatives to antibiotics. Slow release of AMPs using biodegradable polymers can be advantageous in maintaining high peptide levels for topical treatment, especially in the oral environment in which dosage retention is challenged by drug dilution with saliva flow and by drug inactivation by salivary enzymatic activity. Enterococcus faecalis is a multidrug resistant nosocomial pathogen and a persistent pathogen in root canal infections. In this study, four ultra-short lipopeptides (C16-KGGK, C16-KLLK, C16-KAAK and C16-KKK) and an amphipathic α-helical antimicrobial peptide (Amp-1D) were tested against E. faecalis. The antibacterial effect was determined against planktonic bacteria and bacteria grown in biofilm. Of the five tested AMPs, C16-KGGK was the most effective. Next C16-KGGK was formulated with one of two polymers poly (lactic acid co castor oil) (DLLA) or ricinoleic acid-based poly (ester-anhydride) P(SA-RA). Peptide-synthetic polymer conjugates, also referred to as biohybrid mediums were tested for antibacterial activity against E. faecalis grown in suspension and in biofilms. The new formulations exhibited strong and improved anti- E. faecalis activity.

Oxidative stress enhances the expression of sulfur assimilation genes: preliminary insights on the Enterococcus faecalis iron-sulfur cluster machinery regulation

The Firmicutes bacteria participate extensively in virulence and pathological processes. Enterococcus faecalis is a commensal microorganism; however, it is also a pathogenic bacterium mainly associated with nosocomial infections in immunocompromised patients. Iron-sulfur [Fe-S] clusters are inorganic prosthetic groups involved in diverse biological processes, whose in vivo formation requires several specific protein machineries. Escherichia coli is one of the most frequently studied microorganisms regarding [Fe-S] cluster biogenesis and encodes the iron-sulfur cluster and sulfur assimilation systems. In Firmicutes species, a unique operon composed of the sufCDSUB genes is responsible for [Fe-S] cluster biogenesis. The aim of this study was to investigate the potential of the E. faecalis sufCDSUB system in the [Fe-S] cluster assembly using oxidative stress and iron depletion as adverse growth conditions. Quantitative real-time polymerase chain reaction demonstrated, for the first time, that Gram-positive bacteria possess an OxyR component responsive to oxidative stress conditions, as fully described for E. coli models. Likewise, strong expression of the sufCDSUB genes was observed in low concentrations of hydrogen peroxide, indicating that the lowest concentration of oxygen free radicals inside cells, known to be highly damaging to [Fe-S] clusters, is sufficient to trigger the transcriptional machinery for prompt replacement of [Fe-S] clusters.

Performance of the EUCAST disk diffusion method, the CLSI agar screen method, and the Vitek 2 automated antimicrobial susceptibility testing system for detection of clinical isolates of Enterococci with low- and medium-level VanB-type vancomycin resistance: a multicenter study

Different antimicrobial susceptibility testing methods to detect low-level vancomycin resistance in enterococci were evaluated in a Scandinavian multicenter study (n=28). A phenotypically and genotypically well-characterized diverse collection of Enterococcus faecalis (n=12) and Enterococcus faecium (n=18) strains with and without nonsusceptibility to vancomycin was examined blindly in Danish (n=5), Norwegian (n=13), and Swedish (n=10) laboratories using the EUCAST disk diffusion method (n=28) and the CLSI agar screen (n=18) or the Vitek 2 system (bioMérieux) (n=5). The EUCAST disk diffusion method (very major error [VME] rate, 7.0%; sensitivity, 0.93; major error [ME] rate, 2.4%; specificity, 0.98) and CLSI agar screen (VME rate, 6.6%; sensitivity, 0.93; ME rate, 5.6%; specificity, 0.94) performed significantly better (P=0.02) than the Vitek 2 system (VME rate, 13%; sensitivity, 0.87; ME rate, 0%; specificity, 1). The performance of the EUCAST disk diffusion method was challenged by differences in vancomycin inhibition zone sizes as well as the experience of the personnel in interpreting fuzzy zone edges as an indication of vancomycin resistance. Laboratories using Oxoid agar (P<0.0001) or Merck Mueller-Hinton (MH) agar (P=0.027) for the disk diffusion assay performed significantly better than did laboratories using BBL MH II medium. Laboratories using Difco brain heart infusion (BHI) agar for the CLSI agar screen performed significantly better (P=0.017) than did those using Oxoid BHI agar. In conclusion, both the EUCAST disk diffusion and CLSI agar screening methods performed acceptably (sensitivity, 0.93; specificity, 0.94 to 0.98) in the detection of VanB-type vancomycin-resistant enterococci with low-level resistance. Importantly, use of the CLSI agar screen requires careful monitoring of the vancomycin concentration in the plates. Moreover, disk diffusion methodology requires that personnel be trained in interpreting zone edges.

Identification and imaging of peptides and proteins on Enterococcus faecalis biofilms by matrix assisted laser desorption ionization mass spectrometry

The heptapeptide ARHPHPH was identified from biofilms and planktonic cultures of two different strains of Enterococcus faecalis, V583 and ATCC 29212, using matrix assisted laser desorption ionization mass spectrometry (MALDI-MS). ARHPHPH was also imaged at the boundary of cocultured, adjacent E. faecalis and Escherichia coli (ATCC 25922) biofilms, appearing only on the E. faecalis side. ARHPHPH was proteolyzed from κ-casein, a component in the growth media, by E. faecalis microbes. Additionally, top down and bottom up proteomic approaches were combined to identify and spatially locate multiple proteins within intact E. faecalis V583 biofilms by MALDI-MS. The resultant tandem MS data were searched against the NCBInr E. faecalis V583 database to identify thirteen cytosolic and membrane proteins which have functional association with the cell surface. Two of these proteins, enolase and GAPDH, are glycolytic enzymes known to display multiple functions in bacterial virulence in related bacterial strains. This work illustrates a powerful approach for discovering and localizing multiple peptides and proteins within intact biofilms.

Glycopeptide resistance in gram-positive cocci: a review

Vancomycin-resistant enterococci (VRE) have emerged as important nosocomial pathogens in the past two decades all over the world and have seriously limited the choices available to clinicians for treating infections caused by these agents. Methicillin-resistant Staphylococcus aureus, perhaps the most notorious among the nosocomial pathogens, was till recently susceptible to vancomycin and the other glycopeptides. Emergence of vancomycin nonsusceptible strains of S. aureus has led to a worrisome scenario where the options available for treating serious infections due to these organisms are very limited and not well evaluated. Vancomycin resistance in clinically significant isolates of coagulase-negative staphylococci is also on the rise in many setups. This paper aims to highlight the genetic basis of vancomycin resistance in Enterococcus species and S. aureus. It also focuses on important considerations in detection of vancomycin resistance in these gram-positive bacteria. The problem of glycopeptide resistance in clinical isolates of coagulase-negative staphylococci and the phenomenon of vancomycin tolerance seen in some strains of Streptococcus pneumoniae has also been discussed. Finally, therapeutic options available and being developed against these pathogens have also found a mention.

Multivalent artificial opsonin for the recognition and phagocytosis of Gram-positive bacteria by human phagocytes

Hospital-acquired infections (HAIs) remain a leading cause of death in the United States. Unfortunately, treatment of HAIs is complicated by the emergence of antibiotic-resistant bacterial strains. In an effort to enhance the body's natural immune response to infection, we have developed an artificial opsonin to promote the recognition, phagocytosis, and destruction of pathogenic bacteria by human phagocytes. The artificial opsonin is constructed from multivalent conjugates of poly(L-lysine)-graft-poly(ethylene glycol) with vancomycin and human IgG-Fc. Our approach utilizes vancomycin's inherent ability to bind to D-Ala-D-Ala terminated peptides present in the cell wall of Gram-positive bacteria. Here, we show that conjugation of vancomycin to PLL-g-PEG prevents its action as an antibiotic and allows vancomycin to function solely as a recognition molecule. Human IgG-Fc antibody fragment serves as a phagocyte recognition molecule and is recognized by the Fcγ cell surface receptors expressed on professional human phagocytes. Using flow cytometry, we found that a polysaccharide-encapsulated, methicillin-resistant strain of Staphylococcus epidermidis is efficiently recognized by the artificial opsonin (nearly 100% of cells were opsonized) and that opsonin binding is specific since it can be inhibited by the soluble cell wall peptide analog acetyl-Lys-D-Ala-D-Ala. Opsonization of S. epidermidis resulted in an approximate 2-fold increase in phagocytosis by a human neutrophil cell line. Notably, Enterococcus faecalis VanB, a bacterial strain with inducible vancomycin resistance, was used to show that the artificial opsonin does not unintentionally induce antibiotic resistance mechanisms.

Enterococcus faecalis sufCDSUB complements Escherichia coli sufABCDSE

Iron-sulfur [Fe-S] clusters are inorganic prosthetic groups that play essential roles in all living organisms. Iron and sulfur mobilization, formation of [Fe-S] clusters, and delivery to its final protein targets involves a complex set of specific protein machinery. Proteobacteria has three systems of [Fe-S] biogenesis, designated NIF, ISC, and SUF. In contrast, the Firmicutes system is not well characterized and has only one system, formed mostly by SUF homologs. The Firmicutes phylum corresponds to a group of pathological bacteria, of which Enterococcus faecalis is a clinically relevant representative. Recently, the E. faecalis sufCDSUB [Fe-S] cluster biosynthetic machinery has been identified, although there is no further information available about the similarities and/or variations of Proteobacteria and Firmicutes systems. The aim of the present work was to compare the ability of the different Proteobacteria and Firmicutes systems to complement the Azotobacter vinelandii and Escherichia coli ISC and SUF systems. Indeed, E. faecalis sufCDSUB is able to complement the E. coli SUF system, allowing viable mutants of both sufABCDSE and iscRSU-hscBA-fdx systems. The presence of all E. faecalis SUF factors enables proper functional interactions, which would not otherwise occur in proteins from different systems.

Synthesis and combinational antibacterial study of 5''-modified neomycin

A library of 5″-modified neomycin derivatives were synthesized for an antibacterial structure-activity optimization strategy. Two leads exhibited prominent activity against both methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). Antibacterial activities were measured when combined with other clinically used antibiotics. Significant synergistic activities were observed which may lead to the development of novel therapeutic practices in the battle against infectious bacteria.

Genetic Relatedness of Enterococcus Faecalis Isolates with High-level Gentamicin Resistance from Patients with Bacteraemia in the South East of Sweden 1994–2001

High-level gentamicin resistant (HLGR) enterococci (Enterococcus faecalis and Enterococcus faecium) have become a substantial nosocomial problem in many countries. In this study, we investigated the prevalence of HLGR enterococci and their genetic relatedness in blood culture isolates from patients with bacteraemia admitted to the 3 hospitals in Ostergötland, a county in the south east of Sweden, during 1994-2001. 36 of 250 E. faecalis (14%,) and 4 of 106 E. faecium isolates (4%) were shown by PCR to carry the aac(6')-Ie-aph(2")-Ia aminoglycoside modifying gene and these isolates were also classified as HLGR enterococci by the gentamicin antibiotic disk diffusion method. A majority of HLGR E. faecalis isolates (83%) belonged to the same cluster of genetically related isolates, according to the pulsed-field gel electrophoresis (PFGE) patterns, whereas all 4 HLGR E. faecium isolates had unique PFGE patterns. In conclusion, our study showed that in contrast to studies from many other countries, the presence of HLGR enterococci was more common in E. faecalis than in E. faecium and appeared the first time in 1996 and 1999, respectively. Bacteraemia with HLGR enterococci in Ostergötland was mainly due to the spread of a cluster of related E. faecalis strains.

Surprising alteration of antibacterial activity of 5"-modified neomycin against resistant bacteria

A facile synthetic protocol for the production of neomycin B derivatives with various modifications at the 5'' position has been developed. The structural activity relationship (SAR) against aminoglycoside resistant bacteria equipped with various aminoglycoside-modifying enzymes (AMEs) was investigated. Enzymatic and molecular modeling studies reveal that the superb substrate promiscuity of AMEs allows the resistant bacteria to cope with diverse structural modifications despite the observation that several derivatives show enhanced antibacterial activity compared to the parent neomycin. Surprisingly, when testing synthetic neomycin derivatives against other human pathogens, two leads exhibit prominent activity against both methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE) that are known to exert a high level of resistance against clinically used aminoglycosides. These findings can be extremely useful in developing new aminoglycoside antibiotics against resistant bacteria. Our result also suggests that new biological and antimicrobial activities can be obtained by chemical modifications of old drugs.

Detection of genetic elements carrying glycopeptide resistance clusters in Enterococcus by DNA microarrays

Glycopeptide resistance in enterococci is conferred by discrete genetic elements, which can carry mutations, deletions, and partial disruptions. We developed a custom DNA microarray as a tool for typing glycopeptide resistant enterococci. Oligonucleotide DNA microarrays were produced targeting the genes of enterococcal glycopeptide resistance gene clusters. A total of 105 probes selective for 42 genes were designed. The microarrays were validated on DNA extracted from reference strains and used to type 24 enterococcal strains belonging to various species and with different resistance phenotype, detecting all the genes of the resistance clusters which were actually present in each control and study strain. Moreover, this technique was capable of identifying the presence of additional resistant elements masked by a higher or similar resistance phenotype, and also of correcting an erroneous assignment made on the basis of phenotype alone. The use of our microarrays provided additional and complementary information compared to the phenotype alone. Our screening technique compares favourably, in aggregate terms of turnaround time, costs and convenience with other molecular biology techniques such as amplification and sequencing when applied to the study of complex genetic elements composed of several genes, such as glycopeptide resistance elements in enterococci.

Diversity of bacteriocins and activity spectrum in Streptococcus pneumoniae

The production of bacteriocins can be favorable for colonization of the host by eliminating other bacterial species that share the same environment. In Streptococcus pneumoniae, the pnc (blp) locus encoding putative bacteriocins, immunity, and export proteins is controlled by a two-component system similar to the comCDE system required for the induction of genetic competence. A detailed comparison of the pnc clusters of four genetically distinct isolates confirmed the great plasticity of this locus and documented several repeat sequences. Members of the multiple-antibiotic-resistant Spain23F-1 clone, one member of the Spain9V-3 clone, sensitive 23F strain 2306, and the TIGR4 strain produced bactericidal substances active against other gram-positive bacteria and in some cases against S. pneumoniae as well. However, other strains did not show activity against the indicator strains despite the presence of a bacteriocin cluster, indicating that other factors are required for bacteriocin activity. Analysis of strain 2306 and mutant derivatives of this strain confirmed that bacteriocin production was dependent on the two-component regulatory system and genes involved in bacteriocin transport and processing. At least one other bacteriocin gene, pncE, is located elsewhere on the chromosome and might contribute to the bacteriocin activity of this strain.

Convenient selective differential broth for isolation of vancomycin-resistant enterococcus from fecal material

Studies have shown that vancomycin broth enrichment is superior to direct plating for the detection of vancomycin-resistant enterococcus (VRE), but vancomycin selective broth is not generally commercially available. We developed an easy-to-prepare VRE selective differential broth and compared it to direct plating on bile esculin azide (BEA) agar for the isolation of VRE from fecal samples. A total of 528 consecutive rectal swabs and stools were inoculated onto BEA agar and into BEA broth with vancomycin at a concentration of 15 microg/ml (BEA VAN15 microg/ml broth). After 1 to 2 days of incubation, broths were subcultured to BEA VAN6 microg/ml agar. Bile esculin-positive colonies from the direct and broth subculture plates were evaluated for the presence of VRE by standard microbiological techniques. Addition of the broth enrichment step led to the detection of significantly more VRE isolates than did direct plating alone (28 versus 18 VRE isolates, respectively). In all, 30 VRE strains were isolated from 29 cultures, all of which were Enterococcus faecium. MICs of vancomycin ranged from 32 microg/ml (n = 2) to > 256 microg/ml (n = 28). Twenty-two VRE isolates were available for further testing: sixteen exhibited a VanA phenotype and six were of the VanB phenotype. van genotypes were in agreement with phenotypes for all VRE isolates except one, which could not be genotyped. The broth method also resulted in significantly fewer bile esculin-positive, non-VRE isolates requiring further workup. We have thus developed an easily prepared vancomycin selective differential broth that is significantly more sensitive and specific in the detection of VRE than is direct fecal plating to BEA agar.

Antimicrobial proficiency testing of National Nosocomial Infections Surveillance System hospital laboratories

OBJECTIVE

The National Nosocomial Infections Surveillance (NNIS) System personnel report trends in antimicrobial-resistant pathogens. To validate select antimicrobial susceptibility testing results and to identify test methods that tend to produce errors, we conducted proficiency testing among NNIS System hospital laboratories.

SETTING

NNIS System hospital laboratories in the United States.

METHODS

Each laboratory received five organisms (ie, an imipenem-resistant Serratia marcescens, an oxacillin-resistant Staphylococcus aureus, a vancomycin-resistant Enterococcus faecalis, a vancomycin-intermediate Staphylococcus epidermidis, and an extended-spectrum beta-lactamase (ESbetaL)-producing Klebsiella pneumoniae). Testing results were compared with reference testing results from the Centers for Disease Control and Prevention.

RESULTS

Of 138 laboratories testing imipenem against the Serratia marcescens strain, 110 (80%) correctly reported minimum inhibitory concentrations (MICs) or zone sizes in the resistant range. All 193 participating laboratories correctly reported the Staphylococcus aureus strain as oxacillin resistant Of the 193 laboratories, 169 (88%) reported correct MICs or zone sizes for the vancomycin-resistant Enterococcus faecalis. One hundred sixty-two (84%) of 193 laboratories demonstrated the ability to detect a vancomycin-intermediate strain of Staphylococcus epidermidis, however, disk diffusion performed poorly when testing both staphylococci and enterococci with vancomycin. Although laboratory personnel correctly reported nonsusceptible extended-spectrum cephalosporins and aztreonam results for K. pneumoniae, only 98 (51%) of 193 correctly reported this organism as an ESbetaL producer.

CONCLUSION

Overall, NNIS System hospital laboratory personnel detected most emerging resistance patterns. Disk diffusion continues to be unreliable for vancomycin testing of staphylococci and must be used cautiously for enterococci. Further education on the processing of ESbetaL-producing organisms is warranted.

A comparison of high-level aminoglycoside resistance in vancomycin-sensitive and vancomycin-resistant Enterococcus species

The aim of this study was to investigate whether there was a significant difference in high-level aminoglycoside resistance (HLAR) between vancomycin-sensitive enterococci (VSE) and vancomycin-resistant enterococci (VRE). Vancomycin resistance was determined in 116 Enterococcus isolates using brain-heart infusion agar containing 6 micrograms/ml vancomycin. HLAR was determined by both standard agar screening and disk diffusion methods. Streptomycin and gentamicin were used as predictors of HLAR. Vancomycin resistance and HLAR were found in 17 (14.7%) and 41 (35.3%) of the Enterococcus strains, respectively. HLAR was found in 11 of 17 VRE and 30 of 98 VSE strains. HLAR in VRE strains was significantly higher than in VSE. More enterococcal strains were found to be resistant to both gentamicin and streptomycin (29) than to gentamicin (one) or streptomycin (11) alone. The HLAR rate in VRE was two-fold higher than in VSE. The synergistic bactericidal effect of aminoglycosides and beta-lactam or glycopeptide antibiotics is lost if there is high-level resistance to aminoglycosides.

Nosocomial outbreak of vancomycin-resistant Enterococcus faecium at a German university pediatric hospital

Nosocomial Infections caused by vancomycin-resistant enterococci (VRE) are an emerging threat to critically ill patients. At the University Hospital Eppendorf, VRE were isolated from 38 patients between August 1993 and April 1997, of whom 32 were hospitalized at the Department of Pediatrics. Pulsed-field gel electrophoresis revealed that 26 Enterococcus faecium isolates from patients of the Department of Pediatrics were identical or closely related, and that isolates from three additional patients of the same department were possibly related. All of these isolates were of vanA genotype. They were resistant to glycopeptides, ampicillin, ciprofloxacin, clindamycin, and erythromycin. Most isolates displayed high-level resistance to gentamicin, but all remained susceptible to quinupristin/dalfopristin. Implementation of stringent hand disinfection and environmental disinfection policies, as well as measures for patient isolation contained this first outbreak of VRE at a German Children's hospital, which emphasizes the importance of hygienic measures for the control of nosocomial spread of these organisms.

Identification and characterization of vanB2 glycopeptide resistance elements in enterococci isolated in Scotland

Thirty-two vanB glycopeptide-resistant enterococci (28 Enterococcus faecium and 4 Enterococcus faecalis) were collected from hospitalized patients in Glasgow, Edinburgh, Dundee, and Aberdeen, Scotland, and the vanB element in each was compared to vanB1 of E. faecalis strain ATCC 51299. HhaI digestion of PCR fragments of the vanB ligase gene was used to identify vanB subtypes. All E. faecium isolates were vanB2, and all E. faecalis isolates were vanB1. Restriction fragment length polymorphism analysis of a 5,180-bp vanS(B)-vanX(B) long-PCR fragment of the vanB cluster showed the loss of HaeII restriction sites in vanS(B), vanW, and vanX(B) in strains containing a vanB2 ligase gene. Partial sequences of genes in the vanB2 cluster for two genomically distinct Scottish isolates were >99.8% identical to each other. vanS(B2), vanX(B2), and vanB2 sequences differed at the nucleotide level from those of vanS(B), vanX(B), and vanB by 4.2, 4.6, and 4.8%, respectively. The vanB2 resistance element appears to be widespread among VanB glycopeptide-resistant E. faecium strains isolated in Scottish hospitals.

Use of molecular and reference susceptibility testing methods in a multicenter evaluation of MicroScan dried overnight gram-positive MIC panels for detection of vancomycin and high-level aminoglycoside resistances in enterococci

Modified MicroScan gram-positive MIC no. 8 panels (PM-8) were analyzed for their improved ability to detect vancomycin resistance (VR) and high-level aminoglycoside resistance (HLAR) in enterococci. A validation study design that utilized selected challenge strains, recent clinical isolates, and reproducibility experiments in a multicenter format was selected. Three independent medical centers compared the commercial panels to reference broth microdilution panels (RBM) and Synergy Quad Agar (QA). Resistance was verified by demonstration of VR and HLAR genes by PCR tests. The study was conducted in three phases. (i) In the challenge phase (CP), two well-characterized sets of enterococci were obtained from the Centers for Disease Control and Prevention; one set contained 50 isolates for VR testing and one contained 48 isolates for HLAR testing. In addition, a set of 47 well-characterized isolates representing diverse geographic areas, obtained from earlier national surveillance studies, was tested at the University of Iowa College of Medicine (UICM). (ii) In the efficacy phase (EP), each laboratory tested 50 recent, unique clinical isolates by all methods. (iii) In the reproducibility Phase (RP), each laboratory tested the same 10 strains by all methods in triplicate on three separate days. All isolates from the EP were sent to the UICM for molecular characterization of vanA, -B, -C1, -C2-3, and HLAR genes. In the CP, the ranking of test methods by error rates (in parentheses; very major and major errors combined, versus PCR results) were as follows: for high-level streptomycin resistance (HLSR), QA (12.0%) > PM-8 (5.2%) > RBM (1.6%); for high-level gentamicin resistance (HLGR), RBM (3.7%) > PM-8 (3.1%) > QA (2.6%); and for VR, RBM = QA (3.0%) > PM-8 (1.2%). In the EP, agreement between all methods and the reference PCR result was 98.0% for HLSR, 99.3% for HLGR, and 98. 6% for VR. In the RP, the percentages of results +/- 1 log2 dilution of the all-participant mode were as follows: for VR, 100% (PM-8), 98.9% (QA), and 90.0% (RBM); for HLSR, 99.6% (RBM), 98.5% (PM-8), and 82.2% (QA); and for HLGR, 99.6% (RBM), 99.3% (PM-8), and 98.1% (QA). The ability of the PM-8 to detect VR and HLAR in enterococci was comparable to those for reference susceptibility and molecular PCR methods and was considered acceptable for routine clinical laboratory use.

Characterization of staphylococci with reduced susceptibilities to vancomycin and other glycopeptides

During the last several years a series of staphylococcal isolates that demonstrated reduced susceptibility to vancomycin or other glycopeptides have been reported. We selected 12 isolates of staphylococci for which the vancomycin MICs were > or =4 microg/ml or for which the teicoplanin MICs were > or =8 microg/ml and 24 control strains for which the vancomycin MICs were < or =2 microg/ml or for which the teicoplanin MICs were < or =4 microg/ml to determine the ability of commercial susceptibility testing procedures and vancomycin agar screening methods to detect isolates with reduced glycopeptide susceptibility. By PCR analysis, none of the isolates with decreased glycopeptide susceptibility contained known vancomycin resistance genes. Broth microdilution tests held a full 24 h were best at detecting strains with reduced glycopeptide susceptibility. Disk diffusion did not differentiate the strains inhibited by 8 microg of vancomycin per ml from more susceptible isolates. Most of the isolates with reduced glycopeptide susceptibility were recognized by MicroScan conventional panels and Etest vancomycin strips. Sensititre panels read visually were more variable, although with some of the panels MICs of 8 microg/ml were noted for these isolates. Vitek results were 4 microg/ml for all strains for which the vancomycin MICs were > or =4 microg/ml. Vancomycin MICs on Rapid MicroScan panels were not predictive, giving MICs of either < or =2 or > or =16 microg/ml for these isolates. Commercial brain heart infusion vancomycin agar screening plates containing 6 microg of vancomycin per ml consistently differentiated those strains inhibited by 8 microg/ml from more susceptible strains. Vancomycin-containing media prepared in-house showed occasional growth of susceptible strains, Staphylococcus aureus ATCC 29213, and on occasion, Enterococcus faecalis ATCC 29212. Thus, strains of staphylococci with reduced susceptibility to glycopeptides, such as vancomycin, are best detected in the laboratory by nonautomated quantitative tests incubated for a full 24 h. Furthermore, it appears that commercial vancomycin agar screening plates can be used to detect these isolates.