Investigation of the reformulated Remel Synergy Quad plate for detection of high-level aminoglycoside and vancomycin resistance among enterococci

Chemiluminescence determination of amikacin based on the inhibition of the luminol reaction catalyzed by copper

A simple and sensitive method has been proposed for the amikacin sulphate determination. It is based on the inhibition of the chemiluminescence (CL) emission generated from the oxidation of luminol in alkaline medium by H2O2 catalyzed by Cu(II), due to the interaction caused by amikacin, which forms a robust complex with the catalyst. The optimization of the experimental and instrumental variables affecting this CL inhibition effect has been carried out using statistical models, based on the application of two-level full factorial and Box-Behnken designs. The performance characteristics of the proposed method have been established, showing that the method is efficient to determine amikacin sulphate in the linear range of 9.89-20 mg/L with a detection limit of 2.97 mg/L. It has been successfully applied to the amikacin sulphate determination in pharmaceutical formulations.

Comparison of microscan broth microdilution, synergy quad plate agar dilution, and disk diffusion screening methods for detection of high-level aminoglycoside resistance in enterococcus species

We compared the dried MicroScan microdilution panel, Synergy Quad plate agar dilution, and high-potency disk diffusion screening methods for the detection of high-level aminoglycoside resistance in 815 enterococcal bloodstream isolates. Agreement between the three methods was 99% when testing for high-level gentamicin resistance and 96% when testing for high-level streptomycin resistance.

Efficacy of telithromycin (HMR 3647) against enterococci in a mouse peritonitis model

We used a mouse peritonitis model to evaluate the in vivo efficacy of telithromycin (HMR 3647) (TEL) and erythromycin (ERY) against four strains of Enterococcus faecalis and three strains of Enterococcus faecium with differing susceptibilities to TEL. TEL was highly active in vivo against Ery-susceptible (Ery(s)) and -intermediate (Ery(i)) strains (MIC of TEL = 0.015 to 0.062 microg/ml) and showed less efficacy against Ery-resistant (Ery(r)) isolates (MIC of TEL = 4 to 16 microg/ml), although this was overcome in part by a second subcutaneous dose. Quinupristin-dalfopristin was also noted to have less efficacy against Ery(r) versus Ery(s) or Ery(i) E. faecium strains, but this difference was reduced by intravenous administration. In conclusion, TEL was more potent in vivo against enterococci than was ERY; its activity was lowered by the presence of erm(B)-mediated Ery(r).

The efficacy and safety of quinupristin/dalfopristin for the treatment of infections caused by vancomycin-resistant Enterococcus faecium. Synercid Emergency-Use Study Group

A progressive increase in the incidence of vancomycin resistance in strains of Enterococcus faecium (VREF) has severely constrained treatment options for patients with infection caused by this emerging pathogen. Quinupristin/dalfopristin (Synercid), the first injectable streptogramin antibiotic, is active in vitro against VREF, with an MIC90 of 1.0 mg/L. We studied the clinical efficacy and safety of quinupristin/dalfopristin in the treatment of VREF infection. Two prospective studies were conducted simultaneously. The first enrolled only patients with VREF infection; the second included patients with infection caused by other gram-positive bacterial pathogens in addition to VREF. Patients were enrolled if they had signs and symptoms of active infection and no appropriate alternative antibiotic therapy. The recommended treatment regimen of quinupristin/dalfopristin was 7.5 mg/kg i.v. every 8 h for a duration judged appropriate by the investigator. A total of 396 patients with VREF infection were enrolled. The most frequent indications for treatment included intra-abdominal infection, bacteraemia of unknown origin, urinary tract infection, catheter-related bacteraemia, and skin and skin structure infection. This patient population had a high prevalence of severe underlying illness, including a history of diabetes mellitus, transplantation, mechanical ventilation, dialysis, chronic liver disease with cirrhosis and oncological disorders. The mean (+/- S.D.) duration of treatment was 14.5 +/- 10.7 days (range: 1-108). The majority of patients (82.1%) were treated every 8 h, as assessed on day 2 of treatment, while 15.9% were treated every 12 h. The clinical success rate was 73.6% [142/193 clinically evaluable patients; 95% confidence interval (CI): 67.4%, 79.8%], the bacteriological success rate 70.5% (110/156 bacteriologically evaluable patients; 95% CI: 63.4%, 77.7%) and the overall success (both clinical and bacteriological success) rate 65.8% (102/156 bacteriologically evaluable patients; 95% CI: 57.9%, 72.9%). VREF bacteraemia at entry, mechanical ventilation and laparotomy were associated with a worse outcome. Quinupristin/dalfopristin was generally well tolerated. The most common systemic adverse events related to treatment were arthralgias (9.1%) and myalgias (6.6%). Related laboratory abnormalities were infrequent. In these severely ill patients with VREF infection and no other clinically appropriate therapeutic alternatives, quinupristin/dalfopristin demonstrated substantial efficacy and a good nervous system, cardiovascular, gastrointestinal, renal and hepatic tolerability.

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.

Clonal dissemination of vancomycin-resistant Enterococci and comparison of susceptibility testing methods

One hundred fifty clinical isolates of Enterococcus faecalis (88 isolates) and Enterococcus faecium (62 isolates) were tested in vitro for their susceptibility to vancomycin and high-level aminoglycosides (HLA). Remel's Synergy Quad Plates (RSQ) were used as the reference method and compared to Kirby-Bauer disc diffusion test, Vitek GPS-TA card, MicroScan Panel (GP-6), and Etest. Streptomycin susceptibility results for MicroScan GP-6 and RSQ were recorded at 24 and 48 h and all other methods and antibiotics were read at 24 h or less. When compared with the agar screen method, all of the methods demonstrated > 99% agreement. One isolate was falsely sensitive to gentamicin at 24 h, but resistant at 48 h, when tested on both MicroScan and RSQ agar screen. Thirty-nine isolates showed resistance to vancomycin with all methods. These isolates were from three different local hospitals and were identified as E. faecium. Pulse-field gel electrophoresis demonstrated that all of the vancomycin-resistant isolates were derived from the same clone. Of interest is the observation that high-level resistance to aminoglycosides varied between the clonally related isolates.

Comparison of agar dilution, broth microdilution, E-test, disk diffusion, and automated Vitek methods for testing susceptibilities of Enterococcus spp. to vancomycin

An evaluation was undertaken to determine the optimal method for testing the susceptibilities of 100 clinical isolates and two reference strains of Enterococcus spp. to vancomycin in vitro. Six testing methods were studied by using the following media and incubation times: agar screen with the Synergy Quad Plate (Remel, Lenexa, Kans.), an in-house-prepared brain heart infusion (BHI) agar plate, and an in-house-prepared Mueller-Hinton (MH) agar plate, all incubated for 24 or 48 h; broth microdilution (Sensititre Just One Strip; AccuMed International, Inc., West Lake, Ohio) with BHI or cation-adjusted MH broth incubated for 24 or 48 h; agar dilution with BHI or MH agar incubated for 24 or 48 h; epsilometer test (E test; AB BioDisk, Solna, Sweden) with BHI or MH agar incubated for 24 or 48 h; disk diffusion with BHI or MH agar incubated for 24 or 48 h; and the automated Vitek method with the gram-positive susceptibility Staphylococcus aureus card and R02.03 software (bioMerieux, Inc., Hazelwood, Mo.). Growth failures occurred with MH media (n = 6) but not with BHI media. One growth failure occurred with the Vitek method. Results for each testing method for each Enterococcus strain were interpreted as susceptible, intermediate, or resistant according to current National Committee for Clinical Laboratory Standards (NCCLS) criteria and compared to the vancomycin resistance genotype (i.e., vanA, vanB, vanC-1, or vanC-2/3). For all methods, extension of the incubation time from 24 h to 48 h either produced no difference in the results or gave poorer results. The following methods produced no very major or major interpretive errors: broth microdilution with BHI media incubated for 24 h, agar dilution with BHI media incubated for 24 or 48 h, and E test with BHI media incubated for 24 or 48 h. Unacceptable frequencies of very major errors (> 1%) occurred with all methods for which MH media were used. Minor interpretive errors were frequent with all methods. These minor interpretive errors also occurred most frequently with Enterococcus strains with vanC genes, which encoded low-level vancomycin resistance (MIC < or = 8 microg/ml), as opposed to Enterococcus strains which possessed vanA or vanB genes, which encoded higher-level vancomycin resistance (MIC > or = 64 microg/ml). Modification of NCCLS breakpoints, especially for motile Enterococcus spp. (E. casseliflavus, E. flavescens, and E. gallinarum), may resolve this problem; however, in the current study, one E. faecalis strain and one E. faecium strain carried only the vanC gene. The agar screen method may also require reformulation. The current agar screen plate contains 6 microg of vancomycin per ml, which may not detect all low-level resistance associated with vanC genotypes. Nevertheless, the clinical significance of this low-level vancomycin resistance remains unknown.

Vancomycin-resistant enterococcus. Detection, epidemiology, and control measures

VRE have spread rapidly since their initial description in 1988. Although much has been learned about the epidemiology of VRE, further studies are needed to establish the reservoirs of the organism and the relative importance of various modes of transmission. There is considerable anecdotal evidence that nosocomial transmission of VRE can be thwarted by using measures such as those recommended by HICPAC, especially if they are implemented promptly after VRE have been introduced into hospitals.

Evaluation of commercial vancomycin agar screen plates for detection of vancomycin-resistant enterococci

Brain heart infusion-6-micrograms/ml vancomycin agar plates obtained from five commercial sources (B-D Microbiology Systems, Carr-Scarborough Microbiologicals, MicroBio Products, PML Microbiologicals, and REMEL) were evaluated with 714 enterococci for detection of vancomycin resistance. All 465 (100%) vancomycin-resistant enterococci (MIC > or = 32 micrograms/ml) were detected by each manufacturer's agar screen plate, and each manufacturer's agar screen plate detected at least 99% of the 177 vancomycin-susceptible enterococci (MIC < or = 4 micrograms/ml). Detection of the 72 vancomycin-intermediate enterococci (MIC = 6 to 16 micrograms/ml) ranged from 94% for B-D Microbiology Systems to 99% for PML Microbiologicals.

Revised approach for identification and detection of ampicillin and vancomycin resistance in Enterococcus species by using MicroScan panels

The frequency of antimicrobial agent-resistant enterococci is increasing, making accurate identification and screening for susceptibility essential. We evaluated the ability of MicroScan Positive Breakpoint Combo Type 6 panels (Dade MicroScan Inc., West Sacramento, Calif.) to identify Enterococcus species and to detect ampicillin and vancomycin resistance. A total of 398 well-characterized Enterococcus isolates from two institutions were inoculated into MicroScan panels, into conventional biochemical assays, and into ampicillin and vancomycin agar dilution media. Resistance was verified by the broth macrodilution method. MicroScan panels accurately detected resistance to ampicillin in 132 of 132 enterococcal isolates, while three isolates for which the MICs were < 16 micrograms/ml were classified incorrectly by MicroScan panels as resistant. No beta-lactamase-producing enterococci were detected. All 64 isolates showing resistance to vancomycin (MICs > or = 32 micrograms/ml) were correctly classified by MicroScan panels. Seven isolates for which the vancomycin MICs were 8 and 16 micrograms/ml were incorrectly classified as susceptible by MicroScan panels, while eight isolates for which the MICs were 4 micrograms/ml were incorrectly labeled as intermediate. Fourteen of these 15 isolates were subsequently identified as motile enterococci. Overall, there were three major errors in susceptibility testing for ampicillin and 15 minor errors for vancomycin. Conventional testing confirmed the identity of 181 Enterococcus faecalis isolates, 157 E. faecium isolates, and 60 isolates of other species; however, 56 of these 60 isolates were misidentified by the MicroScan panels. After recognition of this problem, a revised approach which included tests for pigment, motility, and sucrose fermentation was devised. In combination with these additional assays, the conventional MicroScan panels accurately identified the 56 originally misidentified isolates. In summary, the ability of MicroScan panels to detect vancomycin and ampicillin resistance in enterococci was confirmed. Our study found that the inability of MicroScan panels to identify enterococci other than E. faecalis and E. faecium can be compensated for by the addition of standard assays.

Factors influencing the vitek gram-positive susceptibility system's detection of vanB-encoded vancomycin resistance among enterococci

Studies were conducted to identify factors contributing to the inability of the Vitek Gram-Positive Susceptibility system (GPS; bioMerieux, Vitek, Inc., Hazelwood, Mo.) to reliably detect vanB-mediated vancomycin resistance among enterococci. To some extent the accuracy of the GPS depended on a particular strain's level of resistance, as all isolates for which vancomycin MICs were > or = 128 mu g/ml were readily detected but detection of resistance expressed by several strains for which MICs were < or = 64 mu g/ml was sporadic. Factors besides the level of resistance were studied in two vanB strains. For one strain (Enterococcus faecium U8304), the ability of GPS to detect resistance was accurate and consistent, while for the other (Enterococcus faecalis V583), GPS results were inconsistent and unreliable. Using these isolates, we established that growth medium had the most notable effect on the detection of resistance. In the absence of vancomycin, Vitek GPS broth supported growth comparable to that obtained with brain heart infusion broth for both E. faecium U8304 and E. faecalis V583. However, in the presence of vancomycin the growth patterns changed dramatically so that neither VanB strain grew well in Vitek broth, and growth of V583 was barely detectable after 8 h of incubation. In contrast, good growth of both strains was observed in brain heart infusion broth supplemented with vancomycin. Additionally, the same medium effect was observed with other inducibly resistant VanB strains. In conclusion, although Vitek broth can support good enterococcal growth, this medium does not sufficiently support expression of vancomycin resistance by certain strains to allow them to be detected by the Vitek automated system. Furthermore, this observation establishes that the type of growth medium used can substantially influence the expression of vancomycin resistance and indicates that medium-based strategies should be explored for the enhancement of resistance detection among commercial systems.