Detection of high-level aminoglycoside resistance in enterococci other than Enterococcus faecalis

Antimicrobial Susceptibility Testing for Enterococci

Enterococci are major, recalcitrant nosocomial pathogens with a wide repertoire of intrinsic and acquired resistance determinants and the potential of developing resistance to all clinically available antimicrobials. As such, multidrug-resistant enterococci are considered a serious public health threat. Due to limited treatment options and rapid emergence of resistance to all novel agents, the clinical microbiology laboratory plays a critical role in deploying accurate, reproducible, and feasible antimicrobial susceptibility testing methods to guide appropriate treatment of patients with deep-seated enterococcal infections. In this review, we provide an overview of the advantages and disadvantages of existing manual and automated methods that test susceptibility of Enterococcus faecium and Enterococcus faecalis to β-lactams, aminoglycosides, vancomycin, lipoglycopeptides, oxazolidinones, novel tetracycline-derivatives, and daptomycin. We also identify unique problems and gaps with the performance and clinical utility of antimicrobial susceptibility testing for enterococci, provide recommendations for clinical laboratories to circumvent select problems, and address potential future innovations that can bridge major gaps in susceptibility testing.

High Level Aminoglycoside Resistance And Distribution Of The Resistance Genes In Enterococcus faecalis And Enterococcus faecium From Teaching Hospital In Malaysia

Background

Enterococcus faecium and Enterococcus faecalis are among the predominant species causing hospital-acquired infections. Currently, enterococcal infections are treated using combination therapy of an aminoglycoside with cell-wall active agents, which led to high level aminoglycoside resistance (HLAR) and vancomycin resistance (VRE) among enterococci. The aim of this study was to determine the prevalence of HLAR and the distribution of the resistance genes among clinical E. faecalis and E. faecium isolates in Malaysia.

Materials and methods

Seventy-five enterococci isolates recovered from different clinical sources were re-identified by subculturing on selective medium, Gram staining, biochemical profiling (API 20 Strep), and 16s rRNA sequencing. Antimicrobial susceptibility testing (AST) was performed using Kirby-Bauer disc diffusion, E-test, and broth microdilution methods. PCR amplification was used to detect the presence of aminoglycoside modifying enzyme (AME) genes [aac(6')-Ie-aph(2")-Ia, aph(2")-Ib, aph(2")-Ic, aph(2")-Id, aph(3')-IIIa]. Descriptive data analysis was used to analyze the antibiotic susceptibility profiles and the distribution of HLAR genes.

Results

The majority of the isolates recovered from the clinical samples are E. faecalis (66.7%), with the highest recovery from the pus. The prevalence of HLGR (51%) is higher when compared to HLSR (45-49%). Analysis of the resistance genes showed that bifunctional genes aac(6')-Ie-aph(2")-Ia and aph(3')-IIIa contributed to the HLAR E. faecalis and E. faecium. The other AME genes [aph(2")-Ib, aph(2")-Ic, aph(2")-Id] were not detected in this study.

Conclusion

This study provides the first prevalence data on HLAR and the distribution of the AME genes among E. faecalis and E. faecium isolates from Malaysia. These highlight the need for continued antibiotic surveillance to minimize its emergence and further dissemination.

High level aminoglycoside resistance and distribution of aminoglycoside resistant genes among clinical isolates of Enterococcus species in Chennai, India

Enterococci are nosocomial pathogen with multiple-drug resistance by intrinsic and extrinsic mechanisms. Aminoglycosides along with cell wall inhibitors are given clinically for treating enterococcal infections. 178 enterococcal isolates were analyzed in this study. E. faecalis is identified to be the predominant Enterococcus species, along with E. faecium, E. avium, E. hirae, E. durans, E. dispar and E. gallinarum. High level aminoglycoside resistance (HLAR) by MIC for gentamicin (GM), streptomycin (SM) and both (GM + SM) antibiotics was found to be 42.7%, 29.8%, and 21.9%, respectively. Detection of aminoglycoside modifying enzyme encoding genes (AME) in enterococci was identified by multiplex PCR for aac(6')-Ie-aph(2'')-Ia; aph(2'')-Ib; aph(2'')-Ic; aph(2'')-Id and aph(3')-IIIa genes. 38.2% isolates carried aac(6')-Ie-aph(2'')-Ia gene and 40.4% isolates carried aph(3')-IIIa gene. aph(2'')-Ib; aph(2'')-Ic; aph(2'')-Id were not detected among our study isolates. aac(6')-Ie-aph(2'')-Ia and aph(3')-IIIa genes were also observed in HLAR E. durans, E. avium, E. hirae, and E. gallinarum isolates. This indicates that high level aminoglycoside resistance genes are widely disseminated among isolates of enterococci from Chennai.

High-level aminoglycoside resistance and virulence characteristics among Enterococci isolated from recreational beaches in Malaysia

We report the first study on the occurrence of high-level aminoglycoside-resistant (HLAR) Enterococci in coastal bathing waters and beach sand in Malaysia. None of the encountered isolates were resistant to high levels of gentamicin (500 μg/mL). However, high-level resistance to kanamycin (2,000 μg/mL) was observed in 14.2 % of tested isolates, the highest proportions observed being among beach sand isolates. High-level resistance to kanamycin was higher among Enterococcus faecalis and Enterococcus faecium than Enterococcus spp. Chi-square analysis showed no significant association between responses to tested antibiotics and the species allocation or source of isolation of all tested Enterococci. The species classification of encountered Enterococci resistance to vancomycin was highest among Enterococcus spp. (5.89 %) followed by E. faecium (4.785) and least among E. faecalis. A total of 160 isolates were also tested for virulence characteristics. On the whole, caseinase production was profoundly highest (15.01 %) while the least prevalent virulence characteristic observed among tested beach Enterococci was haemolysis of rabbit blood (3.65 %). A strong association was observed between the source of isolation and responses for each of caseinase (C = 0.47, V = 0.53) and slime (C = 0.50, V = 0.58) assays. Analysis of obtained spearman's coefficient showed a strong correlation between caseinase and each of the slime production (p = 0.04), gelatinase (p = 0.0035) and haemolytic activity on horse blood (p = 0.004), respectively. Suggestively, these are the main virulent characteristics of the studied beach Enterococci. Our findings suggest that recreational beaches may contribute to the dissemination of Enterococci with HLAR and virulence characteristics.

Expert systems in clinical microbiology

This review aims to discuss expert systems in general and how they may be used in medicine as a whole and clinical microbiology in particular (with the aid of interpretive reading). It considers rule-based systems, pattern-based systems, and data mining and introduces neural nets. A variety of noncommercial systems is described, and the central role played by the EUCAST is stressed. The need for expert rules in the environment of reset EUCAST breakpoints is also questioned. Commercial automated systems with on-board expert systems are considered, with emphasis being placed on the "big three": Vitek 2, BD Phoenix, and MicroScan. By necessity and in places, the review becomes a general review of automated system performances for the detection of specific resistance mechanisms rather than focusing solely on expert systems. Published performance evaluations of each system are drawn together and commented on critically.

Emergence of unusual species of enterococci causing infections, South India

BACKGROUND

Enterococci tend to be one of the leading causes of nosocomial infections, with E. faecalis and E. faecium accounting up to 90% of the clinical isolates. Nevertheless, the incidence of other species of enterococci from clinical sources shows an alarming increase with the properties of intrinsic resistance to several antibiotics including beta-lactams and glycopeptides. Thus proper identification of enterococci to species level is quintessential for management and prevention of these bacteria in any healthcare facility. Hence this work was undertaken to study the prevalence of unusual species of enterococci causing human infections, in a tertiary care hospital in South India.

METHODS

The study was conducted in a tertiary care hospital in South India from July 2001 to June 2003. Isolates of enterococci were collected from various clinical specimens and speciated using extensive phenotypic and physiological tests. Antimicrobial susceptibility testing were performed and interpreted as per NCCLS guidelines. Whole cell protein (WCP) fingerprinting of enterococci were done for species validation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and analyzed computationally.

RESULTS

Our study showed the prevalence of unusual (non-faecalis and non-faecium enterococci) and atypical (biochemical variant) species of enterococci as 19% (46 isolates) and 5% (12 isolates) respectively. The 7 unusual species (46 isolates) isolated and confirmed by phenotypic characterization includes: 15 E. gallinarum (6.2%), 10 E. avium (4.1%), 6 E. raffinosus (2.5%), 6 E. hirae (2.5%), 4 E. mundtii (1.7%), 3 E. casseliflavus-including the two atypical isolates (1.2%) and 2 E. durans (0.8%). The 12 atypical enterococcal species (5%) that showed aberrant sugar reactions in conventional phenotyping were confirmed as E. faecalis, E. faecium and E. casseliflavus respectively by WCP fingerprinting. The antimicrobial susceptibility testing depicted the emergence of high-level aminoglycoside and beta-lactam resistance among different species apart from intrinsic vancomycin resistance by some species, while all the species tested were susceptible for linezolid and teicoplanin.

CONCLUSION

Our study reveals the emergence of multi-drug resistance among unusual species of enterococci posing a serious therapeutic challenge. Precise identification of enterococci to species level enables us to access the species-specific antimicrobial resistance characteristics, apart from knowing the epidemiological pattern and their clinical significance in human infections.

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.

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.

Multicenter laboratory evaluation of the bioMérieux Vitek antimicrobial susceptibility testing system with 11 antimicrobial agents versus members of the family Enterobacteriaceae and Pseudomonas aeruginosa

A four-center study in which a total of 1,082 recent clinical isolates of members of the family Enterobacteriaceae and Pseudomonas aeruginosa were examined versus 11 antimicrobial agents with the bioMérieux Vitek susceptibility test system (Hazelwood, Mo.) and the GNS-F6 card was conducted. In addition, a challenge set consisting of the same 200 organisms was examined in each of the four participating laboratories. Results obtained with the Vitek system were compared to MICs determined by a standardized broth microdilution method. For purposes of comparison, susceptibility categories (susceptible, intermediate, or resistant) were assigned on the basis of the results of both methods. The result of the broth microdilution test was considered definitive. The total category error rate with the Vitek system and the recent clinical isolates (11,902 organism-antimicrobial comparisons) was 4.5%, i.e., 1.7% very major errors, 0.9% major errors, and 1.9% minor errors. The total category error rate calculated from tests performed with the challenge set (i.e., 8,800 organism-antimicrobial comparisons) was 5.9%, i.e., 2.2% very major errors, 1.1% major errors, and 2.6% minor errors. Very major error rates higher than the totals were noted with Enterobacter cloacae versus ampicillin-sulbactam, aztreonam, ticarcillin, and ticarcillin-clavulanate and with P. aeruginosa versus mezlocillin, ticarcillin, and ticarcillin-clavulanate. Major error rates higher than the averages were observed with Proteus mirabilis versus imipenem and with Klebsiella pneumoniae versus ofloxacin. Excellent overall interlaboratory reproducibility was observed with the Vitek system. The importance of inoculum size as a primary determinant in the accuracy of susceptibility test results with the Vitek system was clearly demonstrated in this study. Specifically, when an inoculum density fourfold higher than that recommended by the manufacturer was used, high rates of false resistance results were obtained with cell wall-active antimicrobial agents versus both the Enterobacteriaceae and P. aeruginosa.

Plasmid-borne high-level resistance to gentamicin in Enterococcus hirae, Enterococcus avium, and Enterococcus raffinosus

Enterococcus hirae, E. avium, and E. raffinosus isolated in Romania, Tunisia, and Portugal harbored plasmids pICC8, pIP1700, and pIP1701, respectively, encoding resistance to high levels of gentamicin (Gmr). The Gmr marker was carried on pIP1700 by a Tn4001-like element and on pICC8 and pIP1701 by Tn4001-truncated structures. pICC8 carried, in addition to Gmr, chloramphenicol, erythromycin, and tetracycline-minocycline (TetM) resistance determinants. The gene tetM of pICC8 was carried on a Tn916-like element.

Multilaboratory evaluation of screening methods for detection of high-level aminoglycoside resistance in enterococci. National Committee for Clinical Laboratory Standards Study Group on Enterococci

Since the early 1970s, the synergistic activity of an aminoglycoside with a cell wall-active agent has been predicted by determining the ability of an enterococcus to grow in the presence of high levels of the aminoglycoside (usually > or = 2,000 micrograms/ml). However, a variety of media and concentrations of aminoglycosides has been used for this screening procedure. In the present study, we sought to optimize the agar dilution, broth microdilution, and disk diffusion tests used to detect high-level gentamicin and streptomycin resistance in enterococci. For dilution tests, brain heart infusion agar or broth gave the best growth and performance. For agar dilution, 500 micrograms of gentamicin per ml, 2,000 micrograms of streptomycin per ml, and an inoculum of 1 x 10(6) CFU/ml were optimal, while for broth microdilution, 500 micrograms of gentamicin per ml, 1,000 micrograms of streptomycin per ml, and an inoculum of 5 x 10(5) CFU/ml were best. Growth of more than one colony in the agar dilution test was determined to be the best indicator of high-level resistance. For disk diffusion, Mueller-Hinton agar, 120-micrograms gentamicin disks, and 300-micrograms streptomycin disks with breakpoints of no zone for resistance and > or = 10 mm for susceptibility gave the best sensitivity and specificity if results for strains with zones of 7 to 9 mm are considered inconclusive, indicating that a broth or agar test should be performed to determine susceptibility or resistance.

Detection of aminoglycoside-penicillin synergy against Enterococcus faecium using high-content aminoglycoside disks

Thirty-seven Enterococcus faecium strains were screened for high-level aminoglycoside resistance with an agar diffusion test using high-content aminoglycoside disks (300 micrograms of streptomycin and 120 micrograms of gentamicin, tobramycin, kanamycin or amikacin). The inhibition zones obtained were correlated with results of time-kill penicillin-aminoglycoside synergy studies. An 11 mm breakpoint differentiated strains susceptible or resistant to the synergy of streptomycin plus penicillin. Irrespective of the inhibition zones obtained with tobramycin and kanamycin disks, Enterococcus faecium strains never showed synergy with penicillin in combination with these aminoglycosides. Penicillin-amikacin synergy cannot be predicted by the amikacin disks. Nevertheless, even though kanamycin disks do not predict penicillin-kanamycin synergy, they can be used to predict penicillin-amikacin synergy. In summary, high-content streptomycin, gentamicin and kanamycin disks can be used to predict the susceptibility of Enterococcus faecium strains to the synergistic combination of penicillin plus one of the aminoglycosides (streptomycin, gentamicin or amikacin, respectively).

Species identification and antibiotic susceptibility of enterococci isolated from clinical specimens of hospitalized patients

Over a 4-month period, a total of 315 enterococci were isolated from various clinical specimens of hospitalized patients. By applying an array of biochemical tests, all strains were accurately identified to the species level, and their susceptibilities to clinically relevant antibiotics were determined by a standardized agar dilution technique. E. faecalis and E. faecium accounted for 87.3% and 9.2% of isolates, respectively. E. avium (1%), E. gallinarum (1%), E. durans (0.6%), E. hirae (0.6%), and E. casseliflavus (0.3%) isolates were also identified. Eleven strains of E. faecium and 1 E. hirae isolate were resistant to ampicillin, but none of the isolates produced beta-lactamase. Twenty-three E. faecium and 3 E. faecalis strains as well as 1 E. hirae isolate revealed imipenem resistance. A total of 25.4% enterococci (60 E. faecalis and 19 E. faecium isolates, 1 E. hirae strain) were erythromycin-resistant. Twelve strains (11 E. faecium and 1 E. avium) exhibited ciprofloxacin resistance. High-level resistance to streptomycin was found in 58 (21.1%) E. faecalis, 9 (31%) E. faecium, and both E. hirae strains, whereas high-level gentamicin resistance (HLGR) was exclusively seen in the species E. faecalis (11.6% of isolates belonging to this species). A simple agar screening test containing 500 micrograms of gentamicin per ml proved to be highly reliable for detection of HLGR. The structural gene coding for HLGR was specifically amplified by the polymerase chain reaction in all isolates showing this resistance trait. Moreover, the gene was specifically detected by a nonradioactively labelled oligonucleotide probe in colony blot hybridization assays, indicating the potential application of these molecular approaches as a diagnostic tool.

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

We investigated the accuracy of the recently released Remel Synergy Quad plate, a commercially available agar screening method for detecting high-level aminoglycoside and vancomycin resistance among enterococci that is based on the National Committee for Clinical Laboratory Standards recommended guidelines (National Committee for Clinical Laboratory Standards, M7-A3, 1993). The Synergy Quad correctly determined the gentamicin and streptomycin resistance status for > or = 97% of 147 Enterococcus faecalis and Enterococcus faecium isolates tested. Detection of vancomycin resistance also was reliable, as no false susceptibility occurred with 36 vancomycin-resistant E. faecalis and E. faecium strains and false resistance occurred only once with the 47 susceptible strains tested. One strain each of Enterococcus gallinarum and Enterococcus casseliflavus failed to grow on the screen, but because the true nature and significance of resistance in such isolates is unknown the implication of their screen negativity is uncertain. In summary, the Remel Synergy Quad provides a highly accurate and convenient method for susceptibility testing of enterococci against gentamicin, streptomycin, and vancomycin.

Occurrence of high-level aminoglycoside resistance in environmental isolates of enterococci

High-level resistance to aminoglycosides was observed in environmental isolates of enterococci. Various aquatic habitats, including agricultural runoff, creeks, rivers, wastewater, and wells, were analyzed. Strains of Enterococcus faecalis, E. faecium, E. gallinarum, and other Enterococcus spp. demonstrated multiple antibiotic resistance patterns to aminoglycosides.

High level resistance to aminoglycosides in enterococci from Riyadh

Enterococci with high level of aminoglycosides resistance are being reported from different parts of the world with increasing frequency. Treatment of infections caused by such isolates is associated with a high incidence of failure or relapse. This is attributed to the loss of the synergetic effect of aminoglycosides and cell wall active agents against isolates exhibiting this type of resistance. To determine the prevalence of enterococci with high level resistance to aminoglycosides in Riyadh, Saudi Arabia, 241 distinct clinical isolates were examined by disk diffusion method using high content aminoglycosides disks. Seventy-four isolates (30%) were resistant to one or more of the aminoglycosides tested. The most common pattern of resistance was that to streptomycin and kanamycin. Of the 241 isolates tested, 29 (12%) were resistant to high levels of gentamicin, 35 (15%) to tobramycin, 65 (27%) to kanamycin and 53 (22%) to streptomycin. The highest rate of resistance to a high level of gentamicin was found among enterococcal blood isolates (30%). Eighteen of the isolates were identified as Enterococcus faecium, 13 (72%) of these showed high level resistance to two or more of the aminoglycosides tested.

Transferability and genetic relatedness of high-level gentamicin resistance among enterococci

Gentamicin resistance in six enterococcal species was investigated. Transfer of resistance was observed for the donors E. faecium UC 79, E. avium CC 54, and E. gallinarum B 51, but not for E. raffinosus UC 78 or E. casseliflavus UC 73. Except for E. casseliflavus UC 73, homology was observed between the E. faecalis aac6'-aph2" gene and DNA from other species. Whereas 2.6-kb HindIII fragments encoded resistance in E. faecalis UC 244, its transconjugant, and E. raffinosus UC 78, 3.4-kb fragments encoded resistance in E. faecium UC 79, E. gallinarum B 51, and their transconjugants. A 3.4-kb fragment encoded resistance in E. avium CC 54, but 2.6-kb fragments encoded resistance in the transconjugants. Although many similarities were found among the strains, the heterogeneity in gentamicin resistance exhibited by some isolates indicates diversity among these determinants.

Reliability of the E test for detection of ampicillin, vancomycin, and high-level aminoglycoside resistance in Enterococcus spp

By comparison with agar dilution results, the E test was investigated for the ability to detect high-level aminoglycoside (gentamicin and streptomycin), ampicillin, and vancomycin resistance among strains representing six enterococcal species. For ampicillin and vancomycin, disk diffusion results also were obtained. No false high-level aminoglycoside resistance occurred, and no false gentamicin susceptibility was noted. With the high-range streptomycin E test (2,048 micrograms), 24% of the 38 resistant strains were falsely susceptible. However, these discordances could likely be reconciled by adjustments in incubation duration and by using broth microdilution rather than agar screen breakpoint criteria, or by using the lower-range (1,024-micrograms) strip. For ampicillin, category results obtained by E test and disk diffusion showed good agreement with agar dilution; E test MICs were generally comparable to agar dilution MICs. The E test was more sensitive than disk diffusion for detecting vancomycin-intermediate strains, but for these strains and those exhibiting low-level vancomycin resistance (MIC, 32 to 128 micrograms/ml), disk diffusion and E test inhibition zones must be interpreted with caution. Given the reliability of E test for detecting resistance to anti-enterococcal agents, the decision to use this method should be based on convenience, cost, testing frequency, and satisfaction with currently used methods.

Species identification and determination of high-level aminoglycoside resistance among enterococci. Comparison study of sterile body fluid isolates, 1985-1991

Enterococcus spp. have become the third most common cause of nosocomial infections. High-level aminoglycoside resistance (HLR), an important clinical concern, has been associated with some species of the enterococci. We evaluated the Vitek and API 20S systems for species identification and the Vitek for the detection of HLR. Enterococci from nosocomial infections (208 strains) at the University of Iowa Hospital (1985-1991) were tested by Vitek, API 20S, and reference methods. The error rate for species identification was 6.7% for the API 20S and 5.8% for the Vitek Gram-positive identification (GPI) cards. Both systems tended to incorrectly identify other enterococcal species as Enterococcus faecium. HLR was found in Enterococcus faecalis and E. faecium isolates only. The highest rates of HLR to streptomycin alone (17.9%) and with gentamicin (13.5%) was observed among E. faecalis strains, and to gentamicin alone (7.3%) was found among E. faecium isolates. No apparent differences in HLR rates were found from year-to-year over the 7-year enterococcus sample interval. Susceptibility errors for Vitek were among the streptomycin tests only. Our results demonstrated acceptable performance by the Vitek cards for enterococcal species identification and the detection of HLR. API 20S also provided an acceptable ability to speciate the enterococci within its data base, however, both systems must be improved by adding other clinical important Enterococcus species.

Prevalence of enterococcal high-level aminoglycoside resistance in Japan. Comparative detection by three methods

A total of 250 strains of enterococci isolated in Kumamoto University Hospital, Japan, during the period from January to March 1992 were tested for high-level aminoglycoside resistance. Brain-heart infusion (BHI) agar plates supplemented with 1000 micrograms/ml of gentamicin or 2000 micrograms/ml of streptomycin detected 164 (66%) isolates resistant to either gentamicin or streptomycin alone, or both, and consisted of 107 (43%) resistant to gentamicin and 96 (38%) resistant to streptomycin. The Vitek Gram-Positive Susceptibility card (GPS-TA) revealed high correlations with those by agar screens, the results indicating a sensitivity of 100% and 99% to gentamicin and streptomycin, respectively, and 100% specificity to both. Also, the microdilution tests of the National Committee for Clinical Laboratory Standards (NCCLS) showed 100% and 92% sensitivity to gentamicin and streptomycin, respectively, and no false resistance (100% specificity) when compared with the results by agar screens.

Use of the E test to predict high-level resistance to aminoglycosides among enterococci

The E test and the reference agar dilution methods were compared for detecting high-level aminoglycoside resistance (HLAR) among 71 selected clinical isolates, including 62 Enterococcus faecalis and 9 Enterococcus faecium isolates. High-level gentamicin resistance alone was found in 11% (5 E. faecalis and 3 E. faecium strains) and high-level streptomycin resistance was found in 42% (28 E. faecalis, 2 E. faecium strains) of the strains tested, and 31% of the strains demonstrated high-level resistance to both antimicrobial agents (21 E. faecalis and 1 E. faecium strains). The E test detected all HLAR populations, but the streptomycin strip may require recalibration to achieve absolute MIC comparisons with the reference value (twofold less) or the use of an alternative interpretive resistance breakpoint, e.g., > 1,000 micrograms/ml. By the E test, MIC results indicate that ampicillin, imipenem, penicillin, piperacillin, and vancomycin remain active against the HLAR E. faecalis isolates; however, these tested drugs were less effective on the HLAR E. faecium isolates (< 50%).

Detection of vancomycin resistance in Enterococcus species

Enterococcus faecalis and Enterococcus faecium isolates that are resistant to vancomycin have recently been identified in North America and Europe. Of 155 clinical isolates of enterococci (113 E. faecium and 42 E. faecalis), we found that 98 were resistant, 52 were moderately susceptible, and 5 had intermediate susceptibilities to vancomycin by using broth microdilution susceptibility testing according to the National Committee for Clinical Laboratory Standards (NCCLS) (Approved Standard M7-A2). Using NCCLS disk diffusion methodology (Approved Standard M2-A4), we evaluated the NCCLS supplemental M100-S3 revisions for zone diameter interpretive standards and incubation conditions and found 5.8% minor errors. A total of 234 isolates, which included an additional 79 E. faecium isolates that were moderately susceptible to vancomycin, were used to evaluate the Vitek GPS-TA card (bioMerieux, Inc., Hazelwood, Mo.) and the Pos MIC type 6 panel (MicroScan; Baxter Health Care Corp., West Sacramento, Calif.) for the detection of vancomycin resistance. The Vitek card was 100% specific and 72% sensitive, whereas the MicroScan panel with the Walk/Away system was 98% specific, with a sensitivity of 93% which increased to 99% when readings were performed manually. An agar screen plate method was evaluated with vancomycin concentrations of 6, 8, 10, or 12 micrograms/ml; plates were inoculated so as to obtain a final concentration of 10(5) CFU per spot. This method was found to be 100% sensitive and specific at all concentrations.