Factors influencing determination of high-level aminoglycoside resistance in Enterococcus faecalis

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.

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.

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.

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.

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.

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.

Evaluation of MicroScan rapid panels for detection of high-level aminoglycoside resistance in enterococci

The ability of MicroScan rapid panels to detect high-level aminoglycoside-resistant enterococci was evaluated. By agar dilution, 46 of 139 isolates were susceptible to gentamicin (GNT) and streptomycin (STRP); the rest were highly resistant to one or both agents. Rapid panels detected 97.5% of STRP- and GNT-resistant isolates and had a specificity of 95.6%. Detection of resistance by conventional panels at 18 h was 64.6% for STRP and 90.2% for GNT.

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.

Susceptibility testing of clinical isolates of Enterococcus faecium and Enterococcus faecalis

We collected 103 clinical Enterococcus faecium isolates from across Canada, performed standard broth microdilution susceptibility testing, and compared these results with results from the MicroScan Pos MIC Type 6 panel (Baxter Health Care Corp., West Sacramento, Calif.) and the AMS-Vitek Gram-Positive Susceptibility card (Vitek Inc., St. Louis, Mo.). High-level aminoglycoside resistance to gentamicin and streptomycin was detected by a single-concentration agar method with 1,000 micrograms of each aminoglycoside per ml. In addition, we tested the effect of the lower calcium content in broth media as recommended in National Committee for Clinical Laboratory Standards (NCCLS) guideline M7-A2 on the activity of the highly calcium-dependent agent daptomycin. Of the 103 E. faecium isolates, there were 4 and 30 isolates with high-level gentamicin resistance (HLGR) and high-level streptomycin resistance (HLSR), respectively. An additional 39 (37 with HLGR and 36 with HLSR) E. faecium isolates were tested by both the MicroScan and the AMS-Vitek systems. The AMS-Vitek card demonstrated sensitivities of 95 and 82% for detecting HLGR strains and HLSR strains, respectively. The MicroScan panel demonstrated improved sensitivities for detecting HLGR (42 to 97%) and HLSR (64 to 84%) when readings were performed manually instead of being generated automatically. Ampicillin resistance (MIC, greater than or equal to 16 micrograms/ml) was detected in 23 of the 103 E. faecium isolates. Only 14 and 20 of these were detected by the MicroScan panels and AMS-Vitek cards, respectively. beta-Lactamase activity was not detected in any isolates. The lower calcium content in broth media recommended by NCCLS guideline M7-A2 markedly reduced the in vitro activity of daptomycin against Enterococcus spp.

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

The ability of six screening methods to detect high-level aminoglycoside resistance in enterococcal species other than Enterococcus faecalis was investigated. The 85 Enterococcus isolates, which included 55 E. faecium, 11 E. gallinarum, 9 E. casseliflavus, 5 E. raffinosus, 4 E. avium, and 1 E. mundtii, were tested by using aminoglycoside-supplemented brain heart infusion agar (BHI), Remel EF Synergy Quad plates, high-content aminoglycoside diffusion disks, standard (prepared in-house) microdilution panels, Pasco MIC Gram Positive microdilution panels, and Vitek GPS-TA cards. When tested on BHI, 32 and 35 strains showed resistance to gentamicin and streptomycin, respectively. Resistance profiles obtained with Remel EF Synergy Quad plates were in complete agreement with those obtained on BHI. However, growth on Mueller-Hinton agar-based plates was not as heavy. Some isolates showed only weak growth and required 48 h for resistance to become evident, especially with swab inoculation of quadrants containing 2,000 micrograms of gentamicin per ml. Profiles obtained by use of the agar-based screens were used as the basis for evaluating the other methods. Disk diffusion showed complete agreement. No false resistance occurred by either microdilution method, but 48 h of incubation was needed for detection of some gentamicin-resistant isolates, and 14% of the streptomycin-resistant strains were not detected by standard microdilution. The Vitek GPS-TA card detected 81 and 100% of the gentamicin- and streptomycin-resistant isolates, respectively. In general, most methods used to detect high-level aminoglycoside resistance in E. faecalis appear to be reliable for the testing of the other enterococcal species. However, further investigations with a greater number of resistant E. raffinosus, E. avium, and E. mundtii isolates, when they are available, will be useful for establishing the full range of enterococci that can reliably be tested by the various methods.