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Annals of Clinical & Laboratory Science 37:260-262 (2007)
© 2007 Association of Clinical Scientists

Comparison of the MicroScan System and the Agar Dilution Assay for Quinupristin/Dalfopristin Susceptibility of Enterococcus faecium

Yang-Ree Kim1, Sang-Il Kim2, Ji-Ahn Hur3, Youn-Jeong Kim2, Seong-Heon Wie3, Yeon-Joon Park4 and Moon-Won Kang2
1 Department of Internal Medicine, Uijeongbu St. Mary’s Hospital, Uijeongbu, Korea; 2 Department of Internal Medicine, Kangnam St. Mary’s Hospital, Seoul, Korea; 3 Department of Internal Medicine, St. Vincent’s Hospital, Suwon, Korea; and 4 Department of Laboratory Medicine, Kangnam St. Mary’s Hospital, Seoul, Korea. All are affiliated with the College of Medicine of The Catholic University of Korea, Seoul, Korea

Address correspondence to Yeon-Joon Park, M.D., Department of Laboratory Medicine, The Catholic University of Korea, Kangnam St. Mary’s Hospital, 505 Banpo-dong, Seocho-ku, Seoul, 137-701, South Korea; tel 82 2 590 1604; fax 82 2 592 4190; e-mail yjpk{at}catholic.ac.kr.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results and Discussion
 Acknowledgements
 References
 
We compared the results of Quinupristin/Dalfopristin (Q/D) susceptibility tests by the Positive Combo Panel (Type 11) of the MicroScan Walk Away 96 analyzer (Dade Behring, Inc.) with those obtained by the reference agar dilution method. From September 2003 to August 2004, a total of 410 E. faecium isolates were obtained from clinical samples. Of these, 65 (15.9%) strains were non-susceptible, and 345 (84.1%) strains were susceptible to Q/D. We collected consecutively 65 Q/D non-susceptible E. faecium isolates (42 resistant, 23 intermediate), and randomly selected 32 Q/D susceptible E. faecium isolates using the MicroScan system. The minimal inhibitory concentrations (MICs) of Q/D, vancomycin, and teicoplanin were determined by the agar dilution method according to CLSI guidelines. The agreement rates between the two methods were 100% for Q/D-susceptible strains, 85.7% for Q/D-resistant strains, and 26.1% for Q/D-intermediate strains of E. faecium. The major error rate (S->R) was 11.9%, and the minor error rate (S->I) was 13.0%. No very major errors were found. We conclude that for MicroScan ‘non-susceptible’ test results for Q/D, it is necessary to confirm the result using a reference method. The Q/D-resistance rate was higher in glycopeptide-susceptible (78.0% for vancomycin, 82.0% for teicoplanin) than glycopeptide-resistant E. faecium (22.0% for vancomycin, 16.0% for teicoplanin). Further studies are needed to determine whether Q/D use in hospitals or virginiamycin use in animals, or other factors, are responsible for the high rates of glycopeptide-susceptible and Q/D-resistant E. faecium strains in Korea.

Keywords: quinupristin/dalfopristin, Enterococcus faecium, antimicrobial susceptibility testing


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results and Discussion
 Acknowledgements
 References
 
Enterococcus faecium (E. faecium) is an important nosocomial pathogen and vancomycin resistance in this species is increasing worldwide. Quinupristin/ Dalfopristin (Q/D) is a mixture of the semisynthetic streptogramin A (dalfopristin) and B (quinupristin) compounds, and is an effective antimicrobial for treating vancomycin-resistant E. faecium (VRE). In Korea and Taiwan, the Q/D resistance rate is high (9–15%) [1–3], and an accurate susceptibility test is needed for appropriate therapeutic drug selection for VRE. The MicroScan system is frequently used for identification and antimicrobial susceptibility testing in clinical laboratories, but the results do not always agree with the reference method [4]. In this study, we compared the results of a Q/D susceptibility test by the Positive Combo Panel (Type 11) of the MicroScan Walk Away 96 analyzer (Dade Behring, Inc., Newark, DE, USA) with those obtained by the reference agar dilution method. We also determined the glycopeptide resistance rate among Q/D-susceptible and Q/D-resistant isolates of E. faecium.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results and Discussion
 Acknowledgements
 References
 
From September 2003 to August 2004, a total of 410 E. faecium isolates were obtained from clinical samples. We found that 65 (15.9%) strains were non-susceptible to Q/D, and 345 (84.1%) strains were susceptible. We collected consecutively 65 E. faecium isolates that were non-susceptible to Q/D (42 resistant, 23 intermediate). We also randomly collected 32 E. faecium isolates that were susceptible to Q/D according to the MicroScan system. All isolates were identified in the Kangnam St. Mary’s Hospital using the MicroScan system. The MIC of Q/D, vancomycin, and teicoplanin were determined using the standard agar dilution method according to the CLSI guidelines [5]. The agar dilution tests were performed in duplicate, and E. faecalis ATCC 29212 was included for quality control.


   Results and Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
Acknowledgements
 References
 
Of the 42 Q/D-resistant E. faecium strains, 5 were susceptible to Q/D by the reference method, giving a major error (false resistance) rate of 11.9%. This is above the maximum limit (3%) of major errors recommended by NCCLS [6].

The agreement rate between the agar dilution and MicroScan methods is shown in Table 1Go. The MICs of Q/D resistant isolates by the agar dilution method was 4 µg/ml in all but one isolate. That is considered a low-level resistance. Of the 23 isolates that showed intermediate resistance to Q/D by MicroScan, 3 (13.0%) were susceptible to Q/D when tested by the agar dilution method. In summary, 8 (12.3%) of 65 isolates were falsely non-susceptible to Q/D by MicroScan.


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  		Table 1. Comparison of Q/D susceptibility test by MicroScan and agar dilution methods.
 
Sader et al [7] stated that automated systems, including the MicroScan WalkAway system, showed unacceptable levels of very major errors for selected ß-lactams in strains of Pseudomonas aeruginosa. D’Azevedo et al [4] reported that for enterococci tested with the MicroScan WalkAway and agar MIC tests, the percentages of agreement for the detection of resistance was 90.2% for ampicillin, 90.6% for high-level resistance to streptomycin, 96.4% for high-level resistance to gentamicin, and 100% for vancomycin.

Of 50 Q/D-resistant isolates of E. faecium that we tested, most were susceptible to vancomycin and teicoplanin (78.0% and 82.0%, respectively). The glycopeptide susceptibility rate was much higher in Q/D-resistant E. faecium than in Q/D-susceptible E. faecium (Table 2Go). This observation is consistent with previous reports [1,3,8], and it supports the suggestion that Q/D-resistant E. faecium emerged before Q/D use in medicine because virginiamycin (another streptogramin) had been widely used in agriculture [9]. But there may be other factors leading to Q/D resistance of E. faecium, considering that the Q/D resistance rates are high in Korea (10.0%) and Taiwan (9 to 15%) [1–3], in contrast to low rates (0 to 3.8%) in America and Europe where virginiamycin has also been used [10–14],


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  		Table 2. Glycopeptide susceptibility of E. faecium isolates by agar dilution method.
 
In conclusion, to our knowledge, this is the first publication on the accuracy of the MicroScan Q/D susceptibility test. We found that it has a major error rate of 11.9%, which exceeds the acceptable range (≤3%). The minor error rate (S->I) is also high (13.0%). Until the MicroScan achieves better correlation of antibiotic susceptibility results with those obtained by agar dilution, it may be advisable to confirm Q/D-resistant results by use of a reference method.


   Acknowledgements
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Acknowledgements
References
 
We thank Ms. Hyun Jeong for technical assistance.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Acknowledgements
 References
 

  1. Oh WS, Ko KS, Song JH, Lee MY, Park S, Peck KR, Lee NY, Kim CK, Lee H, Kim SW, Chang HH, Kim YS, Jung SI, Son JS, Yeom JS, Ki HK, Woo GJ. High rate of resistance to Quinupristin-Dalfopristin in Enterococcus faecium clinical isolates from Korea. Antimicrob Agents Chemother 2005;49:5176–5178.[Abstract/Free Full Text]
  2. Hsueh PR, Chen WH, Teng LJ, Luh KT. Nosocomial infections due to methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci at a university hospital in Taiwan from 1991 to 2003: resistance trends, antibiotic usage and in vitro activities of newer antimicrobial agents. Int J Antimicrob Agents 2005;26:43–49.[Medline]
  3. Luh KT, Husueh PR, Teng LJ, Pan HJ, Chen YC, Lu JJ, Wu JJ, Ho SW. Quinupristin-dalfopristin resistance among gram-positive bacteria in Taiwan. Antimicrob Agents Chemother 2000;44:374–380.
  4. d’Azevedo PA, Dias CA, Goncalves AL, Rowe F, Teixeria LM. Evaluation of an automated system for the identification and microbial susceptibility testing of enterococci. Diagn Microbiol Infect Dis 2001;40:157–161.[Medline]
  5. Clinical and Laboratory Standards Institute (CLSI). Performance standards for antimicrobial susceptibility testing, 16th informational supplement. Document M100-S16. CLSI Press, Wayne, PA, 2006.
  6. National Committee for Clinical Laboratory Standards (NCCLS). Development of in vitro susceptibility testing criteria and quality control parameters; tentative guideline M23-T2. NCCLS Press, Villanova, PA,, 1992.
  7. Sade HS, Fritsche TR, Jones RN. Accuracy of three automated systems (MicroScan WalkAway, VITEK, and VITEK2) for susceptibility testing of Pseudomonas aeruginosa against five broad-spectrum beta-lactam agents. J Clin Microbiol 2006;44:1101–1104.[Abstract/Free Full Text]
  8. Ballow CH, Jones RN, Biedenbach DJ. A multicenter evaluation of linezolid antimicrobial activity in North America. Diagn Microbiol Infect Dis 2002;43:75–83.[Medline]
  9. Smith DL, Johnson JA, Harris AD, Furuno JP, Perencevich EN, Morris JG Jr. Assessing risks for a pre-emergent pathogen: virginiamycin use and the emergence of streptogramin resistance in Enterococcus faecium. Lancet Infect Dis 2003;3:241–249.[Medline]
  10. Jevitt LA, Smith AJ, Williams PP, Raney PM, McGowan JE Jr, Tenover FC. In vitro activities of daptomycin, linezolid, and quinupristin-dalfopristin against a challenge panel of staphylococci and enterococci, including vancomycin-intermediate Staphylococcus aureus and vancomycin-resistant Enterococcus faecium. Microb Drug Resist 2003; 9:389–393.[Medline]
  11. Low DE, Keller N, Barth A, Jones RN. Clinical prevalence, antimicrobial susceptibility, and geographic resistance patterns of enterococci: results from the SENTRY antimicrobial surveillance program, 1997–1999. Clin Inf Dis 2001;32(Suppl.2):S133–S145.[Medline]
  12. Sader HS, Jones RN, Ballow CH, Biedenbach DJ, Cered RF, the GSMART Latin America Study Group. Antimicrobial susceptibility of quinupristin/dalfopristin tested against gram-positive cocci from Latin America: results from the global SMART (GSMART) surveillance study. Braz J Infect Dis 2001;5:21–31.[Medline]
  13. Simonsen GS, Bergh K, Bevanger L, Digranes A, Gaustad P, Melby KK, Hoiby EA. Susceptibility to quinupristin-dalfopristin and linezolid in 839 clinical isolates of Gram-positive cocci from Norway. Scand J Infect Dis 2004;35:254–258.
  14. Torres-Vierak, C, Dembry LM. Approaches to vancomycin-resistant enterococci. Curr Opin Infect Dis 2004;17:541–547.[Medline]




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