Annals of Clinical & Laboratory Science 34:83-86 (2004)
© 2004 Association of Clinical Scientists
Evaluation of Vitek System for Susceptibility Testing of Enterococcus faecium Isolates
Hae Kyung Lee,
Yeon-Joon Park,
Hi Jeong Kwon,
Eun Jung Lee,
Byung Kee Kim and
Chang Suk Kang
Department of Clinical Pathology, College of Medicine, Catholic University, Seoul, Korea
Address correspondence to Yeon-Joon Park, M.D., Department of Clinical Pathology, Kangnam St. Mary’s Hospital, 505 Banpo-dong, Seocho-ku, Seoul 137-040, Korea; tel 82 2 590 1604; fax 82 2 592 4190; e-mail yipk{at}catholic.ac.kr.
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Abstract
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Although imipenem is not a first-line drug for treating enterococcal infection, it could well become a useful drug for treating mixed infections that include enterococci. However, there is no NCCLS guideline for susceptibility testing of imipenem versus enterococci. Moreover, there are no statements to indicate that in vitro susceptibility results for other antimicrobial agents can be used to predict the in vitro activity of imipenem against enterococci. In this study, 52 Enterococcus faecium isolates were collected from patients hospitalized at Kangnam St. Mary’s Hospital between March 2002 and December 2002. The sources of the isolates were mainly urine specimens and wounds. For ampicillin, the "major" and "very major" error rates observed with the Vitek system were 0% and 2.0%, respectively. For penicillin, the major and very major error rates observed with the Vitek system were both 0%. For imipenem, the major and very major error rates observed with the Vitek system were 0% and 36.5%, respectively. The MICs of ampicillin and penicillin obtained using the Vitek system were reliable, but that of imipenem was unreliable. In the 52 E. faecium isolates, the in vitro activity of penicillin and ampicillin versus enterococci accurately predicted that of imipenem. Therefore, the MIC of imipenem obtained with the Vitek system must be retested by the agar dilution method, when it disagrees with those of penicillin and ampicillin.
(received 15 May 2003; accepted 2 October 2003)
Keywords: imipenem, Enterococcus faecium, Vitek system, ampicillin, penicillin
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Introduction
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Enterococcus faecium has emerged as a nosocomial pathogen. Increasingly, isolates of E. faecium have been reported to possess high-level resistance to the aminoglycosides, penicillin and vancomycin [1]. In Korea, about 40–50% of the enterococci isolated from clinical specimens are E. faecium. Although imipenem exhibits strong activity against a wide variety of bacteria, including enterococci [2], isolates of E. faecium are less susceptible to imipenem than those of E. faecalis [3]. Imipenem could well become a useful drug for treating mixed infections that include enterococci. However, there is no NCCLS (National Committee for Clinical Laboratory Standards) guideline for the susceptibility testing of imipenem versus enterococci. Moreover, there are no statements to indicate that in vitro susceptibility results for other antimicrobial agents can be used to predict the in vitro activity of imipenem against enterococci.
Weinstein [4] suggested that testing the susceptibilities of isolates of enterococci to penicillin or ampicillin might be used as a method of predicting the in vitro activity of imipenem. On the other hand, ampicillin-susceptible, imipenem-resistant isolates have also been reported [5–7]; the authors of these reports recommended that sensitivity testing with carbapenem should be done when the treatment of enterococcal infections with this type of drug is considered. Controversy continues concerning the testing of imipenem versus enterococci.
We reviewed the susceptibility pattern of E. faecium isolates from patients at Kangnam St. Mary’s Hospital, Seoul, Korea, from March 2002 through December 2002. Of the 165 E. faecium isolates examined, 147 (89%) were resistant to ampicillin. Of these 147 isolates, 112 (76%) were resistant to imipenem, and 35 (24%) were susceptible to imipenem as dertermined by the Vitek GPS card (Bio-Merieux Vitek, Inc., Hazelwood, MO) and by the criteria of the British Society for Antimicrobial Chemotherapy (BSAC) (ie, susceptible, 
4 µg/ml; resistant, 
16 µg/ml). Therefore, we performed this study (a) to compare the MICs of ampicillin, penicillin, and imipenem as determined by the Vitek system with those obtained by the agar dilution method, and (b) to investigate the capacity of the penicillin and ampicillin MICs to predict the in vitro susceptibility of E. faecium versus imipenem.
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Materials and Methods
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Fifty-two consecutive isolates of E. faecium were tested. All isolates were identified in Kangnam St. Mary’s Hospital Clinical Microbiology Laboratory using the Vitek GPI card, and in each case the result was confirmed by conventional microbiological testing [8]. Routine antimicrobial susceptibility testing was performed using the Vitek GPS card. Prior to testing, the isolates were thawed and subcultured to ensure their purity and viability.
As listed in Table 1
, the sources of the isolates were mainly urine specimens and wounds. Each isolate was tested against penicillin, ampicillin, and imipenem. Solutions of all antimicrobials were prepared from standard powders of known potencies obtained either from the manufacturer or from a commercial source (Sigma Co., St. Louis, MO).
The MICs were determined by the agar dilution method according to the NCCLS guidelines [9]. Imipenem MIC determinations were also performed using E-test strips according to the manufacturer’s specification. The MIC endpoint was defined as the lowest antibiotic concentration resulting in the absence of visible growth after 18 to 24 hr of incubation at 37°C. Staphylococcus aureus ATCC 25923 and Enterococcus faecalis ATCC 29212 were included with each run. The breakpoints for ampicillin and penicillin were defined according to the NCCLS guideline (susceptible, 8 µg/ml; resistant, 16 µg/ml)[10] and, for imipenem, they were defined according to the BSAC criteria (susceptible, 4 µg/ml; resistant, 8 µg/ml) [10].
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Results and Discussion
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For ampicillin, the "major" and "very major" error rates observed with the Vitek system were 0% and 2.0%, respectively. For penicillin, the major and very major error rates observed with the Vitek system were both 0% (Table 2). For imipenem, the major and very major error rates observed with the Vitek system were 0% and 36.5%, respectively (Table 3).
The 19 isolates that were found to be imipenem-susceptible according to the BSAC breakpoint of the Vitek system showed resistance to imipenem when tested by the agar dilution method (MIC >512 µg/ml) and the E-test method (MIC 32 >µg/ml). For imipenem, the major and very major error rates of the E-test were both 0%. According to the E-test method, the imipenem MICs were >32 µg/ml for the 49 ampicillin resistant isolates, and 3 µg/ml for the 3 isolates which were ampicillin susceptible according to the agar dilution method (Table 3
).
As listed in Table 4, the penicillin and ampicillin results obtained by the agar dilution method correctly predicted the susceptibility results for imipenem for all 52 isolates.
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Table 4. Correlation of penicillin and ampicillin NCCLS susceptibility breakpoints with imipenem BSAC susceptibility breakpoints assayed by the agar dilution method.
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Although the NCCLS breakpoints for penicillin and ampicillin versus enterococci are identical, the MIC of ampicillin is usually 1 dilution lower than that of penicillin [11]. Therefore, the pattern of susceptibility to ampicillin and penicillin can vary. However, in this study, no such tendency was observed. The MICs of penicillin and ampicillin obtained using the Vitek system and the agar dilution method showed good concordance, but less concordance was observed in the case of imipenem. Of the 52 E. faecium isolates, in only one case did the Vitek system (MIC: 4 µg/ml) and the agar dilution method (MIC: 128 µg/ml) give different susceptibility results to ampicillin. This is probably because the MIC obtained from the Vitek system tends to be lower than that obtained by the agar dilution method [12].
For imipenem, the very major error rate was 36.5% and the major error rate was 0% as determined by the Vitek system. This high value for the very major error rate is probably because of the 19 imipenem resistant isolates that were incorrectly interpreted as being susceptible to imipenem by the Vitek system. The weekly QC results with E. faecalis ATCC 29212 were always within the acceptable range.
In this study, the 3 E. faecium isolates that were susceptible to ampicillin and penicillin were also susceptible to imipenem and the 49 E. faecium isolates that were resistant to ampicillin and penicillin were also resistant to imipenem according to the BSAC breakpoint of the agar dilution method. Although in our study the number of isolates that were susceptible to ampicillin and penicillin was too small to be statistically valid, our results support Weinstein’s report [4] that the in vitro results obtained for penicillin or ampicillin can be used to predict imipenem susceptibility. However, of the 3 E. faecium isolates that were found to be susceptible to ampicillin and penicillin in our study, only one isolate was susceptible to imipenem according to the Swedish Reference Group for Antibiotics (SRGA) breakpoint (susceptible, 1 µg/ml; resistant, 16 µg/ml) [7]. Therefore, we consider the BSAC breakpoint for imipenem to be more concordant than that of the SRGA.
Although imipenem is not a first-line drug for treating enterococcal infection, it could well become a useful drug for treating mixed infections that include enterococci. The results of this study indicate that the MICs of ampicillin and penicillin for E. faecium observed with the Vitek system are reliable, but that the MIC of imipenem obtained using this method is unreliable. Therefore, the imipenem MICs for enterococci observed with the Vitek system should be rechecked by the agar dilution method when they disagree with those of penicillin and ampicillin.
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References
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Abstract
Introduction
