Abstract
We tested the in vitro susceptibilities of 603 enterococcal isolates from eight tertiary-care hospitals in Korea. The quinupristin-dalfopristin resistance rate in Enterococcus faecium was very high (25 isolates, 10.0%). It was suggested that both clonal spread and the sporadic emergence of quinupristin-dalfopristin-resistant isolates may explain the high prevalence of quinupristin-dalfopristin resistance in Korea.
Enterococci have become a more important cause of nosocomial infections with the emergence of multidrug-resistant strains in recent years (17). For instance, infections caused by vancomycin-resistant enterococci have resulted in increased morbidity and mortality due to limited treatment options (15). According to recent nationwide surveillance studies in Korea, the rate of vancomycin-resistant Enterococcus faecium (VREF) isolates has increased from 4% in 1997 to 16% in 2002 (11, 12). Quinupristin-dalfopristin (QD) could be useful in clinical practice as one of a few therapeutic options. Although several surveillance studies have reported increases in resistance to QD, the resistance rate still remains low in most parts (8, 13, 18). This recent multicenter surveillance study reports a high prevalence of QD resistance among E. faecium isolates from Korea.
Enterococcal isolates.
As part of a multicenter surveillance study during 2 months (August and September) in 2004, a total of 603 nonduplicate enterococcal isolates (330 Enterococcus faecalis isolates, 249 E. faecium isolates, and 24 other isolates) were collected from eight tertiary-care hospitals in various regions of Korea. In vitro susceptibility testing was performed by a broth microdilution test according to CLSI guidelines (2). Eleven antimicrobial agents were tested: vancomycin, teicoplanin, ampicillin, tetracycline, erythromycin, ciprofloxacin, chloramphenicol, rifampin, QD, streptomycin, and gentamicin. For streptomycin (1,000 mg/liter) and gentamicin (500 mg/liter), high-level resistance was tested. Susceptibility interpretive criteria used were those established in CLSI standard M100-S15 (2). E. faecalis strain ATCC 29212 and Staphylococcus aureus strain ATCC 29213 were used as control strains. The chi-square test and Fisher's exact test were used to determine the significance of resistance differences where appropriate.
Molecular characterization.
Multilocus sequence typing was performed as described previously (4, 9). To determine the number of variations in the esp A and C repeats, two different primer combinations were used, espfs7F-espfm5R and espfm5F-espfs3R, respectively (10). A genotypic clone was defined by coupling sequence type in multilocus sequence typing and the number of esp A and C repeats (9). Two virulence genes of E. faecium, enterococcal surface protein (esp) and hyaluronidase (hyl) genes, were detected by the duplex PCR method as described previously (23).
The results of the antimicrobial susceptibility test are summarized in Table 1. Sixty-three (25.3%) of 249 E. faecium isolates were resistant to vancomycin, while only 6 (1.8%) of 330 E. faecalis isolates were resistant to vancomycin. Resistance rates to vancomycin in E. faecium markedly varied by hospital, ranging from 0% to 54.7%. Isolates of VREF showed significantly higher resistance rates than vancomycin-susceptible E. faecium strains (VSEF) to teicoplanin, ampicillin, tetracycline, ciprofloxacin, and chloramphenicol (Table 2).
TABLE 1.
Antibiotic resistance of E. faecium and E. faecalis isolates
| Antimicrobial agenta |
E. faecium (n = 249)
|
E. faecalis (n = 330)
|
||
|---|---|---|---|---|
| MIC90 (mg/liter) | Resistance (%) | MIC90 (mg/liter) | Resistance (%) | |
| Vancomycin | >64 | 63 (25.3) | 2 | 6 (1.8) |
| Teicoplanin | 64 | 53 (21.3) | 0.5 | 5 (1.5) |
| Ampicillin | >64 | 233 (93.6) | 8 | 16 (4.8) |
| Tetracycline | 4 | 21 (8.4) | >64 | 262 (79.4) |
| Erythromycin | >32 | 227 (91.2) | >32 | 210 (63.6) |
| Ciprofloxacin | >64 | 235 (94.4) | 64 | 92 (27.8) |
| Chloramphenicol | 16 | 8 (3.2) | 32 | 88 (26.7) |
| Rifampin | >16 | 240 (96.4) | 16 | 122 (37.0) |
| Quinupristin-dalfopristin | 4 | 25 (10.0) | 16 | 269 (81.5) |
| Streptomycin-HLR | NAb | 162 (65.1) | NA | 106 (31.2) |
| Gentamicin-HLR | NA | 228 (91.6) | NA | 178 (53.9) |
HLR, high-level resistance.
NA, not available.
TABLE 2.
Comparison of antimicrobial resistance between VREF and VSEF isolates
| Antimicrobial agentc | No. of resistant isolates (%)
|
P value | |
|---|---|---|---|
| VREF (n = 63) | VSEF (n = 186) | ||
| Teicoplanin | 53 (84.1) | 0 | <0.01a |
| Ampicillin | 63 (100) | 170 (91.4) | 0.01a |
| Tetracycline | 10 (15.9) | 11 (5.9) | 0.01a |
| Erythromycin | 60 (95.2) | 167 (89.8) | 0.19a |
| Ciprofloxacin | 63 (100) | 172 (92.5) | 0.02b |
| Chloramphenicol | 5 (7.9) | 3 (1.6) | 0.03b |
| Rifampin | 62 (98.4) | 178 (95.7) | 0.45b |
| Quinupristin-dalfopristin | 2 (3.2) | 23 (12.4) | 0.01a |
| Streptomycin-HLR | 45 (71.4) | 117 (62.9) | 0.22a |
| Gentamicin-HLR | 60 (95.2) | 168 (90.3) | 0.22a |
Chi-square test.
Fisher's exact test.
HLR, high-level resistance.
In this study, the most prominent piece of data was a high rate of resistance to QD in E. faecium isolates in Korea (10.0%). This rate was significantly higher than those in North America, South America, and Europe, which ranged from 0% to 3.8% (7, 13, 19, 20, 22). Previous data from Korea with 56 E. faecium isolates also showed that only one isolate was resistant to QD (6). Based on previous reports, Taiwan showed very high rates of resistance to QD in E. faecium isolates, ranging from 9% to 51% (5, 14). Recent data from the SENTRY project in the Asian-Pacific region confirmed the high QD resistance rate in E. faecium from Taiwan (19.0%) and also showed increasing resistance to QD (29.4%) in Korea, with 17 isolates of E. faecium (J. M. Bell and J. D. Turnidge, Abstr. 44th Intersci. Conf. Antimicrob. Agents Chemother., abstr. C2-1361, 2004). Our study confirmed the increasing tendency of QD resistance in E. faecium isolates in Korea. Such high QD resistance rates in E. faecium in Taiwan and Korea were not observed in other Asian-Pacific countries (Bell and Turnidge, 44th ICAAC, abstr. C2-1361).
Our data showed a much higher QD resistance rate (12.9%) in VSEF than in VREF (3.2%), which was consistent with previous data (1). This implies that the QD resistance in E. faecium is not associated with the recent use of QD in the hospital for the treatment of vancomycin-resistant enterococci. Actually, the emergence of QD resistance even before its commercial use in the United States suggests that QD resistance might be linked with other reasons. Luh et al. (14) inferred that the high QD resistance rate in E. faecium in Taiwan was due to the use of virginiamycin in animal husbandry for many years. In Korea, virginiamycin has also been frequently used as a growth promoter in food animals, which could partly explain the high prevalence of QD resistance in E. faecium. However, the use of virginiamycin may not be the sole reason for the high rate of resistance to QD in Korea and Taiwan because Europe and the United States, where virginiamycin has also been used in animal husbandry, showed a low rate of QD resistance (16, 21). In addition, transmission of antibiotic-resistant E. faecium isolates from animals to humans is not common (3).
The QD resistance rate in E. faecium isolates was the highest at the Samsung Medical Center (16 of 64 isolates). In this hospital, two clones, ST192-A5-C6 in seven isolates and STnew-A0-C0 in two isolates, were identified (Table 3). This may suggest the clonal spread of the resistant strain within that hospital. In addition, two isolates in the Seoul Veterans Hospital also belonged to the same clone. However, there was no evidence that QD-resistant E. faecium isolates from other Korean hospitals have been clonally disseminated.
TABLE 3.
Genotypic characteristics and antimicrobial resistance in 25 QD-resistant E. faecium isolates from Korea
| Hospitala | Isolate | ST (allelic profile)b | esp repeat (A-C) | hyl | Antimicrobial resistancec |
|---|---|---|---|---|---|
| SMC | 01-27 | 78 (15-1-1-1-1-1-1) | 5-6 | + | Van, Tei, Amp, Pen, Ery, Cip, Rif, QD, Str, Gen |
| 01-34 | 78 (15-1-1-1-1-1-1) | 6-5 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| 01-37 | 192 (15-1-1-1-1-7-1) | 5-6 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| 01-67 | 203 (15-1-1-1-1-20-1) | 8-6 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| 01-93 | 78 (15-1-1-1-1-1-1) | 6-5 | + | Amp, Pen, Ery, Cip, Rif, QD, Str | |
| 01-106 | NEW (1-12-1-1-1-1-1) | —d | − | Amp, Pen, Tet, Ery, Cip, Rif, QD, Str | |
| 01-107 | 78 (15-1-1-1-1-1-1) | 6-5 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| 01-118 | 78 (15-1-1-1-1-1-1) | 6-5 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| 01-121 | 192 (15-1-1-1-1-7-1) | 5-6 | + | Amp, Pen, Ery, Cip, Rif, QD, Str | |
| 01-122 | 78 (15-1-1-1-1-1-1) | 6-5 | − | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| 01-142 | NEW (1-12-1-1-1-1-1) | — | − | Amp, Pen, Tet, Ery, Cip, Rif, QD, Str | |
| 01-148 | 192 (15-1-1-1-1-7-1) | 5-6 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| 01-158 | 192 (15-1-1-1-1-7-1) | 5-6 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| 01-167 | 192 (15-1-1-1-1-7-1) | 5-6 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| 01-176 | 192 (15-1-1-1-1-7-1) | 5-6 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| 01-196 | 192 (15-1-1-1-1-7-1) | 5-6 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| SVH | 02-03 | 78 (15-1-1-1-1-1-1) | 5-6 | − | Tet, Chl, Gen |
| 02-25 | 203 (15-1-1-1-1-20-1) | 5-6 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| 02-34 | 203 (15-1-1-1-1-20-1) | 5-6 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| Kyungpook | 06-14 | 192 (15-1-1-1-1-7-1) | 5-6 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen |
| Chonnam | 08-20 | 78 (15-1-1-1-1-1-1) | 6-5 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen |
| 08-81 | 203 (15-1-1-1-1-20-1) | 5-6 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| Chungbuk | 10-14 | 192 (15-1-1-1-1-7-1) | 5-6 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen |
| 10-24 | 78 (15-1-1-1-1-1-1) | 6-5 | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen | |
| 10-32 | 78 (15-1-1-1-1-1-1) | — | + | Amp, Pen, Ery, Cip, Rif, QD, Str, Gen |
SMC, Samsung Medical Center, SVH, Seoul Veterans Hospital; Kyungpook, Kyungpook National University Hospital; Chonnam, Chonnam National University Hospital; Chungbuk, Chungbuk National University Hospital.
ST, sequence type (atpA-ddl-gdh-purK-gyd-pstS-adk).
Van, vancomycin; Tei, teicoplanin; Amp, ampicillin; Pen, penicillin; Tet, tetracycline; Ery, erythromycin; Cip, ciprofloxacin; Chl, chloramphenicol; Rif, rifampin; Str, streptomycin; Gen, gentamicin.
Absence of esp gene.
Of 249 E. faecium isolates, esp and hyl genes were detected in 184 (73.9%) and 169 (67.9%) isolates, respectively. The esp gene was more frequently found in VREF (58/63 isolates, 92.1%) than in VSEF (67.7%) isolates. The hyl gene was present in 37 (58.7%) and 232 (71.0%) VREF and VSEF isolates, respectively. A dual presence of esp and hyl genes was observed among 31 VREF (49.2%) and 108 VSEF (58.1%) isolates.
In summary, the present study documented a high rate of QD resistance in E. faecium from Korea due to both clonal spread and sporadic emergence. Given the clinical importance of multidrug-resistant enterococci, continuous surveillance of QD resistance in E. faecium is strongly warranted if QD is to be used to treat E. faecium.
Acknowledgments
This study was partly supported by the Korean Food & Drug Administration (KFDA) and the Asian-Pacific Research Foundation for Infectious Diseases (ARFID).
The eight tertiary-care hospitals participating in this study were the Samsung Medical Center (SMC, Seoul), Seoul Veterans Hospital (Seoul), Kangbuk Samsung Hospital (Seoul), Dong-A University Hospital (Busan), Kyungpook National University Hospital (Daegu), Chungnam National University Hospital (Daejeon), Chonnam National University Hospital (Gwangju), and Chungbuk National University Hospital (Chungju).
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