Abstract
The susceptibility of 14 species of 115 Gram-positive anaerobic cocci (GPAC) was determined for 14 antibiotics. To assure correct identification, strains were genotypically identified by fluorescence in situ hybridization and sequencing. Susceptibility differences (MIC50 and MIC90) for penicillin G, clindamycin, tigecycline, levofloxacin, amoxicillin-clavulanic acid, cefoxitin, ertapenem, meropenem, metronidazole, and doxycycline were found for the three clinically most relevant GPAC species: Finegoldia magna, Parvimonas micra, and Peptoniphilus harei.
Gram-positive anaerobic cocci (GPAC) are part of the commensal microbiota and account for about one-third of the anaerobic isolates recovered from clinical materials (14). It is a heterogeneous group, which in the last decade has undergone an extensive taxonomic change. The species Peptostreptococcus micros and Peptostreptococcus magnus were transferred to two new genera, Micromonas and Finegoldia, respectively, with each being the only species present in their respective genus (15). The genus Micromonas has recently been replaced by Parvimonas, with Parvimonas micra (Pa. micra) being the only species present (20). Ezaki et al. (7) divided the remaining peptostreptococci into three phylogenetic groups, Peptoniphilus gen. nov., Anaerococcus gen. nov., and Gallicola gen. nov., with Gallicola barnesae being the only species present in the latter genus. The species left in the genus Peptostreptococcus include Peptostreptococcus anaerobius (Pe. anaerobius) and a recently described new species, Pe. stomatis (6). Song et al. (19) described three new species: Peptoniphilus gorbachii (Pt. gorbachii) sp. nov., Pt. olsenii sp. nov., and Anaerococcus murdochii sp. nov. The most commonly found GPAC in clinical material are Finegoldia magna, Pa. micra, Pt. harei (21), and Pe. anaerobius (22). The data on the antimicrobial susceptibility of the different species of GPAC is often based on GPAC in general, even though several authors describe a difference in antimicrobial susceptibility between species (3-5, 11, 12, 18). In these studies, the strains were identified phenotypically. However, for some species it is difficult to obtain a reliable phenotypic identification, e.g., in the past Pt. harei has often been misidentified as Pt. asaccharolyticus (21), probably due to the fact that these two species share the same biochemical characteristics (10).
In the present study, we assessed the susceptibility of 115 isolates of GPAC against 14 different antibiotics. Isolates were genotypically identified by using fluorescence in situ hybridization (FISH) (21) or sequencing, thus allowing more accurate insight into the distribution of susceptible and resistant strains within the different species.
MATERIALS AND METHODS
Isolates.
Strains were obtained from the diagnostic laboratory of the University Medical Center Groningen and collected in the years 2002 to 2004. All strains were isolated from human clinical samples from a variety of anatomical sites, e.g., from abdominal, head and neck, and soft tissue infections. Strains were stored at −80°C and subcultured on brucella blood agar (BBA) prior to susceptibility testing.
Identification.
Strains were genotypically identified by using 16S rRNA-based probes (21) and sequencing. Shortly thereafter, bacterial cells were harvested from BBA using a sterile loop and fixed in 1:1 phosphate-buffered saline (8 g of NaCl, 0.2 g of KCl, 1.44 g of Na2HPO4, and 0.24 g of KH2PO4 per liter) and ethanol 96% (vol/vol). Fixed cells were spotted on slides and, if necessary, permeabilized using proteinase K. Strains were hybridized by using probes directed against F. magna, Pa. micra, Pt. harei, Pe. anaerobius, A. vaginalis, Pt. asaccharolyticus, A. lactolyticus, and Pt. ivorii. The addition of new species to the genera Peptoniphilus and Anaerococcus (19) showed that the probes directed against A. lactolyticus and Pt. harei were also positive, with A. murdochii and Pt. gorbachii, respectively (data not shown). Strains that were negative with the probes or positive with the probes directed against A. lactolyticus and Pt. harei were sequenced. DNA was isolated as described previously (2), and the 16S genes were amplified and sequenced using universal 16S rRNA-specific primers (9). Sequences were compared to those in the GenBank database by performing a BLAST search (National Center of Biotechnology Information) (1).
Susceptibility testing.
The antimicrobial susceptibility using penicillin G, amoxicillin-clavulanic acid, cefotetan, cefoxitin, ertapenem, meropenem, levofloxacin, moxifloxacin, clindamycin, metronidazole, linezolid, chloramphenicol, doxycycline, and tigecycline was determined by using Etest (AB Biodisk, Sweden). Suspensions of approximately 2 McFarland standards were made in prereduced brucella broth and applied onto a prereduced BBA. All culture handlings were performed in an anaerobic chamber. Plates with Etest strips were incubated for 48 h at 37°C in an anaerobic chamber before reading the MIC. In each batch a quality control strain Bacteroides fragilis ATCC 25285 was included.
A difference in susceptibility was defined as at least a two-dilution-step (with one dilution step being a difference of 2-fold dilutions with a precision of a 0.5 dilution) difference between the MIC's of the different species.
RESULTS
The quality control strain B. fragilis ATCC 25285 was tested 10 times with all 14 antibiotics. The obtained MICs are summarized in Table 1.
TABLE 1.
MIC values determined from the quality control tests on B. fragilis ATCC 25285
| Antibiotic | MIC (mg/liter) |
|
|---|---|---|
| MICs (no. of tests) | Expected MIC rangea | |
| Penicillin G | 12 (2), 16 (7), 24 (1) | 8-32 |
| Amoxicillin-clavulanic acid | 0.19 (2), 0.25 (5), 0.38 (3) | 0.125-0.5† |
| Cefotetan | 6 (7), 8 (3) | 4-16 |
| Cefoxitin | 4 (1), 6 (7), 8 (2) | 4-16 |
| Ertapenem | 0.125 (4), 0.19 (6) | 0.064-0.25 |
| Meropenem | 0.094 (2), 0.125 (4), 0.19 (4) | 0.064-0.25 |
| Levofloxacin | 1 (1), 1.5 (9) | 1* |
| Moxifloxacin | 0.19 (1), 0.25 (1), 0.38 (6), 0.5 (2) | 0.125-0.5 |
| Clindamycin | 1.5 (2), 2 (4), 3 (4) | 0.5-2 |
| Metronidazole | 0.25 (4), 0.38 (4), 0.5 (2) | 0.25-1 |
| Linezolid | 4 (1), 6 (5), 8 (3), 12 (1) | 2-8* |
| Chloramphenicol | 6 (3), 8 (7) | 2-8 |
| Doxycycline | 0.25 (3), 0.38 (5), 0.5 (2) | 0.25-0.5* |
| Tigecycline | 0.25 (2), 0.5 (2), 0.75 (6) | 0.125-1* |
The expected range is derived from CLSI standards for B. fragilis for reference agar dilution testing, except as indicated: *, expected range derived from literature; and †, expected range derived from the manufacturer.
All results of the clinical isolates are summarized in Table 2 and Table 3. The MIC50 and MIC90 values were only calculated for species for which more than 10 strains were present in the study, i.e., F. magna, Pa. micra, and Pt. harei. Upon comparing the MIC50 and MIC90 values for these three species, F. magna had the highest MIC50 and MIC90 values for penicillin G, amoxicillin-clavulanic acid, clindamycin, and tigecycline. It has the highest MIC50 values for cefotetan, cefoxitin, meropenem, linezolid, and chloramphenicol and the highest MIC90 values for levofloxacin and moxifloxacin. Pa. micra has the lowest MIC50 and MIC90 for levofloxacin, metronidazole, and doxycycline and the lowest MIC90 for amoxicillin-clavulanic acid. Pt. harei has the highest MIC50 for levofloxacin and doxycycline. It has the lowest MIC50 and MIC90 for cefoxitin, ertapenem, and meropenem and the lowest MIC90 for chloramphenicol.
TABLE 2.
MICs and range for GPAC against 14 antibiotics
| Organism (no. of strains)a | Antibiotic | MIC (mg/liter) |
||
|---|---|---|---|---|
| Range | MIC50 | MIC90 | ||
| F. magna (31) | Penicillin G | 0.023-0.38 | 0.125 | 0.25 |
| Amoxicillin-clavulanic acid | 0.094-2 | 0.25 | 0.5 | |
| Cefotetan | 0.25-4 | 2 | 2 | |
| Cefoxitin | 0.38-3 | 1 | 1.5 | |
| Ertapenem | 0.016-0.19 | 0.064 | 0.125 | |
| Meropenem | 0.064-0.25 | 0.125 | 0.19 | |
| Levofloxacin | 0.094-64 | 0.75 | 64 | |
| Moxifloxacin | 0.047-64 | 0.19 | 6 | |
| Clindamycin | 0.125->256 | 1 | 3 | |
| Metronidazole | 0.094-1.5 | 0.38 | 1 | |
| Linezolid | 2-6 | 3 | 3 | |
| Chloramphenicol | 4-16 | 6 | 8 | |
| Doxycycline | 0.75-24 | 2 | 24 | |
| Tigecycline | 0.064-1 | 0.25 | 0.75 | |
| Pa. micra (27) | Penicillin G | 0.016-0.125 | 0.016 | 0.047 |
| Amoxicillin-clavulanic acid | 0.016-0.75 | 0.032 | 0.094 | |
| Cefotetan | 0.125-2 | 0.38 | 1.5 | |
| Cefoxitin | 0.125-3 | 0.5 | 2 | |
| Ertapenem | 0.008-0.19 | 0.047 | 0.125 | |
| Meropenem | 0.008-0.38 | 0.047 | 0.19 | |
| Levofloxacin | 0.125-3 | 0.25 | 0.5 | |
| Moxifloxacin | 0.094-1.5 | 0.19 | 0.38 | |
| Clindamycin | 0.047-2 | 0.38 | 1.5 | |
| Metronidazole | 0.032->256 | 0.094 | 0.25 | |
| Linezolid | 0.125-3 | 1 | 3 | |
| Chloramphenicol | 0.75-6 | 3 | 6 | |
| Doxycycline | 0.047-4 | 0.125 | 1 | |
| Tigecycline | 0.016-0.38 | 0.064 | 0.125 | |
| Pt. harei (16) | Penicillin G | 0.016-0.19 | 0.023 | 0.032 |
| Amoxicillin-clavulanic acid | 0.016-0.38 | 0.023 | 0.25 | |
| Cefotetan | 0.38-8 | 0.5 | 1 | |
| Cefoxitin | 0.023-1.5 | 0.094 | 0.5 | |
| Ertapenem | 0.006-0.023 | 0.012 | 0.016 | |
| Meropenem | 0.004-0.032 | 0.008 | 0.032 | |
| Levofloxacin | 2-64 | 4 | 6 | |
| Moxifloxacin | 0.125-1.5 | 0.19 | 0.38 | |
| Clindamycin | 0.094-1.5 | 0.25 | 1.5 | |
| Metronidazole | 0.032-2 | 0.38 | 1.5 | |
| Linezolid | 0.5-2 | 0.75 | 1.5 | |
| Chloramphenicol | 1.5-4 | 3 | 3 | |
| Doxycycline | 0.064-24 | 8 | 16 | |
| Tigecycline | 0.023-0.25 | 0.094 | 0.25 | |
| A. vaginalis (8) | Penicillin G | 0.016-0.094 | ||
| Amoxicillin-clavulanic acid | 0.016-0.125 | |||
| Cefotetan | 0.094-0.5 | |||
| Cefoxitin | 0.032-0.125 | |||
| Ertapenem | 0.023-0.19 | |||
| Meropenem | 0.006-0.125 | |||
| Levofloxacin | 24-64 | |||
| Moxifloxacin | 0.5-2 | |||
| Clindamycin | 0.023->256 | |||
| Metronidazole | 0.047-0.5 | |||
| Linezolid | 0.38-1.5 | |||
| Chloramphenicol | 1.5-3 | |||
| Doxycycline | 0.125-16 | |||
| Tigecycline | 0.047-1.5 | |||
| Pt. ivorii (5) | Penicillin G | 0.016-0.047 | ||
| Amoxicillin-clavulanic acid | 0.016-0.032 | |||
| Cefotetan | 0.125-1 | |||
| Cefoxitin | 0.125-0.75 | |||
| Ertapenem | 0.004-0.032 | |||
| Meropenem | 0.002-0.016 | |||
| Levofloxacin | 0.38-64 | |||
| Moxifloxacin | 0.094-64 | |||
| Clindamycin | 0.094-2 | |||
| Metronidazole | 0.094-0.25 | |||
| Linezolid | 0.19-2 | |||
| Chloramphenicol | 1-3 | |||
| Doxycycline | 0.064-16 | |||
| Tigecycline | 0.032-0.25 | |||
| Pe. anaerobius (4) | Penicillin G | 0.064-2 | ||
| Amoxicillin-clavulanic acid | 0.125-4 | |||
| Cefotetan | 0.5-24 | |||
| Cefoxitin | 0.19-3 | |||
| Ertapenem | 0.032-0.75 | |||
| Meropenem | 0.023-1 | |||
| Levofloxacin | 0.38-1.5 | |||
| Moxifloxacin | 0.19-0.25 | |||
| Clindamycin | 0.032-1 | |||
| Metronidazole | 0.032-0.25 | |||
| Linezolid | 0.38-1.5 | |||
| Chloramphenicol | 1-3 | |||
| Doxycycline | 0.5-4 | |||
| Tigecycline | 0.064-0.125 | |||
| Pt. lacrimalis (4) | Penicillin G | 0.016-0.125 | ||
| Amoxicillin-clavulanic acid | 0.016-0.25 | |||
| Cefotetan | 0.016-0.38 | |||
| Cefoxitin | 0.016-0.25 | |||
| Ertapenem | 0.002-0.012 | |||
| Meropenem | 0.002-0.016 | |||
| Levofloxacin | 3-8 | |||
| Moxifloxacin | 0.002-0.38 | |||
| Clindamycin | 0.016-0.38 | |||
| Metronidazole | 0.023-0.38 | |||
| Linezolid | 0.19-2 | |||
| Chloramphenicol | 0.75-3 | |||
| Doxycycline | 0.125-4 | |||
| Tigecycline | 0.023-0.25 | |||
| Pt. gorbachii (4) | Penicillin G | 0.016-0.19 | ||
| Amoxicillin-clavulanic acid | 0.016-0.064 | |||
| Cefotetan | 0.5-1.5 | |||
| Cefoxitin | 0.064-0.5 | |||
| Ertapenem | 0.012-0.023 | |||
| Meropenem | 0.004-0.064 | |||
| Levofloxacin | 3-64 | |||
| Moxifloxacin | 0.19-0.5 | |||
| Clindamycin | 0.125-0.75 | |||
| Metronidazole | 0.023-0.5 | |||
| Linezolid | 0.75-1.5 | |||
| Chloramphenicol | 2-3 | |||
| Doxycycline | 0.064-0.38 | |||
| Tigecycline | 0.016-0.094 | |||
| A. murdochii (3) | Penicillin G | 0.016-0.75 | ||
| Amoxicillin-clavulanic acid | 0.032-0.25 | |||
| Cefotetan | 0.75-8 | |||
| Cefoxitin | 0.125-1 | |||
| Ertapenem | 0.19-2 | |||
| Meropenem | 0.125-0.75 | |||
| Levofloxacin | 1.5-4 | |||
| Moxifloxacin | 0.25 | |||
| Clindamycin | 0.016-0.5 | |||
| Metronidazole | 0.19-0.5 | |||
| Linezolid | 0.38-0.75 | |||
| Chloramphenicol | 1-3 | |||
| Doxycycline | 0.25-16 | |||
| Tigecycline | 0.047 | |||
| At. parvulum (4) | Penicillin G | 0.094-0.25 | ||
| Amoxicillin-clavulanic acid | 0.064-0.25 | |||
| Cefotetan | 2-8 | |||
| Cefoxitin | 1.5-3 | |||
| Ertapenem | 0.032-0.19 | |||
| Meropenem | 0.125-0.25 | |||
| Levofloxacin | 0.38-0.5 | |||
| Moxifloxacin | 0.19-0.38 | |||
| Clindamycin | 1.5-6 | |||
| Metronidazole | 0.19-0.5 | |||
| Linezolid | 0.75-2 | |||
| Chloramphenicol | 4-16 | |||
| Doxycycline | 1-2 | |||
| Tigecycline | 0.064-0.5 | |||
| A. tetradius (2) | Penicillin G | 0.023-0.032 | ||
| Amoxicillin-clavulanic acid | 0.032-0.064 | |||
| Cefotetan | 0.25-0.5 | |||
| Cefoxitin | 0.19-0.38 | |||
| Ertapenem | 0.094-0.125 | |||
| Meropenem | 0.094-0.125 | |||
| Levofloxacin | 2-3 | |||
| Moxifloxacin | 0.19-0.38 | |||
| Clindamycin | 1-4 | |||
| Metronidazole | 0.25-0.75 | |||
| Linezolid | 1-1.5 | |||
| Chloramphenicol | 3-3 | |||
| Doxycycline | 2-8 | |||
| Tigecycline | 0.125-0.19 | |||
| Pt. octavius (1) | Penicillin G | 0.125 | ||
| Amoxicillin-clavulanic acid | 0.064 | |||
| Cefotetan | 0.5 | |||
| Cefoxitin | 0.25 | |||
| Ertapenem | 0.094 | |||
| Meropenem | 0.094 | |||
| Levofloxacin | 4 | |||
| Moxifloxacin | 0.5 | |||
| Clindamycin | 0.047 | |||
| Metronidazole | 0.38 | |||
| Linezolid | 0.75 | |||
| Chloramphenicol | 2 | |||
| Doxycycline | 0.19 | |||
| Tigecycline | 0.064 | |||
| R. gnavus (1) | Penicillin G | 1 | ||
| Amoxicillin-clavulanic acid | 0.19 | |||
| Cefotetan | 32 | |||
| Cefoxitin | 4 | |||
| Ertapenem | 0.38 | |||
| Meropenem | 0.125 | |||
| Levofloxacin | 64 | |||
| Moxifloxacin | 6 | |||
| Clindamycin | 0.38 | |||
| Metronidazole | 0.094 | |||
| Linezolid | 2 | |||
| Chloramphenicol | 3 | |||
| Doxycycline | 0.25 | |||
| Tigecycline | 0.094 | |||
| A. lactolyticus (1) | Penicillin G | 0.125 | ||
| Amoxicillin-clavulanic acid | 0.125 | |||
| Cefotetan | 2 | |||
| Cefoxitin | 0.5 | |||
| Ertapenem | 1 | |||
| Meropenem | 0.38 | |||
| Levofloxacin | 6 | |||
| Moxifloxacin | 0.19 | |||
| Clindamycin | 0.047 | |||
| Metronidazole | 0.25 | |||
| Linezolid | 0.38 | |||
| Chloramphenicol | 1 | |||
| Doxycycline | 0.38 | |||
| Tigecycline | 0.094 | |||
| GPAC (4) | Penicillin G | 0.023-0.125 | ||
| Amoxicillin-clavulanic acid | 0.016-0.094 | |||
| Cefotetan | 1-4 | |||
| Cefoxitin | 0.125-1 | |||
| Ertapenem | 0.006-2 | |||
| Meropenem | 0.008-0.75 | |||
| Levofloxacin | 0.5-2 | |||
| Moxifloxacin | 0.064-0.38 | |||
| Clindamycin | 0.094-0.125 | |||
| Metronidazole | 0.064-0.38 | |||
| Linezolid | 0.5-1 | |||
| Chloramphenicol | 1.5-3 | |||
| Doxycycline | 0.094-1 | |||
| Tigecycline | 0.023-0.19 | |||
Genus abbreviations: Pe., Peptostreptococcus; Pa., Parvimonas; Pt., Peptoniphilus; A., Anaerococcus; R., Ruminococcus; At., Atopobium.
TABLE 3.
Overall resistance of GPAC against 15 antibioticsa
| Antibiotic | MIC (mg/liter) |
||
|---|---|---|---|
| Range | MIC50 | MIC90 | |
| Penicillin G | 0.016-2 | 0.047 | 0.19 |
| Amoxicillin-clavulanic acid | 0.016-4 | 0.094 | 0.38 |
| Cefotetan | 0.016-32 | 0.75 | 3 |
| Cefoxitin | 0.016-4 | 0.5 | 2 |
| Ertapenem | 0.002-2 | 0.064 | 0.19 |
| Meropenem | 0.002-1 | 0.064 | 0.25 |
| Levofloxacin | 0.094-64 | 0.75 | 64 |
| Moxifloxacin | 0.002-64 | 0.25 | 1.5 |
| Clindamycin | 0.016->256 | 0.38 | 2 |
| Metronidazole | 0.023->256 | 0.19 | 0.75 |
| Linezolid | 0.125-6 | 1.5 | 3 |
| Chloramphenicol | 0.75-16 | 3 | 8 |
| Doxycycline | 0.047-24 | 1 | 16 |
| Tigecycline | 0.016-1.5 | 0.094 | 0.38 |
The overall resistance of GPAC (n = 115) against various antibiotics is indicated.
DISCUSSION
Since GPAC can show poor growth, we used a McFarland 2 inoculum. The MICs obtained with the quality control strain B. fragilis ATCC 25285 show that most of these values are within the expected range. Comparison between a McFarland standard 1 and 2 inoculum using the quality control strain gave the same MIC value (data not shown). However, 4 of the 10 MIC values obtained for clindamycin were just above the expected range obtained using McFarland standard 2. Since GPAC show poor growth compared to B. fragilis, this is not expected to affect our set of data. A practical approach is to use a higher McFarland turbidity as recommended by the manufacturer of Etest.
In the present study strains were identified genotypically, since phenotypic identification is not always reliable for all species (21). It is difficult to compare our results to other published resistance data, since authors may use different breakpoints. For example, some did use breakpoints advised by the Clinical and Laboratory Standards Institute (CLSI), while others used those advised by EUCAST. Therefore, we have chosen to base a difference in susceptibility on the MIC50 and MIC90 values, instead of the percentage resistant strains. However, the interpretation of our results using CSLI and EUCAST breakpoints is provided in the supplemental material.
The clinically most important GPAC in our study are F. magna, Pa. micra, and Pt. harei. The latter can be especially difficult to identify phenotypically, since its biochemical features resemble those of Pt. asaccharolyticus (10). In the past, Pt. harei was probably often misidentified as Pt. asaccharolyticus, resulting in limited susceptibility data for this species. Brazier et al. (4) included 44 clinical isolates of Pt. harei in a European study; all of them were phenotypically identified. No resistance was reported. In a susceptibility study in England and Wales (5), four clinical isolates of Pt. harei were included; all of these were also phenotypically identified. Resistance (MIC > 256) was reported to clindamycin. In our study, the MIC50 and MIC90 values for clindamycin were 0.25 and 1.5, respectively. The latter was the highest MIC found for Pt. harei.
Our study is the first to include Pt. gorbachii and A. murdochii, although the numbers are low. It is worth mentioning that one strain of A. murdochii had high MIC values for 4 of the 14 antibiotics: doxycycline, ertapenem, levofloxacin, and penicillin G.
Differences in susceptibility to antibiotics were described for Pe. anaerobius and Pe. stomatis (12). Pe. anaerobius has higher MIC values for amoxicillin, amoxicillin-clavulanic acid, cefoxitin, ertapenem, azithromycin, clindamycin, metronidazole, and moxifloxacin than Pe. stomatis; only the MIC90 of azithromycin and moxifloxacin was not two dilution steps higher. Brazier et al. (5) also suggest that some GPAC species are more resistant to antibiotics than others. For example, Pe. anaerobius had a higher MIC50 for tetracycline but had lower MIC values for erythromycin than did F. magna. Roberts et al. (18) showed that Pe. anaerobius has higher MIC50 and MIC90 values for amoxicillin-clavulanic acid, piperacillin-tazobactam, cefoxitin, cefotetan, and meropenem than did F. magna, Pa. micra, and Pt. asaccharolyticus. Koeth et al. (11) showed that F. magna has a higher MIC50 for clindamycin as Pa. micra and Pe. anaerobius, while Pe. anaerobius has the highest MIC90 for amoxicillin-clavulanic acid.
Metronidazole is often the drug used for empirical treatment of anaerobic infections. However, GPAC strains are described which are resistant to this drug (11, 13, 16). We encountered one strain of Pa. micra that was resistant to metronidazole (MIC > 256). Microbiologists should be aware of this possibility. It is remarkable to notice the difference in susceptibility to the different antibiotics between the three most clinically important GPAC: F. magna, Pa. micra, and Pt. harei. Therefore, it is important to identify clinical isolates of GPAC. F. magna and Pa. micra can be reliably phenotypically identified by using a commercially available enzymatic kit such as Rapid ID 32A (21). However, Pt. harei cannot be phenotypically distinguished from Pt. asaccharolyticus (10, 21). The combination of diminished antimicrobial susceptibility, its prevalence, and the described virulence factors (8) gives F. magna a special position among the GPAC.
Supplementary Material
Acknowledgments
We are grateful to AB Biodisk for providing the Etest strips.
Footnotes
Published ahead of print on 28 December 2010.
Supplemental material for this article may be found at http://aac.asm.org/.
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