LETTER
We recently reported Klebsiella pneumoniae carrying blaIMP-type metallo-β-lactamase gene variants showing a paradoxical resistance phenotype: resistance to virtually all β-lactams except imipenem (1, 2). Here we report the isolation of Acinetobacter spp. showing a similar resistance phenotype and report the first case of Acinetobacter baumannii carrying the Tripoli metallo-β-lactamase 1 (TMB-1) gene.
Two Acinetobacter spp. were isolated from blood samples from two patients, one in November 2009 (A1) and another in August 2012 (MS5320). These two cases were from patients with no history of international travel who were from the same hospital but lacking any apparent epidemiological link. The Acinetobacter isolates were identified by sequencing the 16S rRNA genes (3), in the first case as Acinetobacter baumannii (A1) and in the second case as Acinetobacter calcoaceticus (MS5320). These isolates were resistant to almost all β-lactams, including meropenem, but showed intermediate resistance to imipenem according to CLSI document guidelines (4): the A1 MIC for imipenem was 8 μg/ml and for meropenem was 64 μg/ml, and the MS5320 MIC for imipenem was 2 μg/ml and for meropenem was 64 μg/ml (Table 1). The metallo-β-lactamase phenotype test was positive as judged by the disk approximation method using sodium mercaptoacetic acid (5). Screening for the presence of known carbapenemase genes using PCR with 11 universal primer sets (6) gave negative results. PCR detection of class 1 integron cassettes using a primer set targeting the 5′ conserved segment (CS) and 3′ CS resulted in an amplicon of 1.1 kb. Direct sequencing of the amplicon showed a single cassette containing a gene sequence that had 100% identity with the blaTMB-1 nucleotide sequence of Achromobacter xylosoxidans isolated in Tripoli, Libya (7). The blaTMB-1-harboring integron in this study (GenBank accession number AB846838) was identical to the already-reported blaTMB-2-harboring integron (8), with the exception only of the single nucleotide substitution converting TMB-1 into TMB-2. Multilocus sequence typing (MLST) (9) revealed that the A1 isolate belongs to ST726, is not related to MDR clone CC92, and is new and most similar to ST170 isolated in Korea (10). The failures of electroporation and conjugation with A1 and MS5320 using streptomycin-resistant (SMr) Escherichia coli HB101 with meropenem or ceftazidime and streptomycin for selection (11) suggest that the blaTMB-1 may be on the chromosome, but the possibility that the gene is on a non-self-transmissible plasmid remains to be elucidated.
TABLE 1.
Antibiotic susceptibility profilesa
| Antibiotic | MIC (mg/liter) |
|
|---|---|---|
| A1 (Acinetobacter baumannii) TMB-1 positive | MS5320 (Acinetobacter calcoaceticus) TMB-1 positive | |
| Ceftazidime | 256 | 256 |
| Amikacin | 2 | 4 |
| Tobramycin | 1 | 2 |
| Imipenem | 8 | 2 |
| Meropenem | 64 | 64 |
| Biapenem | 2 | 0.5 |
| Doripenem | 64 | 64 |
| Panipenem | 8 | 8 |
| Minocyclin | 0.25 | 0.25 |
Susceptibility tests were performed by the microdilution method (4).
The kcat/Km value of TMB-1 was significantly lower against imipenem than meropenem, suggesting that TMB-1 is less active in hydrolyzing imipenem (7). Suzuki et al. recently identified blaTMB-2 in non-baumannii Acinetobacter spp. in Japan (8). They artificially created blaTMB-1 by site-directed mutagenesis and demonstrated that the MICs of imipenem and meropenem against E. coli DH5α carrying blaTMB-1 were 1 and 32 μg/ml, respectively. This suggests that the enzyme characteristics of TMB-1 may have conferred the paradoxical phenotype to Acinetobacter spp.
To our knowledge, this is the first report of clinical isolation of a blaTMB-1-positive Acinetobacter sp. Together with the report of Suzuki et al. (8), the results show that dissemination of the blaTMB gene in Acinetobacter spp. may be occurring in Japan and requires further investigation.
Nucleotide sequence accession number.
The nucleotide sequence of the blaTMB-1-harboring integron determined in this study has been submitted to GenBank (accession number AB846838).
ACKNOWLEDGMENTS
We thank Jim Nelson and Larry Strand for editorial assistance.
Footnotes
Published ahead of print 21 January 2014
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