First Occurrence of a Pseudomonas aeruginosa Isolate in the United States Producing an IMP Metallo-β-Lactamase, IMP-18

Metallo-β-lactamases hydrolyze most β-lactam antibiotics except aztreonam. Therefore, many pathogens that produce these enzymes at high levels are resistant to the majority of β-lactam antibiotics, including the carbapenems. The first report of an imported metallo-β-lactamase described a Pseudomonas aeruginosa isolate obtained from a Japanese patient in 1988 (6). Since then, the occurrence of mobile genetic elements encoding metallo-β-lactamases has extended beyond P. aeruginosa to include many types of gram-negative organisms distributed throughout the world (5). Areas which have reported these types of isolates include several countries in Asia and Europe; the Americas, including Brazil, Canada, and the United States; and Australia (5).

Based on recent reports, there are two major families of imported metallo-β-lactamases, IMP and VIM, that are carried on mobile gene cassettes inserted into integrons (5). Including those in this report, there are 18 variants of IMP metallo-β-lactamases and 11 variants of VIM metallo-β-lactamases (5). The only two published reports on metallo-β-lactamases from the United States identified VIM-2 and VIM-7 (1, 4).

In this study, we investigated a carbapenem-resistant P. aeruginosa isolate for metallo-β-lactamase production. The isolate was recovered from the tracheal aspirate of a motorcycle accident victim in the southwestern United States. It was found to produce a new member of the IMP family of metallo-β-lactamases.

The isolate was identified by the Phoenix system (BD Diagnostic Systems, Sparks, Md.). Antibiotic susceptibility was determined by broth microdilution and interpreted by using CLSI criteria (2). These data are presented in Table 1. Metallo-β-lactamase production was phenotypically investigated by using the Etest metallo-β-lactamase strip (AB BIODISK, Solna, Sweden). By Etest there was a ≥32-fold reduction in the MIC of imipenem in the presence of EDTA (≥256 to 8 μg/ml).

TABLE 1.

Microdilution MICs (μg/ml) of selected antimicrobial agents against Pseudomonas aeruginosa (IMP-18)

Antimicrobial agent	Microdilution MIC (μg/ml)
Imipenem	64
Ceftazidime	>64
Ceftriaxone	>64
Cefepime	>64
Aztreonam	32
Ampicillin-sulbactam	>128
Piperacillin-tazobactam	128
Amikacin	64
Gentamicin	>32
Levofloxacin	>16
Polymyxin B	4
Minocycline	>128
Tigecycline	32

The bla_IMP-1-like gene was detected by PCR as previously described (3), using IMP-1-family-specific primers internal to the gene. Amplification of the full-length product of 781 bp was obtained by PCR, using an annealing temperature of 42°C, a magnesium concentration of 2 mM, and primers which flanked the gene. The forward primer was 5′-GTTAGAAAAGGAAAAGTATG-3′, and the reverse primer was 5′-TGCTGCAACGACTTGTTAG-3′. PCR products were generated on at least two separate occasions and sequenced by automated cycle sequencing, using an ABI Prism 3100-Avant Genetic analyzer. Sequence alignments and analyses were performed online using the BLAST program (www.ncbi.nlm.nih.gov). Sequence translation was performed using the DNASIS analysis program (Hatachi Software Engineering Co.). The GenBank accession number for the gene is AY780674, and the enzyme has been designated IMP-18.

Imp-8 and IMP-14 were the β-lactamases most closely related to IMP-18. IMP-18 differed from IMP-14 by 21 amino acids, indicating 91% identity, while IMP-8 differed by 29 amino acids, representing 88% identity. Therefore, IMP-18 represents a distinct variant of this family group.

The acquisition of metallo-β-lactamases by P. aeruginosa is disturbing because this resistance mechanism reduces the few available options for therapy for infections caused by this pathogen. In addition, the mobility of the genetic elements from which metallo-β-lactamases are expressed increases the ability of this resistance mechanism to spread to other gram-negative pathogens (5).