Nosocomially acquired, carbapenem-resistant K. pneumoniae
isolates were first seen at the Tisch Hospital in April 2000, being recovered from the respiratory secretions of patients in ICUs. K. pneumoniae
ranks fourth as a cause of hospital-acquired pneumonia in the United States (1
), with its principal nosocomial reservoirs reported to be contaminated medical equipment, the hands of hospital staff, and the gastrointestinal tracts of patients (26
). PFGE of isolates from 24 patients identified one predominant strain from 19 patients, although other resistant strains were also identified. The isolates were obtained from debilitated patients who had received multiple intensive antibiotic treatments, and antibiotic pressure may have been a factor selecting for initial colonization and the development of further resistance.
Three representatives of the outbreak strain were studied and shown to produce a carbapenem-hydrolyzing enzyme; sequencing revealed it to be a novel member of the KPC family, designated KPC-3. These isolates also lacked OmpK35, which may have reduced permeability and have potentiated their resistance. We also found an Ile(31)→Val substitution in waaE, a gene associated previously with imipenem resistance in a laboratory mutant of K. pneumoniae (Tomas et al., 41st ICAAC), but this substitution was considered unlikely to contribute to the carbapenem resistance of the outbreak strain.
In addition to their carbapenem resistance, these isolates were mostly resistant to cephalosporins, penicillins, and aminoglycosides. Variant CL 5762A showed greater susceptibility to carbapenems and other β-lactams, but no explanation for this behavior was found; the organism had a similar carbapenemase-specific activity and a similar OMP profile to the other three isolates.
Using transformation, conjugation, and hybridization studies, we located the blaKPC
allele on a ca. 75-kb plasmid and showed that this allele also carried an aminoglycoside resistance determinant conferring resistance to amikacin and low-level resistance to gentamicin—a resistance profile consistent with an AAC(6′) variant. MICs for KPC-3-producing transformants of porin-proficient E. coli
DH5α tended to be lower than those reported for KPC-1-producing transformants of the same strain (38
). This finding may reflect either alterations in the substrate specificity of KPC-3 contingent on amino acid differences or the quantity of enzyme produced. The transformant was also much less resistant than the original isolate, probably owing to the lack of any permeability lesion.
Carbapenem-resistant klebsiellae remain rare. No such isolates were reported to The Surveillance Network from 1998 to 2001 (12
), and a 24-hospital U.S. survey of K. pneumoniae
from 1996 to 2000 revealed only four resistant isolates (from a single center) among 1,123 isolates examined; these isolates produced the KPC-2 enzyme (19
). Nevertheless, since 2000, KPC family enzymes have been detected in the New England and Mid-Atlantic regions of the United States, predominantly in Klebsiella
spp. but also in a variety of enteric organisms. Recent reports of KPC-2 include a Salmonella
isolate in Maryland (18
), K. oxytoca
and K. pneumoniae
isolates in New York (3
), and an E. cloacae
isolate in Massachusetts (A. Hossain, M. J. Ferraro, R. M. Pino, R. B. Dew III, E. S. Moland, T. J. Lockhart, K. S. Thomson, R. V. Goering, and N. D. Hanson, Proc. Abstr. 43rd Intersci. Conf. Antimicrob. Agents Chemother., abstr. C1-664, 2003). The KPC-3 enzyme, the variant described here, has also been identified in E. coli
in New Jersey (T. Hong, E. S. Moland, B. Abdalhamid, N. D. Hanson, J. Wang, C. Sloan, D. Fabian, A. Farrajallah, J. Levine, and K. S. Thomson, Proc. Abstr. 43rd Intersci. Conf. Antimicrob. Agents Chemother., abstr. C1-665, 2003). Transfer of these enzymes has been demonstrated in vitro both here and in one other study (20
), and it may be that plasmids encoding them are beginning to spread in the clinical setting. If so, this would be a disturbing development, owing to the multiresistance of the resulting strains. Clearly, surveillance of these and other carbapenemases is warranted. The main selective factors remain uncertain; however, all members of the KPC family identified thus far (18
) confer greater resistance to cephalosporins than to carbapenems. Cephalosporin use, therefore, may be as selective for their spread as carbapenem use. Furthermore, whereas the presence of this enzyme alone is sufficient to result in cephalosporin resistance, resistance to carbapenems appears to require the presence of an additional mechanism, such as decreased permeability.