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Antimicrobial Agents and Chemotherapy, April 2009, p. 1660-1664, Vol. 53, No. 4
0066-4804/09/$08.00+0     doi:10.1128/AAC.01172-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Surveillance of Carbapenem-Resistant Pseudomonas aeruginosa Isolates from Puerto Rican Medical Center Hospitals: Dissemination of KPC and IMP-18 β-Lactamases{triangledown}

Daniel J. Wolter,1 Noha Khalaf,1,{dagger} Iraida E. Robledo,2 Guillermo J. Vázquez,2 Maria I. Santé,3 Edna E. Aquino,2 Richard V. Goering,4 and Nancy D. Hanson1*

Center for Research in Anti-Infectives and Biotechnology, Creighton University School of Medicine, Omaha, Nebraska,1 University of Puerto Rico School of Medicine, Department of Microbiology and Medical Zoology, San Juan, Puerto Rico,2 University of Puerto Rico School of Medicine, Department of Pathology and Laboratory Medicine, San Juan, Puerto Rico,3 Creighton University School of Medicine, Omaha, Nebraska4

Received 2 September 2008/ Returned for modification 23 October 2008/ Accepted 18 January 2009


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ABSTRACT
 
During a 6-month period, 37/513 (7.2%) Pseudomonas aeruginosa isolates belonging to 13 pulsed-field gel electrophoresis (PFGE) groups from Puerto Rican hospitals were carbapenem nonsusceptible. Seven of 37 isolates from four PFGE groups carried blaIMP-18, and 25/37 isolates from seven PFGE groups carried blaKPC. The results indicated the clonal spread of blaKPC-positive P. aeruginosa isolates into several Puerto Rican hospitals and the dissemination of blaIMP-18 and blaKPC into genetically unrelated isolates.


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INTRODUCTION
 
Pseudomonas aeruginosa is a problematic nosocomial pathogen because it can cause complicated infections in immunocompromised and critically ill patients and can survive therapy through chromosomal mutations or acquisition of resistance-encoding genes. Carbapenems, including imipenem and meropenem, are β-lactam antibiotics used extensively for the treatment of P. aeruginosa infections. However, carbapenem resistance can emerge and involves several mechanisms, including porin downregulation, efflux pump overexpression, and the production of carbapenem-hydrolyzing enzymes (11, 12). In the absence of hydrolyzing enzymes, loss of the porin, OprD, can result in imipenem resistance and decreased meropenem susceptibility (10). Meropenem resistance requires OprD loss coupled with mexAB-oprM pump overexpression (7).

During a 2006 surveillance study of nosocomial pathogens from U.S. medical centers, 10.7 and 6.4% of the P. aeruginosa isolates were resistant to imipenem and meropenem, respectively (15). This report did not include hospitals from Puerto Rico, a U.S. commonwealth with close social and economic ties to the United States. In 1998, a study comprising seven different Puerto Rican hospitals reported that 11.3% of P. aeruginosa isolates were imipenem resistant (5). A retrospective surveillance study in 2000 at the Puerto Rico Medical Center (PRMC) found that 46.9% of P. aeruginosa isolates were imipenem resistant, but this high percentage may be an overestimation due to multiple isolations from individual patients (18).

The mechanism(s) associated with carbapenem resistance in P. aeruginosa isolates from Puerto Rico is unknown. The objectives of this study were to collect imipenem-resistant P. aeruginosa isolates from hospitals associated with the PRMC, determine the genetic relatedness of the isolates, and identify the mechanism(s) associated with the carbapenem-resistant phenotype.

The PRMC is located in the San Juan metropolitan area and includes six teaching hospitals, an emergency room, and a central laboratory. Between October 2006 and March 2007, 513 P. aeruginosa isolates were isolated at the PRMC laboratory. During this period, 37/513 (7.2%) unique, consecutive P. aeruginosa isolates were identified as being imipenem nonsusceptible, according to microdilution testing using the Vitek DCS-R5 system. MICs for piperacillin-tazobactam, ceftazidime, imipenem, and meropenem were obtained for the 37 isolates by using agar dilution (3). Thirty-one of 37 and 29/37 isolates had high MICs (≥32 µg/ml) to imipenem and meropenem, respectively (Table 1). The remaining isolates had MICs ranging from 8 to 16 µg/ml for both carbapenems, corresponding with an intermediate or resistant phenotype. Twenty-eight of 37 isolates were piperacillin-tazobactam resistant (MIC ≥ 128 µg/ml), while 31/37 isolates were ceftazidime resistant (MIC ≥ 32 µg/ml) (Table 1). Of the 37 carbapenem-nonsusceptible isolates, 70.2% (26/37) were resistant to all four β-lactams tested.


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  		TABLE 1. Characteristics of the P. aeruginosa isolates from Puerto Rico

Genetic relatedness of the carbapenem-resistant P. aeruginosa isolates from Puerto Rico was determined using pulsed-field gel electrophoresis (PFGE) (8). The 37 isolates were divided into 13 major PFGE groups (Fig. 1). Group 11 contained the largest number of isolates (25/37), was comprised of four highly related (i.e., ≥93% similarity) subgroups (11A to 11D), and was resistant to all β-lactams tested. Groups 1, 3, 4, 7, 9, and 10 consisted of at least two isolates cultured from different patients. Isolates in each respective group shared a common susceptibility pattern, except for isolates belonging to group 9. PS1 (group 9C) had a PFGE pattern that was 90% similar to those of PS4 (group 9B) and PS33 (group 9A) but had ceftazidime MICs that were at least 64-fold lower and carbapenem MICs fourfold lower than those of PS4 and PS33 (Table 1). Groups 2, 5, 6, 8, 12, and 13 were represented by single isolates.


Figure 1
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  		FIG. 1. PFGE of the 37 isolates analyzed. The names and PFGE types of the isolates are shown at the right of the figure. The dendrogram depicting genetic relatedness and generated using BioNumerics is shown at the left of the figure. The scale for the dendrogram represents percentage of relatedness. Staphylococcus aureus strain NCTC 8325 was used for the gel-to-gel normalization required to generate the dendrogram.

A majority of isolates (19/37) were collected from hospital 1 and belonged to eight distinct PFGE groups (Table 1). Hospital 4 had the second highest number of isolates (11/37) but consisted only of two groups (groups 3 and 11), with group 11 isolates comprising the majority (10/11). The remaining hospitals had isolated ≤2 carbapenem-resistant P. aeruginosa isolates during the 6-month period. Group 11D isolates were from patients in three different hospitals, hospitals 1, 4, and 5 (Table 1), suggesting the clonal spread of this highly resistant isolate. Group 3 isolates were also present in three different hospitals, group 3A in hospitals 3 and 4 and group 3B in hospital 1.

All 37 isolates were evaluated for the presence of metallo-β-lactamases, using the MBL Etest strip (AB Biodisk, Solna, Sweden). Thirty-two of 37 isolates were positive for a metallo-β-lactamase (data not shown). The MBL Etest-negative isolates were PS1, PS3, PS9, PS22, and PS26. All 37 isolates were screened by PCR for blaIMP, blaVIM, and blaKPC (17), using the following primers: IMP-1F (5'-GGAATAGAGTGGCTTAATTC-3') and IMP-1R (5'-CAACCAGTTTTGCCTTACC-3'); VIM1F (5'-GGTGTTTGGTCGCATATCGC-3') and VIM1R (5'-CCATTCAGCCAGATCGGCATC-3'); KPCF2 (5'-GTATCGCCGTCTAGTTCTGC-3') and KPCR2 (5'-GGTCGTGTTTCCCTTTAGCC-3'). Seven of 37 isolates, representing four genetically distinct PFGE groups from patients in two different hospitals and the emergency room, were positive for blaIMP (Table 1). Sequence analysis of blaIMP from representatives of each group identified the gene as blaIMP-18 (Table 1). Twenty-five of 37 isolates were positive for blaKPC (Table 1). The blaKPC isolates were collected from four different locations and represented seven genetically distinct (<80% similarity) PFGE groups (Table 1). Sequence analysis of blaKPC from representatives of each group revealed that six isolates contained blaKPC-2 while one isolate harbored a new variant, blaKPC-5 (GenBank accession number EU400222) (21). All of the isolates were PCR negative for blaVIM. Interestingly, all IMP and KPC PCR-positive isolates were metallo-β-lactamase positive by the MBL Etest, as mentioned above. Upon retesting of several isolates from different PFGE groups by the MBL Etest, the IMP PCR-positive strains (PS2, PS4, PS39, and PS44) were positive, as previously determined. However, seven KPC PCR-positive strains (PS5, PS10, PS17B, PS23, PS28, PS40, and PS43) were negative after repeat testing, indicating that the initial results were false positives.

Twelve isolates belonging to different PFGE groups were further evaluated by isoelectric focusing (17), including seven blaKPC-positive isolates, four blaIMP-positive isolates, and one isolate negative for both. Six KPC PCR-positive isolates produced a β-lactamase with a pI of 6.7, whereas no band was observed for KPC PCR-negative isolates at this pI (data not shown). PS28, containing blaKPC-5, produced a β-lactamase with a pI of 7.65, differing from the other KPC-positive isolates. All KPC enzymes were partially inhibited by clavulanate (data not shown). In addition to KPC, these isolates produced chromosomal AmpC (pI ≥ 8.5). PS40 and PS43 also produced an enzyme with a pI of 8.0, indicative of an OXA-2-like β-lactamase (Table 1). The presence of blaOXA-2 was verified by PCR, using OXA-2 family-specific primers (13), and sequencing the generated product. Of the IMP PCR-positive isolates, PS2 had a β-lactamase with a pI of 8.0, correlating with the presence of blaOXA-2 (Table 1). PS4, PS39, and PS44 produced at least three additional β-lactamases (Table 1). PS26 was negative for both blaKPC and blaIMP but produced an enzyme with a pI of 8.0, corresponding to OXA-2.

To determine if OprD downregulation and/or increased mexAB-oprM pump expression contributed to the observed carbapenem MICs, mexA and oprD expression (20) and OprD production (20) were analyzed in 10 selected P. aeruginosa isolates. These isolates represented eight genetically distinct (<80% identity) PFGE groups from five different locations. Regardless of location or genetic relatedness, 8/10 isolates had a drastic decrease (between ~65- and 3,000-fold) in oprD transcript (Table 1), correlating with OprD protein loss, compared to wild-type strain PAO1. Two isolates, PS43 and PS44, had a small decrease (two- to threefold) in oprD expression; however, OprD protein was absent (Table 1). OprD loss in PS44 resulted from a mutation within the structural gene (G580->T substitution) creating a premature stop codon. No mutations were observed in oprD of PS43. The oprD genes from PS5 and PS39 contained a nucleotide insertion (C73) or a base substitution (G580->T) resulting in a frameshift or premature stop codon, respectively. Sequence analysis revealed the insertion of IS element IS1394 1 bp upstream of the oprD translational start codon in PS17B. IS1394 insertion between the promoter and start codon most likely contributed to the significant decrease (2,500-fold) in oprD expression.

Three isolates (PS1, PS39, and PS44) belonging to PFGE groups 9C, 8, and 13, respectively, produced mexA transcript levels three- to fourfold higher than that produced by the wild-type isolate, PAO1 (Table 1), which was comparable to the level produced by the mexAB-oprM overexpressing control strain, PS1454 (4.3-fold higher) (data not shown). PS1 had a meropenem MIC of 8 µg/ml, corresponding with OprD loss and mexAB-oprM overexpression. PS39 and PS44 had higher meropenem MICs (≥32 µg/ml), which were attributed to a combination of OprD loss, mexAB-oprM overexpression, and IMP-18 production.

Characteristics of the 37 patients with carbapenem-resistant P. aeruginosa isolates included a mean age of 50 years (range, 1 month to 99 years), with a statistically significant male gender distribution (70% male versus 30% female; P ≤ 0.05) (Table 1). The most common anatomical site of infection was the respiratory tract (41%), followed by the urinary system (25%) and blood (19%).

P. aeruginosa can utilize numerous resistance mechanisms to counteract therapeutic selective pressures, as evident in the isolates examined from the PRMC hospitals. Although KPC β-lactamases have been predominantly encountered in the Enterobacteriaceae (1, 2, 4), the first report of KPC-producing P. aeruginosa isolates was described in three genetically related isolates from Colombia in 2007 (19). Therefore, we were surprised to find 25/37 blaKPC-positive P. aeruginosa isolates representing seven genetically unrelated PFGE groups, collected from patients of various ages, from multiple sites of infection, and from four different PRMC hospitals. KPC enzymes have recently been identified in Klebsiella pneumoniae and Escherichia coli isolates from PRMC hospitals (16). Whether these organisms represent the source of blaKPC transmission to P. aeruginosa in Puerto Rico remains unknown. However, the spread of blaKPC into different genera is most likely associated with its residence within mobile genetic elements on plasmids of various sizes (14).

Little information is available regarding the metallo-β-lactamase, IMP-18. blaIMP-18-positive single isolates of P. aeruginosa were identified in hospitalized patients from Mexico and the southwestern United States (6, 9). The spread of IMP-18 into multiple unrelated isolates from Puerto Rico indicates the movement of blaIMP-18 by mobile genetic elements.

The OprD porin was absent in all of the isolates examined. In the absence of acquired carbapenemases, carbapenem MICs were ≤16 µg/ml. The presence of KPC in OprD-deficient isolates often correlated with increased carbapenem MICs (>64 µg/ml). PS43, with imipenem and meropenem MICs of 16 µg/ml and 32 µg/ml, respectively, was the only exception. The presence of IMP-18 in OprD-deficient isolates PS39 and PS44 also corresponded with increased carbapenem MICs (32 to >64 µg/ml). In addition, these isolates overexpressed the mexAB-oprM pump, which may have further elevated the meropenem MICs. The absence of OprD coupled with KPC or IMP-18 appeared to have a synergistic effect on the carbapenem MICs. Since all KPC/IMP-18-positive isolates were also OprD deficient, the effect of these enzymes alone on carbapenem susceptibilities has yet to be determined.

In Puerto Rico, the carbapenem resistance rates are similar to those observed in the continental United States (15), but the high percentage of P. aeruginosa isolates producing a KPC is unique to this region. Our study has shown the dissemination of blaKPC into several unrelated isolates of P. aeruginosa in addition to the spread of a clonal isolate into multiple PRMC hospitals. Therefore, the widespread carbapenem resistance observed in these P. aeruginosa isolates was not exclusively a hospital infection control issue.


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ACKNOWLEDGMENTS
 
This research was conducted with support from the Investigator-Sponsored Study Program of AstraZeneca.


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FOOTNOTES
 
* Corresponding author. Mailing address: Center for Research in Anti-Infectives and Biotechnology, Department of Medical Microbiology and Immunology, Creighton University School of Medicine, 2500 California Plaza, Omaha, NE 68178. Phone: (402) 280-5837. Fax: (402) 280-1875. E-mail: ndhanson{at}creighton.edu Back

{triangledown} Published ahead of print on 2 February 2009. Back

{dagger} Present address: Helwan University, Helwan, Egypt. Back


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REFERENCES
 
    1
  1. Bradford, P. A., S. Bratu, C. Urban, M. Visalli, N. Mariano, D. Landman, J. J. Rahal, S. Brooks, S. Cebular, and J. Quale. 2004. Emergence of carbapenem-resistant Klebsiella species possessing the class A carbapenem-hydrolyzing KPC-2 and inhibitor-resistant TEM-30 beta-lactamases in New York City. Clin. Infect. Dis. 39:55-60.[CrossRef][Medline]
    2. 2
  2. Bratu, S., D. Landman, M. Alam, E. Tolentino, and J. Quale. 2005. Detection of KPC carbapenem-hydrolyzing enzymes in Enterobacter spp. from Brooklyn, New York. Antimicrob. Agents Chemother. 49:776-778.[Abstract/Free Full Text]
    3. 3
  3. Clinical and Laboratory Standards Institute. 2008. Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically, 6th ed. Approved standard M7-A6. Clinical and Laboratory Standards Institute, Wayne, PA.
    4. 4
  4. Deshpande, L. M., P. R. Rhomberg, H. S. Sader, and R. N. Jones. 2006. Emergence of serine carbapenemases (KPC and SME) among clinical strains of Enterobacteriaceae isolated in the United States medical centers: report from the MYSTIC program (1999-2005). Diagn. Microbiol. Infect. Dis. 56:367-372.[CrossRef][Medline]
    5. 5
  5. Doern, G. V., R. N. Jones, M. A. Pfaller, M. Erwin, C. Ramirez-Rhonda, et al. 1998. Multicenter evaluation of the in vitro activity of six broad-spectrum beta-lactam antimicrobial agents in Puerto Rico. Diagn. Microbiol. Infect. Dis. 30:113-119.[CrossRef][Medline]
    6. 6
  6. Garza-Ramos, U., P. Tinoco, J. Silva-Sanchez, R. Morfin-Otero, E. Rodriguez-Noriega, G. Leon-Garnica, H. S. Sader, and R. N. Jones. 2008. Metallo-beta-lactamase IMP-18 is located in a class 1 integron (In96) in a clinical isolate of Pseudomonas aeruginosa from Mexico. Int. J. Antimicrob. Agents 31:78-80.[CrossRef][Medline]
    7. 7
  7. Giske, C. G., C. Boren, B. Wretlind, and G. Kronvall. 2005. Meropenem susceptibility breakpoint for Pseudomonas aeruginosa strains hyperproducing mexB mRNA. Clin. Microbiol. Infect. 11:662-669.[CrossRef][Medline]
    8. 8
  8. Goering, R. V. 2004. Pulsed-field gel electrophoresis, p. 185-195. In D. H. Persing et al. (ed.), Molecular microbiology: diagnostic principles and practice. ASM Press, Washington, DC.
    9. 9
  9. Hanson, N. D., A. Hossain, L. Buck, E. S. Moland, and K. S. Thomson. 2006. First occurrence of a Pseudomonas aeruginosa isolate in the United States producing an IMP metallo-β-lactamase, IMP-18. Antimicrob. Agents Chemother. 50:2272-2273.[Free Full Text]
    10. 10
  10. Kohler, T., M. Michea-Hamzehpour, S. F. Epp, and J. C. Pechere. 1999. Carbapenem activities against Pseudomonas aeruginosa: respective contributions of OprD and efflux systems. Antimicrob. Agents Chemother. 43:424-427.[Abstract/Free Full Text]
    11. 11
  11. Lister, P. D. 2002. Chromosomally-encoded resistance mechanisms of Pseudomonas aeruginosa: therapeutic implications. Am. J. Pharmacogenomics 2:235-243.[CrossRef][Medline]
    12. 12
  12. Livermore, D. M. 2002. Multiple mechanisms of antimicrobial resistance in Pseudomonas aeruginosa: our worst nightmare? Clin. Infect. Dis. 34:634-640.[CrossRef][Medline]
    13. 13
  13. Moland, E. S., N. D. Hanson, J. A. Black, A. Hossain, W. Song, and K. S. Thomson. 2006. Prevalence of newer β-lactamases in gram-negative clinical isolates collected in the United States from 2001 to 2002. J. Clin. Microbiol. 44:3318-3324.[Abstract/Free Full Text]
    14. 14
  14. Naas, T., G. Cuzon, M. V. Villegas, M. F. Lartigue, J. P. Quinn, and P. Nordmann. 2008. Genetic structures at the origin of acquisition of the β-lactamase blaKPC gene. Antimicrob. Agents Chemother. 52:1257-1263.[Abstract/Free Full Text]
    15. 15
  15. Rhomberg, P. R., L. M. Deshpande, J. T. Kirby, and R. N. Jones. 2007. Activity of meropenem as serine carbapenemases evolve in US medical centers: monitoring report from the MYSTIC program (2006). Diagn. Microbiol. Infect. Dis. 59:425-432.[CrossRef][Medline]
    16. 16
  16. Robledo, I. E., E. S. Moland, E. A. Aquino, G. J. Vázquez, M. I. Santé, J. Bertrán, and N. D. Hanson. 2007. First report of a KPC-4 and CTX-M producing K. pneumoniae (Kp) isolated from Puerto Rico (PR), abstr. C2-1933. Abstr. 47th Intersci. Conf. Antimicrob. Agents Chemother., Chicago, IL.
    17. 17
  17. Smith Moland, E., N. D. Hanson, V. L. Herrera, J. A. Black, T. J. Lockhart, A. Hossain, J. A. Johnson, R. V. Goering, and K. S. Thomson. 2003. Plasmid-mediated, carbapenem-hydrolysing β-lactamase, KPC-2, in Klebsiella pneumoniae isolates. J. Antimicrob. Chemother. 51:711-714.[Abstract/Free Full Text]
    18. 18
  18. Vázquez, G. J., I. E. Robledo, A. Arroyo, E. Nadal, R. Rodríguez, and M. Bermúdez. 2003. A comparison of the antimicrobial resistance patterns of gram-negative bacilli isolated from community-private and university-affiliated hospitals from Puerto Rico. P. R. Health Sci. J. 22:265-271.[Medline]
    19. 19
  19. Villegas, M. V., K. Lolans, A. Correa, J. N. Kattan, J. A. Lopez, and J. P. Quinn. 2007. First identification of Pseudomonas aeruginosa isolates producing a KPC-type carbapenem-hydrolyzing β-lactamase. Antimicrob. Agents Chemother. 51:1553-1555.[Abstract/Free Full Text]
    20. 20
  20. Wolter, D. J., N. D. Hanson, and P. D. Lister. 2005. AmpC and OprD are not involved in the mechanism of imipenem hypersusceptibility among Pseudomonas aeruginosa isolates overexpressing the mexCD-oprJ efflux pump. Antimicrob. Agents Chemother. 49:4763-4766.[Abstract/Free Full Text]
    21. 21
  21. Wolter, D. J., P. M. Kurpiel, N. Woodford, M. F. Palepou, R. V. Goering, and N. D. Hanson. 2009. Phenotypic and enzymatic comparative analysis between the novel KPC variant, KPC-5, and its evolutionary variants, KPC-2 and KPC-4. Antimicrob. Agents Chemother. 53:557-562.[Abstract/Free Full Text]

Antimicrobial Agents and Chemotherapy, April 2009, p. 1660-1664, Vol. 53, No. 4
0066-4804/09/$08.00+0     doi:10.1128/AAC.01172-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.



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  • Akpaka, P. E., Swanston, W. H., Ihemere, H. N., Correa, A., Torres, J. A., Tafur, J. D., Montealegre, M. C., Quinn, J. P., Villegas, M. V. (2009). Emergence of KPC-Producing Pseudomonas aeruginosa in Trinidad and Tobago. J. Clin. Microbiol. 47: 2670-2671 [Abstract] [Full Text]  

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