Bum Sik Chin, et al. days apart revealed identical PFGE patterns in 7 MBL pro- ducing PA. A direct contact between the two patients was not identified. However, one source of major concern is the ability of MBLs to spread rapidly which has been observed in various clinical circumstances.2,25,26 The above cases could be an example of the potential clonal spread of MPs, although the majority of MPs occurred sporadically in our institution, and contribution of MBL to carbapenem resis- tance was limited, similar to other study performed in the same country.27 In conclusion, the contribution of MBL to imipenem re- sistance in PA/AB isolated from sputum and urine was rela- tively limited in our institution, and there was no identifi- able risk factor associated with MPs acquisition, compared with INMNPs. However, the limited susceptibility to aztre- onam in the MP group implies that MBL producing strains commonly carry other resistance mechanisms such as ES- BLs, AmpC β-lactamases, and other non-enzymatic mecha- nisms. Our study has several limitations. First, the number of MPs used for the study was small, and that could have impaired our ability to identify significant risk factors associated with acquisition of MPs. Subgroup analysis according to isola- tion sources was also not feasible because of the small size of the population under investigation. Second, we included all isolates from the respiratory and urinary tract, therefore, our results might not be generalized as overt clinical infec- tions. Further analysis of a significant number of infection cases is required to more accurately assess the clinical im- pact of MPs. ACKNOWLEDGEMENTS This work was supported by the grant of Intramural Fund of Korea National Institute of Health and Korea Healthcare Technology R&D Project, Ministry for Health, Welfare & Family Affairs, Republic of Korea (A084063). REFERENCES 1. Cornaglia G, Akova M, Amicosante G, Cantón R, Cauda R, Doc- quier JD, et al. Metallo-beta-lactamases as emerging resistance determinants in Gram-negative pathogens: open issues. Int J Anti- microb Agents 2007;29:380-8. 2. Zavascki AP, Barth AL, Gonçalves AL, Moro AL, Fernandes JF, Martins AF, et al. The influence of metallo-beta-lactamase produc- tion on mortality in nosocomial Pseudomonas aeruginosa infec- tions. J Antimicrob Chemother 2006;58:387-92. 3. Zavascki AP, Barth AL, Gaspareto PB, Gonçalves AL, Moro AL, Fernandes JF, et al. Risk factors for nosocomial infections due to Pseudomonas aeruginosa producing metallo-beta-lactamase in two tertiary-care teaching hospitals. J Antimicrob Chemother 2006;58:882-5. 4. Hirakata Y, Yamaguchi T, Nakano M, Izumikawa K, Mine M, Aoki S, et al. Clinical and bacteriological characteristics of IMP- type metallo-beta-lactamase-producing Pseudomonas aeruginosa. Clin Infect Dis 2003;37:26-32. 5. Zavascki AP, Barth AL, Fernandes JF, Moro AL, Gonçalves AL, Goldani LZ. Reappraisal of Pseudomonas aeruginosa hospital-ac- quired pneumonia mortality in the era of metallo-beta-lactamase- mediated multidrug resistance: a prospective observational study. Crit Care 2006;10:R114. 6. Lautenbach E, Weiner MG, Nachamkin I, Bilker WB, Sheridan A, Fishman NO. Imipenem resistance among pseudomonas aerugi- nosa isolates: risk factors for infection and impact of resistance on clinical and economic outcomes. Infect Control Hosp Epidemiol 2006;27:893-900. 7. Kwon KT, Oh WS, Song JH, Chang HH, Jung SI, Kim SW, et al. Impact of imipenem resistance on mortality in patients with Aci- netobacter bacteraemia. J Antimicrob Chemother 2007;59:525-30. 8. Murray EJ, Baron EJ, Pfaller MA, Tenover FC, Yolken RH. Man- ual of clinical microbiology. 7th ed. Washington DC: ASM Press; 1999. p.442-58. 9. Performance standards for antimicrobial susceptibiliy testing; Nine- teenth Informational Supplement. 2009;CLSI document M100-S19. 10. Lee K, Lim YS, Yong D, Yum JH, Chong Y. Evaluation of the Hodge test and the imipenem-EDTA double-disk synergy test for differentiating metallo-beta-lactamase-producing isolates of Pseu- domonas spp. and Acinetobacter spp. J Clin Microbiol 2003;41: 4623-9. 11. Lee K, Yum JH, Yong D, Lee HM, Kim HD, Docquier JD, et al. Novel acquired metallo-beta-lactamase gene, bla(SIM-1), in a class 1 integron from Acinetobacter baumannii clinical isolates from Korea. Antimicrob Agents Chemother 2005;49:4485-91. 12. Lee K, Ha GY, Shin BM, Kim JJ, Kang JO, Jang SJ, et al. Metal- lo-beta-lactamase-producing Gram-negative bacilli in Korean Na- tionwide Surveillance of Antimicrobial Resistance group hospitals in 2003: continued prevalence of VIM-producing Pseudomonas spp. and increase of IMP-producing Acinetobacter spp. Diagn Mi- crobiol Infect Dis 2004;50:51-8. 13. Nouér SA, Nucci M, de-Oliveira MP, Pellegrino FL, Moreira BM. Risk factors for acquisition of multidrug-resistant Pseudomonas aeruginosa producing SPM metallo-beta-lactamase. Antimicrob Agents Chemother 2005;49:3663-7. 14. Kim YA, Choi JY, Kim CK, Kim CO, Kim MS, Choi SH, et al. Risk factors and outcomes of bloodstream infections with metallo- beta-lactamase-producing Acinetobacter. Scand J Infect Dis 2008;40:234-40. 15. Horianopoulou M, Legakis NJ, Kanellopoulou M, Lambropoulos S, Tsakris A, Falagas ME. Frequency and predictors of coloniza- tion of the respiratory tract by VIM-2-producing Pseudomonas ae- ruginosa in patients of a newly established intensive care unit. J Med Microbiol 2006;55:1435-9. 16. Laupland KB, Parkins MD, Church DL, Gregson DB, Louie TJ, Conly JM, et al. Population-based epidemiological study of infec- tions caused by carbapenem-resistant Pseudomonas aeruginosa in 356 Yonsei Med J http://www.eymj.org Volume 52 Number 2 March 2011