https://imcjms.com/public Ibrahim Medical College Journal of Medical Science https://imcjms.com/public/registration/journal_full_text/193 Thu, 20 Apr 2017 10:22:20 +0000 The rapid spread of Metallo-b-lactamase (MBL) producing Gram negative bacilli represents a matter of great concern worldwide. The study analyzed the occurrence of MBL production in carbapenem resistant Pseudomonas and Acinetobacter isolates over one year period. A total of 132 Pseudomonas and 76 Acinetobacter isolates were obtained from two tertiary care hospitals of Dhaka city. A total of 53 Pseudomonas and 29 Acinetobacter isolates were selected because of their resistance to carbapenem specially imipenem (IPM). Screening for MBL production was performed in these isolates by IPM-EDTA microdilution MIC method. 44 (83%) IPM resistant Pseudomonas and 19 (65.5%) Acinetobacter isolates were MBL producer by IPM-EDTA microdilution MIC method. These results suggest that MBL producing Pseudomonas and Acinetobacter isolates are emerging in our country and it is essential to screen  carbapenem resistant isolates for MBL production. Ibrahim Med. Coll. J. 2010; 4(2): 63-65 Introduction There are several mechanisms of resistance for carbapenem such as lack of drug penetration due to mutation in the porin channel, loss of outer membrane proteins, efflux mechanisms and Ambler class B carbapenemase or Metallo-b-lactamases (MBL). MBL require divalent cations of zinc as cofactor for their enzymatic activity and they are susceptible to inhibition by metal chelators such as EDTA and thiol based compounds like 10-phenanthroline, 2-mercaptopropionic acid (2-MPA), etc. MBLs are most important because they confer high resistance to all b lactams except aztreonam. They are not inhibited by beta-lactamase inhibitors like clavulunate, salbactam, tazobactam. MBL production is typically associated with resistance to aminoglycoside and quinolones further compromising the therapeutic options. They are often expressed in combinations with other b lactamases like AmpC b lactamase and extended spectrum b lactamases (ESBL). The genes for MBL are inserted in integron structures that reside on mobile genetic elements like plasmid, transposons having the potential for rapid and generalized dissemination.2 In view of the above, the present study was undertaken to determine the prevalence of MBL producing Pseudomonas and Acinetobacter isolates in two tertiary care hospitals of Dhaka city by IPM-EDTA MIC method. Materials and Methods All the 208 isolates (132 Pseudomonas and 76 Acinetobacter) from sputum, urine, tracheal aspirate, blood, wound swab were obtained from the patient admitted in ICU, ward and outpatient department of Bangabandhu Sheikh Mujib Medical university (BSMMU) and Bangladesh Institute of Research and Rehabilitation for Diabetes, Endocrine and Metabolic Disorders (BIRDEM). Samples were collected from January 2009 to December 2009. Isolation, identification and antimicrobial susceptibility testing of organisms All the isolates were tested for susceptibility to IPM by disk diffusion method of Kirby-Bauer4 and as per the recommendations of the NCCLS.5 The antibiotic testing disks were obtained from Oxoid Ltd (Basingstoke, Hampshire, UK). Antibiotic potency of the disks were standardized against the reference Pseudomonas ATCC 25853 strain. Tests for MBL-production by EDTA-IPM microdilution MIC test   A total of 132 Pseudomonas and 76 Acinetobacter were studied of which 90 Pseudomonas were isolated from BSMMU (53 non ICU and 37 ICU) and 42 were from BIRDEM (13 non ICU and 29 ICU). Out of 76 Acineobacter 62 (36 non ICU and 26 ICU) from BSMMU and 14 were from BIRDEM (6 non ICU and 8 ICU). Amongst them, 53 (40.1%) Pseudomonas and 29 (38.1%) Acinetobacter isolates were resistant to IPM. These IPM resistant isolates were tested for MBL production by IPM-EDTA microdilution MIC method. Table 1: Rate of IPM resistance among Pseudomonas and Acinetobacter collected from different hospitals and detection of their MBL production by EDTA-IPM microdilution MIC test     In this study 53 (40.1%) Pseudomonas and 29 (38.1%) Acinetobacter were found to be IPM resistant. This finding was consistent with other studies. Noyal et al showed high prevalence of imipenem resistant Pseudomonas spp (31.1%) and Acinetobacter spp (59%) in India in 2008.7 Indiscriminate use of carbapenems could have resulted in the increase in carbapenem resistant Pseudomonas and Acinetobacter spp.7 The IPM resistant MBL negative isolates also showed high MIC, the reason for their resistance may be due to mechanism other than MBL production, like hyper production of serine beta lactamases and /or a change in the membrane permeability in the bacteria, efflux pump or mutation in the porin.9 In this study, one Pseudomonas and one Acinetobacter isolate having MIC of 4mg/ml and 8mg/ml respectively were MBL positive though both were within sensitive range for IPM. But both were considered as IPM resistant by disk diffusion method. It has been reported that over 30% MBL carrying isolates, particularly Enterobacteriaceae, were found to be IPM sensitive by MIC method though they were IPM resistant in disk diffusion method.2 These carbapenem sensitive MBL producer may carry “hidden” MBL gene or they may be low level MBL producer. As a result of difficulties in their detection, these organisms may pose a significant risk due to their unnoticed spread within the hospital and their ability to transfer resistant gene to other organisms.2   1.   Gupta V, Datta P and Chander J. Prevalence of metallo-b-lactamase (MBL) producing Pseudomonas spp and Acinetobacter spp. in a tertiary care hospital in India. J Inf 2006; 52: 311-314. 3.   Forbes BA, Sham DF, Weissfeld AS. Laboratory methods and strategies for antimicrobial susceptibility testing. In Forbes BA et al -eds Bailey and Scott’s diagnostic Microbiology, 12th ed. Mosby, New York 2007; 187-213. 5.   National Committee for Clinical Laboratory Standards. Performance standards for Antimicrobial Susceptibility Testing: Eleventh informational supplement. NCCLS document M100-S11 NCCLS. Wayne, Pennsylvania, USA 2001. 7.   Noyal M, Menezes G, Harish B, Sujatha S, & Parija S. Simple screening tests for detection of carbapenemases in clinical isolates of nonfermentative Gram-negative bacteria. Indian J Med Res 2009; 129: 707-712. 9.   Livermore DM and Woodford N. Carbapenemases: a problem in waiting? Current opinion in Microbiology 2000; 3: 489-495. 2026 Ibrahim Medical College. All rights reserved.