Background & objective: The production of carbapenemases is an important mechanism responsible for the carbapenem resistance. A simple and inexpensive testing method for screening of carbapenemase producers is essential. A prospective study was undertaken to detect metallo-beta-lactamases (MBLs) and AmpC beta-lactamases in nonfermentative Gram negative bacteria and to evaluate the various methods for detection of carbapenemases and MBLs.
Methods: A total of 100 Acinetobacter spp. (78 A. baumannii and 22 A. lwoffii) and 140 Pseudomonas spp. (103 P. aeruginosa and 37 other Pseudomonas spp.) were screened for meropenem resistance by Kirby-Bauer disc diffusion method. Modified Hodge test, EDTA disk synergy (EDS) test and AmpC disk test were used for the detection of carbapenemases, MBLs and AmpC beta-lactamases, respectively.
Results: Forty six (59.0%) A. baumannii, 7 (31.8%) A. lwoffii, 32 (31.1%) P. aeruginosa and 7 (18.9%) Pseudomonas spp. were resistant to meropenem. Among the 32 meropenem resistant P. aeruginosa, 15 (46.9%) were AmpC beta-lactamase producers, 16 (50.0%) MBL producers by EDS test, but only 9 (28.1%) found positive for carbapenemases by modified Hodge test. Among the 46 meropenem resistant A. baumannii, 31 (67.4%) were AmpC beta-lactamase producers, 3 (6.5%) MBL producers, but only 1 (14.3%) was positive for carbapenemases by modified Hodge test. One P. aeruginosa was positive for carbapenemase by modified Hodge test, but was negative for MBL and AmpC beta-lactamase.
Interpretation & conclusion: MBL production is an important mechanism of carbapenem resistance among Pseudomonas species but not among Acinetobacter species. EDS is more sensitive for detection of MBLs than modifi ed Hodge test. Both EDTA-meropenem and EDTA-ceftazidime combination must be used to detect all the MBL producers. Carbapenemases other than MBL may also be responsible for carbapenem resistance. AmpC beta-lactamase is also a contributory factor for carbapenem resistance among the isolates in the hospital.