To the Editor—Carbapenems (imipenem, meropenem, and doripenem) are used as the drugs of last resort to treat patients with resistant bacterial infections. These drugs possess a broad range of activity against numerous β-lactamases. The increasing plight of carbapenem-resistant Enterobacteriaceae has become a serious healthcare concern.Reference Gupta, Limbago, Patel and Kallen1 The current antimicrobial susceptibility testing is based on breakpoints and does not emphasize the resistant mechanism involved. From March1 through September 30, 2013, we performed an E-test for all 3 carbapenems (imipenem, meropenem, and doripenem) for a total of 74 nonduplicate clinical isolates. These isolates consisted of urine (28), blood (13), stool (9), sputum (8), tissue (6), synovial fluids (4), bile (2), wound swab specimens (2), bronchoalveolar lavage specimens (1), and other (1). For bacterial identification, susceptibility testing was performed as per Clinical and Laboratory Standards Institute guidelines2 of 2013 and the E-test was performed as per manufacturer’s instructions. An in-house multiplex polymerase chain reaction test was designed to identify the predominant carbapenemases bla NDM, bla OXA, bla KPC, bla VIM, and bla IMP.Reference Kazi, Drego and Nikam3 Of the 74 clinical isolates, 32 (43%) consisted of Klebsiella spp., 25 (34%) of Enterobacter spp., and 17 (23%) of Escherichia coli. Table 1 demonstrates the relation between minimum inhibitory concentrations and carbapenemase mechanisms in the predominant Enterobacteriaceae.
Table 1 Relation of Minimum Inhibitory Concentrations to Carbapenemase Mechanisms in the Predominant Enterobacteriaceae
NOTE. NDM-1, New Delhi Metallo-beta-lactamase-1; OXA 48/181, Oxacillinase 48/181; VIM, Verona integron-encoded metallo-β-lactamase.
The present study highlights 2 issues: First, the absence of Klebsiella pneumoniae carbapenemase in our region demonstrates epidemiologic differences in geographic areas. Second, most isolates with dual carbapenemase producers (New Delhi Metallo-beta-lactamase-1 together with Oxacillinase 48/181 and New Delhi Metallo-beta-lactamase-1 together with Verona integron-encoded metallo-β-lactamase) had minimum inhibitory concentration values greater than 32 µg/mL. Though New Delhi Metallo-beta-lactamase-1 producers also showed higher minimum inhibitory concentrations, they did demonstrate a wider range. However, Oxacillinase-48/181 carbapenemases alone tended to have lower minimum inhibitory concentrations. In countries where colistin is used with carbapenems, this has clinical implications in the choice of the carbapenem.
Acknowledgments
We thank the National Health and Education Society, P. D. Hinduja National Hospital and Medical Research Centre, for their encouragement and support. We also thank the hospital staff of the Department of Laboratory Medicine for their support and cooperation.
Financial support. The National Health and Education Society, P. D. Hinduja National Hospital and Medical Research Centre.
Potential conflicts of interest. All authors report no conflicts of interest relevant to this article.
