Coexistence of blaOXA-23 with blaNDM-1 and armA in clinical isolates of Acinetobacter baumannii from India

Abstract

Sir, The production of carbapenemases is the most common mechanism responsible for carbapenem resistance in Acinetobacter baumannii. They include metallo-β-lactamases (VIM, IMP and SIM types), which have been sporadically reported in some parts of the world, and acquired OXA-type carbapenemases which have been more frequently identified worldwide and are clustered in three major subfamilies (blaOXA-23, blaOXA-24 and blaOXA-58).1 High levels of resistance to aminoglycosides due to the production of plasmid-encoded 16S rRNA methylase in Enterobacteriaceae, Pseudomonas aeruginosa and Acinetobacter spp. have been documented since 2003.2 The present study was undertaken to determine the mechanism responsible for the carbapenem resistance. Three carbapenem-non-susceptible A. baumannii were isolated from patients in the intensive care unit of a tertiary care hospital in Chennai, India in April 2010. Species identification and antibiotic testing were carried out using an automated machine (VITEK-2). The MICs of carbapenems and aminoglycosides were determined using the CLSI agar dilution method, while tigecycline MICs were determined using the EUCAST broth microdilution method. All the isolates were resistant to all the β-lactams, aminoglycosides and quinolones, and were only susceptible to tigecycline and colistin. Double disc synergy test (DDST) and modified Hodge test (MHT) were used for detection of metallo-β-lactamases and other carbapenemases, respectively. PCR screening was performed for OXA-type carbapenemases (blaOXA-23, -24, -51 and -58-like) and the known metallo-β-lactamase genes.3–5 All three isolates showed positivity for both DDST and MHT, and PCR yielded the products with expected sizes for blaOXA-51, 23-like and blaNDM-1 (Table 1). Sequencing of both blaOXA-23 and blaNDM-1 genes showed 100% identities with previously reported genes. In all three isolates, the blaOXA-23 gene was adjacent to insertion element ISAba1, which provides the promoter required for expression of linked resistance genes; blaOXA-51-like genes were also found in all of the isolates, but were not activated by ISAba1. Conjugation experiments using Escherichia coli J53 as the recipient were unsuccessful. Plasmids of Enterobacteriaceae were not detected among A. baumannii by PCR-based replicon typing.6 Analysis of the three A. baumannii by MIC, DDST, MHT and PCR for resistance determinants IPM, imipenem; MEM, meropenem; AMK, amikacin; GEN, gentamicin; NET, netilmicin; TOB, tobramycin; TGC, tigecycline; DDST, double disc synergy test; MHT, modified Hodge test. Analysis of the three A. baumannii by MIC, DDST, MHT and PCR for resistance determinants IPM, imipenem; MEM, meropenem; AMK, amikacin; GEN, gentamicin; NET, netilmicin; TOB, tobramycin; TGC, tigecycline; DDST, double disc synergy test; MHT, modified Hodge test. Susceptibility testing showed increased MICs of carbapenems and an unusual phenotype of broad-spectrum high-level resistance to aminoglycosides, including amikacin, gentamicin, netilmicin and tobramycin (MICs > 256 mg/L), with susceptibility to tigecycline (MIC = 0.5–1 mg/L). PCR screening was performed for 16S rRNA methylase-encoding genes (armA, rmtA, rmtB, rmtC, rmtD and npmA).2,7–9 All three isolates showed positivity for the armA gene (Table 1). NDM-1 metallo-β-lactamase-mediated resistance to carbapenems in Enterobacteriaceae has been found in many parts of India.5,10,11 However, to our knowledge, this is the first report of the coexistence of the blaOXA-23 gene with blaNDM-1 and armA in clinical isolates of A. baumannii in India. A. baumannii isolates have been found to carry mixed carbapenemase genes giving very broad-spectrum antibiotic resistance profiles. The emergence of these powerful coexistent resistance mechanisms described here will further seriously limit future therapeutic options. This study was supported by the University Grant Commission, New Delhi, India. None to declare.