Carbapenem-hydrolyzing oxacillinase in high resistant strains of Acinetobacter baumannii isolated from India

Acinetobacter baumannii, a Gram negative bacterium causes nosocomial infections including bacteremia, secondary meningitis and urinary tract infections. Increased resistance of A. baumannii has been global concern. Till recently, carbapenems, latest generation of β-lactams are used for treating infections caused by A. baumannii. Emerging resistance to carbapenem class is an immediate threat to mankind. The objective of present study is to understand the growing carbapenem resistance of A. baumannii. By using iso-electric focusing followed by (in-gel) nitrocefin assay of carbapenem resistant strains of A. baumannii, we could identify three β-lactamases with pIs in the range 5.4-9.5. Expression of the β-lactamase with a pI ≈ 8.5, was found only in very high carbapenem resistant (MIC for imipenem 128 μg/ml) strains. On PCR analysis and sequencing of PCR product, this β-lactamase was confirmed to be OXA-51. Identification of this protein from IEF gel was reconfirmed with the help of Liquid chromatography and Tandem mass spectrometry (LC-MS/MS). Based on the amino acid sequence, OXA-51 found to be a 30 kDa β-lactamase containing conserved functional motifs of class D serine β-lactamase. In the present study, we have established the emergence of OXA-51 in clinical strains of A. baumannii in India which suggests its role in carbapenem resistance.