Detection of ISAba1 in association with a novel allelic variant of the β-lactamase ADC-82 and class D β-lactamase genes mediating carbapenem resistance among the clinical isolates of MDR A. baumannii

Purpose. The objective of the present study is to investigate the diverse resistance determinants, their association with insertion sequence mobile elements and predilection of a particular clone for such associations in Acinetobacter baumannii. Methodology. Fifty-four consecutive isolates collected during 2011–2012 from a tertiary care hospital were subjected to susceptibility testing followed by PCR screening of commonly reported β-lactamases and 16S rRNA methyltransferase encoding genes. The integrity of resistance–nodulation–cell division efflux pump-related genes in their respective operons was also investigated. Results. β-Lactamase genes such as bla_ADC (100 %), bla_OXA-23 (81 %), bla_PER-1 (81 %), bla_IMP-1 (31 %) and bla_NDM-1 (15 %) were found to be present more frequently while blaVIM-2 and bla_OXA-24 were not observed in our study population. ISAba1 was associated only with bla_OXA-51-like like (30 %), bla_OXA-23-like (55 %) and bla_ADC-like (33 %). armA was found in 87% of isolates and ISAba1 linked with one novel variant of ADC, namely bla_ADC-82, which was identified to have 15 nucleotide differences with bla_ADC-79, and this finding is of much significance. In many isolates, efflux pump genes were not intact, resulting in severely altered effluxing functions. For the first time, we have identified ISAba1-mediated disruption of adeN among the isolates of ST 195^B, which would have led to overexpression of AdeIJK efflux pump causing elevated resistance. Multilocus sequence typing revealed the predominance of CC 92^B (IC-II^B) and CC 447^B clonal complexes. Conclusion. High incidence of IC-II clones, novel resistance determinants (ADC-82) and elevated resistance mediated by ISAba1 reported here will be of enormous importance while assessing the emergence of extremely resistant A. baumannii in India.