Molecular epidemiology of multidrug-resistant Acinetobacter baumannii

Abstract

Acinetobacter baumannii is an important nosocomial pathogen worldwide because of its remarkable ability to acquire antibiotic resistance. The global emergences of multidrug-resistant A. baumannii (MDR-AB) clones are predominated by a number of widely disseminated clones, namely clonal complex (CC) 1, CC2, and CC3. In early 2010, we reported two major clones of MDR-AB, designated HKU1 and HKU2 belong to sequence types (ST) 96 and ST92, widely disseminating in our hospitals. ST92 is a predominant clone that is prevalent in more than 30 countries, whereas ST96 has been identified recently and is geographically confined to certain parts of China. Our previous study only investigated the isolates collected in the year 2005-2006. We therefore extended our investigation over a six-year period (2005-2010) to generate a more complete picture of the molecular epidemiology and resistance mechanisms in A. baumannii.

Firstly, we performed the susceptibility test on various antimicrobial agents and employed molecular methods to characterize the epidemiology of the target A. baumannii isolates. For the entire study period, increased resistance rates were noted for the seven antimicrobial agents, namely imipenem, piperacillin-tazobactam, cefoperazone, ticarcillin-clavulanate, ciprofloxacin, gentamicin and amikacin (P <0.01). Worryingly, an increased trend was also observed for the pandrug-resistant rate, from 0.2% in the year 2005-2006, to 1.9-2.9% in the year 2007-2008 and up to 6.0-8.1% in the year 2009-2010 (chi square for trend, P <0.001). Pulsed-field gel electrophoresis and multilocus sequence typing (PFGE/MLST) categorized 100 out of 108 (92.6%) isolates into four clones (PFGE/MLST), namely HKU2/ST92 (n = 14), HKU3/ST254 (n = 73), HKU4/ST137 (n = 5), and HKU5/ST362 (n = 8), respectively. PCR showed that 88.9% (96/108) of the amikacin-resistant isolates were armA positive and all isolates were found to harbour at least one of the OXA-type carbapenemases with frequencies as follows: OXA-51-like (98/108, 90.7%), OXA-23-like (85/108, 78.7%), OXA-58-like (9/108, 8.3%) and OXA-24-like (8/108, 7.4%).

Secondly, we compared the biological fitness of the circulating clones by performing the doubling time and adhesion experiment. The results demonstrated that HKU3/ST254 has a higher capability for replication and adherence to human bronchial epithelial cells. Together with the higher antibiotic resistance rate, the selective advantages in terms of biological fitness may facilitate the clonal expansion and wide dissemination of this lineage.

Finally, whole genome sequence data showed a high amount of resistance genes intermixed with various insertion sequence (IS) elements, integrons and transponsons clustering inside the resistance islands. The presence of a second genomic resistance island conferring aminoglycoside and sulphonamide resistance, additional loci outside the resistance islands harbouring resistance genes and the high amount of antibiotic efflux pumps in various A. baumannii genomes demonstrated that resistance islands contribute a significant part to the multidrug-resistant phenotype in A. baumannii but are not the only factor. The correlation analysis further demonstrated the significance of IS elements in the dissemination of antibiotic resistance genes in the A. baumannii genomes. As a whole, whole genome sequence data may provide an informative and efficient approach to generating a more comprehensive picture to study the resistance mechanism of the epidemic strains.