A comparative study of class 1 integrons in Acinetobacter baumannii

• Integrons could be transferred across the two evolutionary lineages.
• The origins of A. baumannii class 1 integrons may be the soil environment.
• The gain of integrons in A. baumannii strains started before the antibiotic era.

Multidrug resistance (MDR) in Acinetobacter baumannii is increasingly reported and has become a significant public concern. The method responsible for the acquisition of resistance genes via integrons from the environment or intra-species in A. baumannii remains to be understood. This study was performed to investigate the transmission route of these integrons using a comparative analysis of published A. baumannii complete genomes. The phylogenetic analysis of A. baumannii type 1 integrases (IntI1) showed that the integrons could be transferred across the two evolutionary lineages, the international clone I (IC I) and clone II (IC II) strains. In addition, the integrons in A. baumannii strains were mainly responsible for the transfer of resistance genes for two types of long-term usage antibiotics and antiseptics, such as aminoglycosides, chloramphenicol and the quaternary-ammonium-compound family. The in silico comparative analysis of known integron integrases revealed that the intI genes were phylogenetically related among A. baumannii strains and some microorganisms living in a sediment community, implicating that the integrons of A. baumannii might have originated from those microorganisms belonging to the β-preoteobacterial class in the sediment environment. The data suggest that the gain of class 1 integrons in A. baumannii strains may have started before the antibiotic era. This report shows that the origins of A. baumannii class 1 integrons may be the soil environment and that the resistance genes included in integrons are horizontally transferred across all the A. baumannii genomes, including IC I and IC II.