The evolutionary tale of eight novel plasmids in a colistin-resistant environmental Acinetobacter baumannii isolate

Acinetobacter baumannii is an important opportunistic pathogen known for its high levels of resistance to many antibiotics, particularly those considered last resorts such as colistin and carbapenems. Plasmids of this organism are increasingly associated with the spread of clinically important antibiotic resistance genes. Although A. baumannii is a ubiquitous organism, to date, most of the focus has been on studying strains recovered from clinical samples ignoring those isolated in the environment (soil, water, food, etc.). Here, we analysed the genetic structures of eight novel plasmids carried by an environmental colistin-resistant A. baumannii (strain E-072658) recovered in a recycled fibre pulp in a paper mill in Finland. It was shown that E-072658 carries a new variant of the mcr-4 colistin resistance gene (mcr-4.7) in a novel Tn3-family transposon (called Tn6926) carried by a novel plasmid p8E072658. E-072658 is also resistant to sulphonamide compounds; consistent with this, the sul2 sulphonamide resistance gene was found in a pdif module. E-072658 also carries six additional plasmids with no antibiotic resistance genes, but they contained several pdif modules shared with plasmids carried by clinical strains. Detailed analysis of the genetic structure of all eight plasmids carried by E-072658 showed a complex evolutionary history revealing genetic exchange events within the genus Acinetobacter beyond the clinical or environmental origin of the strains. This work provides evidence that environmental strains might act as a source for some of the clinically significant antibiotic resistance genes.