A tale of two clones; multiple antibiotic resistance in Acinetobacter baumannii .
The importance of Acinetobacter baumannii in causing nosocomial infections was only recognised in the mid 1980s, making it the least studied of the bacteria that are now resistant to most of the antibiotics that are most important for treatment of such infections. We assembled a large collection of A. baumannii isolates recovered between 1999 and 2011 at hospitals in Sydney, Canberra, Brisbane, Melbourne, Newcastle and Adelaide. All but a few of those that are resistant to multiple antibiotics belong to one of two clonal groups that have recently been found to be globally distributed. Hence, these clones, global clone 1 and 2, have been in Australia at least since the mid 1990s.
Representatives of hospitals and of groups with different carbapenem and aminoglycoside resistance patterns in our collection have been sequenced together with the GC1 and GC2 reference strains from the early 1980s. Trees based on single nucleotide polymorphisms reveal significant diversity in the Australian isolates from one clone and little in the other. Most of the genes conferring resistance to older antibiotics are in the chromosome clustered in one island in GC1 and two in GC2 isolates. However, each of these islands is continually evolving, losing and gaining resistance genes. Further variation arises from the acquisition of different plasmids carrying further resistance genes. Other major but unexpected differences arising within the clones affect the exopolysaccharides. The capsule is an important virulence determinant, and substitution of large chromosomal segments leads to many distinct loci for capsule biosynthesis in each clone.