Acinetobacter baumannii that are extensively-antibiotic resistant (XAR) or pan-resistant pose a significant treatment challenge. Most Australian XAR A. baumannii belong to global clone 2 (GC2), one of the globally disseminated clones. In GC2 isolates, most of the resistance genes are located in two genomic islands, AbGRI1 and AbGRI2. However, their structures are not conserved. AbGRI2-1 (19.5 kb) was found in an Australian isolate and contains the genes conferring resistance to sulphonamides, ampicillin, neomycin, kanamycin, gentamicin, streptomycin and spectinomycin. To understand how current strains have become so resistant we investigated the origin of this island through examining the genome sequence of A320. A320 is the reference strain for GC2s and is the oldest GC2 isolate known to be in collections throughout the world. It is multiply-antibiotic resistant (MAR) and was isolated in 1982.
A320 was known to carry a form of AbGRI1 and analysis of its genome sequence revealed an AbGRI2-type island, called AbGRI2-6. AbGRI2-6 was 39.6 kb and, in addition to the resistance genes found in AbGRI2-1 it also contained a chloramphenicol resistance gene and a mercury resistance region. A320 contains an ancestral form of AbGRI2 and AbGRI2-1 can be derived from AbGRI2-6 via two deletions. However, assembly of AbGRI2-6 using PCR indicated that it was in two pieces, AbGRI2-6a and AbGRI2-6b, which had been separated via the inversion of approximately a third of the chromosome. In addition, examination of the chromosomal sequence flanking AbGRI2-1 revealed that a large deletion had occurred to one side. This deletion was not present in A320s’ chromosome.
It appears that the two resistance islands were already present in the earliest known GC2 isolate. Current XAR isolates have maintained versions of AbGRI1 and AbGRI2 but have evolved from the older MAR isolates by gaining extra resistance genes in plasmids or via chromosomal mutations.