Extensively antibiotic resistant (XAR) Acinetobacter baumannii isolates belonging to globally disseminated clone 2 (GC2) were first detected in Australia in 1999. The GC2 complex is composed of many different lineages based on sequence types (ST) and the content of the capsule gene cluster (KL). However, most isolates from the eastern seaboard of Australia are closely related to one another and are ST208 (Oxford scheme) and KL2. This suggests a single import around 1999.
To investigate a possible origin of the Australian isolates, a selection of XAR GC2 isolates recovered at Singapore General Hospital between 1996 and 2011 were examined. Genome data of seven isolates was available. Contigs carrying resistance genes were assembled by PCR and confirmed through sequencing. MLST of the isolates was determined and the ISMapper program was used locate the copies of ISAba1 present.
All isolates contained a version of the resistance islands AbGRI1 and AbGRI2 that are found in all GC2 isolates. Four versions of AbGRI2 were characterised but in three isolates, the structure of AbGRI2 was as seen in Australian isolates. Two of these isolates were ST208 and one was isolated in 1996. Multiple copies of ISAba1 (from 10-21) were detected in each isolate. Their location was determined and compared to those positions present in the Australian isolates (S Nigro, unpublished data). The 1996 isolate shared 13 out of the 14 ISAba1 positions that are common to the vast majority of the Australian isolates. The other six isolates shared at most only 10 of these.
The earliest isolate (1996) from SGH most closely resembled the lineage of GC2 A. baumnnii present in the eastern seaboard of Australia. It not only shares the characteristic AbGRI2 island structure and ST, but it is only missing a single ISAba1 position. Hence, it is closely related to the progenitor of the Australian isolates, consistent with an Asian origin for the Australian outbreak.