Carbapenems and aminoglycosides are important classes of antibiotics used in the treatment of Acinetobacter baumannii infections. In Australian isolates we have detected transposons carrying either oxa23 (imipenem and meropenem resistance) or aphA6 (amikacin, kanamycin and neomycin resistance) associated with conjugative plasmids of the repAci6 family. We found that there were two distinct backbones in this family that are represented by pAb-G7-2 or pA85-3. The transposons have been found in different locations in the plasmid backbones making the backbone type and the transposon location a valuable epidemiological marker.
To examine how widely distributed the repAci6 plasmids are, a representative group of extensively antibiotic resistant global clone 2 (GC2) isolates from Singapore General Hospital were analysed. Available draft genomes of 7 isolates from 1996 to 2009 were searched for known plasmid backbones as well as pertinent resistance genes. PCR was used to assemble relevant contigs.
Five isolates contained a repAci6 family plasmid carrying AbaR4 (consists of oxa23 in Tn2006 which is inserted in Tn6022). Two resembled pA85-3 while the others were like pAb-G7-2. AbaR4 is in 5 different locations, only one of which had been detected previously. Two of the three pAb-G7-2 plasmids had TnaphA6 in the same location as in pAb-G7-2. A new transposon was present on one of these. The third pAb-G7-2 type plasmid had a copper resistance transposon inserted in its backbone. TnaphA6 was also present in this isolate but on a different small, potentially mobilisable plasmid.
The conjugative repAci6 plasmid backbones present in A. baumannii isolates from Singapore are closely related to those detected in Australia but have some unique locations for TnaphA6 and AbaR4 highlighting that these transposons have been repeatedly acquired. They also carry copper resistance transposons. Hence, the location of these transposons distinguishes lineages within the repAci6 family.