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Steven J. Nigro, Virginia Post, Ruth M. Hall, The multiresistant Acinetobacter baumannii European clone I type strain RUH875 (A297) carries a genomic antibiotic resistance island AbaR21, plasmid pRAY and a cluster containing ISAba1-sul2-CR2-strB-strA, Journal of Antimicrobial Chemotherapy, Volume 66, Issue 8, August 2011, Pages 1928–1930, https://doi.org/10.1093/jac/dkr213
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Sir,
The multiply antibiotic-resistant Acinetobacter baumannii isolate RUH875,1 which is lso known as A297,2 has been used in many studies as the type strain for European clone I. The clone is now known to be distributed globally, and has been referred to as worldwide or global clone 1 (GC1). RUH875 is from Dordrecht, The Netherlands, and was recovered from a urinary tract infection in a surgery ward in 1984. RUH875 is recorded as ST109 in the Oxford multilocus sequence typing (MLST) scheme and carries the blaOXA-69 allele of the intrinsic A. baumannii blaOXA-Ab gene.2 In the Institut Pasteur MLST scheme, it is ST1 and clonal complex 1 (CC1).3 The strain is recorded as resistant to ampicillin/sulbactam, piperacillin, gentamicin and tobramycin, tetracycline and co-trimoxazole but susceptible to ceftazidime, imipenem and ofloxacin.3 It has previously been shown to carry the aminoglycoside resistance genes aadB and aphA1 and contain a class 1 integron that yielded a cassette amplicon of 0.7 kb that did not correspond to the size for the aadB cassette.4 RUH875 also carries the tet(A) tetracycline resistance determinant.5
A297 was kindly supplied by Dr K. Towner (Nottingham University Hospitals NHS Trust, Nottingham, UK) and the resistance profile was confirmed and extended using a disc diffusion assay, as described previously.6 A297/RUH875 was also resistant to sulfamethoxazole, trimethoprim, streptomycin and spectinomycin, kanamycin and neomycin and susceptible to amikacin and netilmicin. Chloramphenicol and florfenicol resistances, which are intrinsic resistances for A. baumannii, were also detected. The presence of transposon Tn6020 carrying the aphA1b kanamycin and neomycin resistance gene,7 and of the catA1 chloramphenicol resistance gene, the sul1 sulphonamide resistance gene, the blaTEM gene encoding the TEM β-lactamase, all of which are present in the AbaR3 genomic resistance island,8 was demonstrated using published primer pairs.6,7 Other features of AbaR3-type genomic resistance islands, including the junctions between the chromosome and Tn6019 backbone and between different components of the composite transposon structure, were also found in RUH875 using published primer pairs.6 A297/RUH875 contains an AbaR insertion in the chromosomal comM gene (represented by the dashed line at each end of the resistance island shown in Figure 1), which was detected using PCRs J1 and J2, described previously,6 which detect the comM–AbaR junctions. Hence, the island is in the same position as other AbaR.6–8 The arsB gene and Tn6018, and all four junctions between Tn6018 and the adjacent sequence shown in Figure 1, were also detected using PCR reactions J3–J6 described previously.6 The complete island was mapped using overlapping PCRs followed by digestion of the amplicons with restriction enzymes to confirm their identity, as described previously.6,7 The configuration of the AbaR was as in AbaR38 except for the cassettes in the class 1 integron (circled in Figure 1). The cassette was sequenced and identified as the dfrA5 gene cassette (identical to GenBank accession number X162168), which confers resistance to trimethoprim. To confirm that the dfrA5 cassette was part of the island and had replaced the aacC1-orfP-orfQ-aadA1 cassettes found in AbaR3 or aacC1-orfP-orfP-orfQ-aadA1 found in most Australian GC1 isolates,6,7 PCR was used to demonstrate linkage between dfrA5 and Tn6020. This composite transposon was named AbaR21. The sequence of the dfrA5 gene cassette and part of the integron has been deposited in Genbank under accession number JN121384. AbaR3 has also been detected in a GC1 isolate from 1977,9 indicating that this island was fully formed early in the evolution of the GC1 complex and suggesting that the dfrA5 cassette substitution is a specific modification in the AbaR21 island.
In addition to the resistance genes found in AbaR21, the aadB gene, which confers resistance to gentamicin, kanamycin and tobramycin, the sul2 sulphonamide resistance gene and strA and strB streptomycin resistance genes were also detected using gene-specific PCRs. The aadB gene was shown to be in the same location as it is in pRAY using a PCR detection method reported recently.10 This method specifically detects the aadB cassette in the secondary site it occupies in pRAY. The clonal type of the strain from which pRAY was isolated is not known, but pRAY has recently been found in another GC1 isolate8 and identical or closely related plasmids have been found in a group of GC2 isolates that are resistant to gentamicin and tobramycin.10
PCR was used to link the remaining genes, strA, strB and sul2. The sul2 gene was found to be located adjacent to the recognizable ori end of the small mobile element CR2, as has been observed previously, and the strAB gene pair was on the other side of CR2. The insertion sequence ISAba1 was detected on the upstream side of sul2 and is oriented such that it can supply a promoter for sul2. The overall configuration was ISAba1-sul2-CR2-strB-strA.
Hence, RUH875 carries at least 10 acquired antibiotic resistance genes borne in three distinct entities, the chromosomally located resistance island AbaR21, plasmid pRAY and an additional gene cluster whose location remains to be established.
Funding
No specific funding was received for this study. R. M. H. was supported by NHMRC Fellowship grant 358713. S. J. N. is supported by a University of Sydney Postgraduate Award, V. P. was supported by a University of Sydney International Research Scholarship.
Transparency declarations
None to declare.
Acknowledgements
We thank Dr Kevin Towner for supplying the A297 isolate used in this study.
References
Author notes
Present address: AO Research Institute Davos, Clavadelerstrasse 8, 7270 Davos, Switzerland.