Acinetobacter in Denmark: II. Molecular studies of the Acinetobacter calcoaceticus-Acinetobacter baumannii complex.

The Acinetobacter calcoaceticus-Acinetobacter baumannii complex consists of four closely related "genospecies" or DNA groups: DNA group 1 (A. calcoaceticus), DNA group 2 (A. baumannii), DNA 3, and Tjernberg & Ursing's DNA group 13. Strains in this complex are so similar phenotypically that it is often impossible to identify them to the DNA group level by the use of biochemical tests. Twenty-three Danish clinical strains from 23 patients phenotypically identified to the A. calcoaceticus-A. baumannii complex were studied by ribotyping, plasmid profiling, and DNA/DNA hybridization. Multiple isolates were recovered from four patients. These were identical in each patient as judged by phenotype, ribotype and plasmic profile. Seventeen different ribotypes were observed among the 23 strains, and by using this method 19 out of the 23 strains could be identified to the DNA group level. Five strains were allocated to DNA group 2 (A. baumannii), eight to DNA group 3, and six to DNA group 13. These findings were confirmed by DNA/DNA hybridization. Two of the four unidentified strains were genotypically most closely related to but different from DNA groups 1 and 3. The last two strains were most closely related to DNA group 13. These four strains represent two new DNA groups within the A. calcoaceticus-A. baumannii-complex. One to four plasmids in the size range 2.1 kb- > kb were detected in 13 of the strains. Nine plasmid profiles were seen, indicating the usefulness of this typing method if the strains contain plasmids. The study also indicates that ribotyping is useful both for typing and for identification purposes, and that the genetic relationship in this area are more diverse than hitherto perceived. Taxonomic reconsiderations are warranted.