Phylogenetic structures of the genus Acinetobacter based on gyrB sequences: comparison with the grouping by DNA-DNA hybridization

The phylogenetic relationships of 49 Acinetobacter strains, 46 of which hav e previously been classified into 18 genomic species by DNA-DNA hybridizati on studies, were investigated using the nucleotide sequence of gyrB, the st ructural gene for the DNA gyrase B subunit. The phylogenetic tree showed li nkages between genomic species 1 (Acinetobacter calcoaceticus), 2 (Acinetob acter baumannii), 3 and TU13; genomic species 6, BJ15, BJ16 and BJ17; genom ic species 5, BJ13 (synonym of TU14) and BJ14; genomic species 7 (Acinetoba cter johnsonii), 10 and 11; and genomic species 8 and 9, The phylogenetic g rouping of Acinetobacter strains based on gyrB genes was almost congruent w ith that based on DNA-DNA hybridization studies. Consequently, gyrB sequenc e comparison can be used to resolve the taxonomic positions of bacterial st rains at the level of genomic species. However, minor discrepancies existed in the grouping of strains of genomic species 8, 9 and BJ17. The phylogene tic tree for these strains was reconstructed from the sequence of rpoD, the structural gene for the RNA polymerase sigma(70) factor. The latter tree w as 100% congruent with the grouping based on DNA-DNA hybridization. The rel iability of DNA-DNA hybridization may be superior to that of sequence compa rison of a single protein-encoding gene in resolving closely related organi sms since the former method measures the homologies between the nucleotide sequences of total genomic DNAs. Three strains that have slot been characte rized previously by DNA-DNA hybridization seem to belong to two new genomic species, one including strain ATCC 33308 and the other including strains A TCC 31012 and MBIC 1332.