DNA base composition of Neisseria, Moraxella, and Acinetobacter, as determined by measurement of buoyant density in CsCl gradients

The guanine + cytosine contents (%(G + C)) of DNAs from 75 strains of asaccharolytic Neisseria, Moraxella, and Acinetobacter have been determined by measuring their buoyant densities in the CsCl gradient. The main purpose was to provide supporting evidence for taxonomic conclusions based on assay of genetic transformation to streptomycin resistance among the same strains and species.Three groups of neisseriae can be recognized, both by determination of %(G + C) and by transformation assay: (i) Neisseria flavescens and N. cinerea (46.5–49%), (ii) N. catarrhalis (41–42.5%), and (iii) N. caviae and N. ovis (44.5–45%). There is no transformation compatibility between group (i) and the other groups, whereas groups (ii) and (iii) show mutual transformation interactions. N. catarrhalis, N. caviae, and N. ovis, therefore, can also be considered as one group of nonpigmented, asaccharolytic neisseriae.Four groups of moraxellae can be distinguished: (i) Moraxella nonliquefaciens, M. lacunata, M. liquefaciens, and M. bovis (40–43%), (ii) M. osloensis (43–43.5%), (iii) M. phenylpyrouvica (43–43.5%), and (iv) M. kingii (44.5%). Although groups (ii) and (iii) are identical in terms of %(G + C), they exhibit low transformation compatibility, of the same order as between groups (i) and (ii). The distinctly higher G + C content of M. kingii DNA is consistent with its lack of transformation compatibility with other moraxellae.The similar %(G + C) range for the moraxellae and the nonpigmented, asaccharolytic neisseriae is consistent with the finding of some transformation compatibility between most of these organisms (except M. kingii) and provides additional support for the assumption that they are closely related.The Acinetobacter strains studied exhibit a wide range of G + C contents (38–45%) and hardly any transformation compatibility with neisseriae and moraxellae, even if some of them have matching %(G + C).It can be concluded that determination of the G + C contents of bacterial DNAs provides useful supplementary taxonomic data, but has only limited value as a sole taxonomic criterion.