Taxonomic studies were carried out on 145 strains of Gram-negative methanol-utilizing bacteria, and they were divided into the following four major groups on the basis of morphological characteristics, assimilation of carbon compounds, cellular fatty acid composition, and coenzyme Q system. Bacteria of group 1 were polarly flagellated rods, and showed the presence of coenzyme Q
8 and cellular fatty acid composition consisting of a large amount of C
16:0 acid and C
16:1 acid (Type A). The so-called "obligate methylotrophs" are included in this group. Bacteria of group 2 were polarly flagellated rods, and showed the presence of coenzyme Q
10 and cellular fatty acid composition consisting of a large amount of C
18:1 acid (Type B). Bacteria of group 3 belonged to the genus
Microcyclus on the basis of morphological characteristics. They showed the presence of coenzyme Q
10 and Type B of cellular fatty acid composition. Bacteria of group 4 belonged to the genus
Hyphomicrobium on the basis of morphological characteristics. They showed the presence of coenzyme Q
9 and Type B of cellular fatty acid composition. Bacteria of groups 1 and 2 differed from
Pseudomonas in point of cellular fatty acid composition and coenzyme Q system. Taxonomic implication of cellular fatty acid composition was discussed from the viewpoint of the change in the composition according to cultural conditions.
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