Proposal to reject the genus Hydrogenomonas: Taxonomic implications

Fifty-six strains of “hydrogen bacteria” and related nonautotrophic bacteria, including nearly all existing named Hydrogenomonas spp., have been compared. It is proposed that the genus Hydrogenomonas should be rejected, since its type species H. pantotropha, appears to be a nomen dubium; and that the various species of “hydrogen bacteria” should be assigned to other genera, not specifically characterized by the ability to grow autotrophically with H2. The two species of hydrogen bacteria most frequently isolated by enrichment show a peritrichous or degenerate peritrichous flagellar arrangement; one is nonpigmented, the other produces yellow (carotenoid) cellular pigments. Of the various possible generic assignments for these two species, assignment to the genus Alcaligenes is proposed. The nonpigmented species, previously named Hydrogenomonas eutropha, but never legitimately described, is here described as A. eutrophus. The yellow species which includes both facultatively autotrophic and nonautotrophic strains, is described as a new species, A. paradoxus. The Gram-negative, coccoid hydrogen bacterium, formerly known as Micrococcus denitrificans, is placed in a new genus, Paracoccus. The polarly flagellated species of hydrogen bacteria, including the previously named species Hydrogenomonas facilis, H. flava, H. ruhlandii and Pseudomonassa saccharophila, are all assigned to the genus Pseudomonas.