This taxonomic study addresses the correct classification of a hydrogen bacterium originally named Hydrogenomonas ruhlandii by Packer and Vishniac in 1955. The organism was subsequently transferred to Pseudomonas by Davis and colleagues in 1969, but the authors did not document their observations on flagellation. Aragno and Schlegel used electron microscopy to examine strain ATCC 15749, the type strain of H. ruhlandii, and determined that it possesses peritrichous flagella (2-10 per cell), measuring 17-18 nm in diameter with distinctive sheaths. This peritrichous arrangement excludes the organism from Pseudomonas, which contains polarly flagellated species. The authors demonstrate that the strain belongs in the genus Alcaligenes based on its peritrichous flagellation, strict aerobic metabolism, and hydrogen-oxidizing capability. They propose the new combination Alcaligenes ruhlandii. The organism closely resembles Alcaligenes eutrophus but differs by lacking the ability to utilize aromatic compounds like phenol and benzoate, by lacking denitrification capacity, and by possessing distinctly sheathed flagella with smaller diameter and spiral structure compared to A. eutrophus.

Key findings

• Strain ATCC 15749 (H. ruhlandii) possesses peritrichous rather than polar flagellation, requiring reclassification from Pseudomonas to Alcaligenes
• The organism's flagella are 17-18 nm in diameter with distinctive sheaths that do not extend over the basal hook
• A. ruhlandii differs from the closely related A. eutrophus by inability to use aromatic compounds and lack of denitrification capacity
• Electron microscopy revealed 2-10 flagella per cell, with numbers varying by substrate and culture age
• The new combination Alcaligenes ruhlandii (Packer and Vishniac) comb. nov. is formally proposed for this hydrogen bacterium