This paper examines eighteen strains of gram-negative, aerobic, denitrifying bacteria classified as or resembling Pseudomonas denitrificans. Through phenotypic characterization and DNA/ribosomal RNA homology studies, the authors determined these strains belong to several distinct species across at least two genera. Two unnamed Pseudomonas species (strains NCIB 10465 and NCIB 9496) are described in detail, differing in approximately 40 phenotypic characters and DNA composition. One strain resembles Alcaligenes species, others cluster with P. fluorescens biotypes B and C, and some represent potentially new taxa. The authors conclude that P. denitrificans is a nomen ambiguum—an ambiguous name applied inconsistently by different researchers to morphologically and biochemically distinct organisms. This confusion arose from changing genus descriptions in successive editions of Bergey's Manual and the lack of a clear type strain. The authors recommend abandoning the name P. denitrificans to prevent further taxonomic confusion and request the Judicial Commission formally reject it.

Key findings

• Eighteen strains identified as P. denitrificans represent multiple distinct species belonging to at least two genera (Pseudomonas and Alcaligenes), not a single species
• Two previously undescribed Pseudomonas species (NCIB 10465 and NCIB 9496) differ from each other in approximately 40 phenotypic characters and DNA composition
• Most fluorescent, denitrifying, gelatinase-negative strains examined are atypical members of P. fluorescens biotypes B and C rather than a separate species
• P. denitrificans lacks a valid type strain and historical descriptions changed across editions of Bergey's Manual, enabling inconsistent strain identifications
• The name P. denitrificans should be formally abandoned as a nomen ambiguum to eliminate taxonomic confusion