SUMMARY: Two hundred and seventy Gram-negative strains, representing aquatic members of the genus , were isolated and compared with 48 related clinical isolates and reference strains from a variety of genera. For each isolate, a total of 96 coded characters derived from 89 characteristics was determined using morphological, physiological, nutritional and biochemical features, in addition to sensitivities to several antibiotics and inhibitory agents. The data were analysed by computer to obtain a simple matching coefficient for each pair of strains. Clustering was performed by the unweighted pair-group method of association.

Two major phenons were formed which excluded the oxidase-positive, motile or facultatively anaerobic strains. Within each phenon, three ‘subphenons’ were delimited. The two phenons, comprising 291 isolates, were tentatively differentiated at the species level, while their shared characteristics indicated that both phenons should be included in the genus Phenon 2 contained most of the clinical isolates and corresponded to the type species as described originally by Baumann, Doudoroff & Stanier (1968). Phenon 1 was composed almost entirely of aquatic isolates and may prove to represent a second species of a less biochemically-active nature.

Distinguishing characters have been suggested as diagnostic criteria for the differentiation of these two phenons.


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