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Two pseudomonad strains that produce a yellow cellular pigment, in addition to a diffusible fluorescent pigment on Kings medium B, were isolated from cankers on walnut trees. Biochemical properties, such as a positive oxidase reaction, a negative arginine dihydrolase reaction, and the production of a fluorescent pigment, in addition to the results of an extensive nutritional characterization study and DNA-DNA hybridization experiments, indicated that these strains belong to a new Pseudomonas rRNA group I species. This conclusion was supported by the results of a determination of the sequence of the PCR-amplified 16S rRNA gene and a comparison with the 16S rRNA genes of other bacterial species. The genomic DNAs of the strains had a base composition of 63 mol% G+C. The name Pseudomonas flavescens sp. nov. is proposed. Strain B62 (= NCPPB 3063) is the type strain of the species.

Three-dimensional models of the relationships among fluorescent pseudomonads were prepared from appropriately transformed percent DNA homology values. The transformation selected was f(x) = ( (1 - HOM/HOM)200, where HOM = fractional DNA homology and 200 is a scaling factor. Model accuracy was quite good as a plot of transformed DNA homology values versus model distances was essentially linear for homology values greater than 30%. The model suggested that bacterial strains within the fluorescent pseudomonads appear to be related in a three-dimensional continuum with no clear and easy "natural" demarcation into groups (i.e., species).

Phytopathogenic pseudomonads were placed into four major groups on the basis of nutritional and physiological characteristics. Group I consists of 86 strains of phytopathogens distinguishable from other fluorescent pseudomonads by low growth rates, ability to induce hypersensitivity on tobacco, absence of arginine dihydrolase, and relatively limited ranges of carbon sources. Most of these strains cannot utilize benzoate, 2-ketogluconate, spermine, beta-alanine, l-isoleucine, l-valine, and l-lysine. Most of the organisms in group I clustered into a small number of subgroups, each of which generally corresponded to a previously recognized nomenspecies. These subgroups differ with respect to the number of substrates used. As a rule, the organisms that utilize the fewest substrates have the most limited host ranges. The fluorescent pseudomonads of group II are arginine dihydrolase-positive and utilize a considerably larger number of carbon sources. Most pathogens of group II are similar to Pseudomonas fluorescens biotype A. Groups III and IV consist of nonfluorescent pseudomonads. These two groups can be distinguished by the number of carbon sources used and by pigmentation. An amended description of the flurescent pseudomonads and their internal subdivision is presented.

Most of 58 isolates of phytopathogenic and related bacteria comprising 24 species in the genera Agrobacterium, Erwinia, Corynebacterium, Pseudomonas, and Xanthomonas exhibited beta-glucosidase activity, especially the gall-nonforming pathogenic pseudomonads and soft rot organisms. The gall-forming pseudomonads and P. fluorescens exhibited no beta-glucosidase activity, with the exception of one isolate of P. savastanoi which showed slight activity on an inorganic nitrogen-arbutin medium. The best medium for demonstrating beta-glucosidase activity contained peptone as the nitrogen source and arbutin. beta-Glucosidase activity in this medium was indicated by either acid production or browning. P. syringae, in contrast to other bacteria tested, produced most beta-glucosidase in a medium containing large amounts of glucose. Chromatographic analyses confirmed that splitting of the glucoside occurred at the glucosidic linkage. Reaction of sonically treated bacterial cells with indican or p-nitrophenyl-beta-D-glucoside proved a rapid method for assaying relative amounts of beta-glucosidase among bacterial species. Harda's paper-strip method of detecting beta-glucosidase also was useful in revealing the distribution and relative amounts of beta-glucosidase in most bacteria, but did not indicate the relatively greater amount of beta-glucosidase in P. syringae.