SUMMARY: A study of phytopathogenic pseudomonads was begun, but it was found that they could not easily be differentiated from the commonly occurring soil-and water-inhabiting fluorescent pseudomonads. A collection of 169 isolates, including 24 named cultures from various collections, was studied. Both old and new kinds of diagnostic characters, cytological, physiological and biochemical, were investigated under standard conditions. Recordings were made at frequent intervals during a 28-day incubation period. Each character was investigated a t least three times during a period of at least five years in order to assess its stability.A clear differentiation between Pseudomonas hydrophila (NCTC '7810) and P. icthyosmia (NCTC 8049) and the rest of the collection was apparent; these two isolates belong to the genus A m m a s Kluyver & van Niel. The use of the vibriostatic pteridine derivative 0/129 revealed a close relationship between them and Vibrio icthyodermis PL 1.Of the 169 isolates (including a t least 20 named species) 165 had 26 common definable and stable characters; these are offered for an extended definition of the genus Pseudomonas. A further 43 characteristics were not common to all 165 isolates. No two of these characters were correlated. One isolate had 42/43 positive characters and one had only 4, and there was a very great range of variation. Among 58 selected isolates there were 21 small groups of identical isolates, but the largest number in any group was 5; the remaining 107 isolates each had a different combination of the 43 characters. Even 10 fluorescent isolates, originally obtained from a single plating of 1 loopful of river water, showed 9 different combinations of characters.A small group of 5 isolates appeared to conform with Pseudomonas aeruginosa as defined by Haynes (1951) and Gaby (1955) but many fluorescens-type isolates were closely related when considered on the basis of all the characters investigated.Several of the named phytophathogens showed a close relationship, or apparent identity, with soil and water isolates ; many phytophathogens, however, had a conspicuous number of negative characters, suggesting loss of adaptability in a more selective environment. The 134 soil-and water-type isolates had 11 positive characters, and 9 others almost always positive. On this basis, and adopting the view that all these characters merit equal emphasis (see Tanner, 1918; Sneath, 1957a, b) it is thought that further divisions into subgroups (species?) are not justified, and that these characters should be used to describe accurately P. fluorescens Migula 1894. A definition of this species is suggested.

SUMMARY: A modification of Fontana's silver-plating method was devised and used for the cytological study of 214 isolates of Pseudonzonas spp. Many features, such as flagella, cross-walls or septa, capsules and slime were thereby clearly demonstrated simultaneously and easily. Comparisons were made with other published methods for staining flagella, capsules and slime, and good agreement was obtained. No totally non-flagellate isolate was found, nor were flagella lost during maintenance for 3-5& years in the laboratory. The number of flagella/rod and also bipolar flagellation were found to be characters of little value for species differentiation. The flagellation of young cultures was not found to be significantly different from that observed in older cultures ; the polar position of the flagella was confirmed. Flagella curvature (wavelength and amplitude) as seen in silver-plated organisms prepared under reasonably standardized conditions, was noted and considered to have no usefulness for subgeneric classification.

SUMMARYA comparative study of 50 strains of Acetomourn and 165 strains of Pseudomonas showed that the two genera could be easily and rapidly differentiated by the use of four criteria; these were: (a) production of acid on ethanol + CaCO, agar plates,

SUMMARYData which had been used to characterize the species Pseudomonas Jlzcorescens (Rhodes, 1959), as distinct from the data used to define the genus Pseudornonas, have now been used to obtain an assessment of the over-all similarity of 169 isolates of polarly flagellate bacteria (mainly of the P . Jluorescelzs species-group) by means of an electronic computer. The results of this analysis are compared with the arrangement of isolates previously favoured by the author. It was found that the two arrangements were often closely similar ; the discrepancies are discussed. The two aeromonads of the collection were picked out by the computer, which also satisfactorily replaced an accidentally misplaced pseudomonad isolate. One hundred and thirty-four isolates previously considered to belong to the P. JEzcorescens species-group were positioned together by the computer in a group with more than 80.0 yo over-all similarity throughout. This confirmed previous conclusions about the validity of this taxonomic unit, even though the description of it may appear imprecise because each character was not possessed by every isolate. The possibility of further species differentiation within this group is discussed, particularly with regard to P . mruginosa. The results suggest that P . aeruginosa may be regarded as a variety of P . Jluorescens. Other evidence was obtained which suggests that further work on the phytopathogenic pseudomonads may well reveal significant differences from P . Jluorescens. Factors governing the choice of a type strain or culture are discussed.

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