QUANTITATIVE APPROACH TO THE STUDY OF BACTERIAL SPECIES1

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Liston, J. (University of Washington, Seattle), W. Wiebe, and R. R. Colwell. Quantitative approach to the study of bacterial species. J. Bacteriol. 85:1061–1070. 1963.—The stability of strains of various species of gram-negative, rod-shaped bacteria was investigated by subculturing each strain through ten different media, testing the substrains so produced through a standard series of tests, and analyzing the resulting data by electronic computer. Strain stability was shown to be high [similarity coefficient (S) ≥ 80%], and some evidence was obtained for species stability (S ≥ 75%). The limits of variability for a species were studied by use of Pseudomonas aeruginosa; 33 P. aeruginosa strains (from the American Type Culture Collection) were tested and found to share a high level of similarity. An analysis was made of the distribution of positive characters among the strains, and the group was found to have a mean total of 35 characters (out of a possible 78 which were coded and included in the computer analyses) with a range of 33 to 39 positive characters in individual organisms. The frequency of occurrence of each positive character and the probability of mutual occurrence of pairs of positive characters were calculated. By use of the quantitative and qualitative data so obtained, a hypothetical Median Organism for the P. aeruginosa group was derived. The species may then be defined as consisting of all strains which show an S relationship of 75% or more with the Median Organism. The general applicability of this concept is discussed.

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