Summary: A collection of Gram-positive filamentous and/or diphtheroidal organisms was classified by means of numerical phenetic methods. The collection comprised 76 new isolates from dental plaques, upper respiratory tracts and urinary tracts, and 47 reference strains. The organisms were characterized according to 77 different features. The data were subjected to computer analysis in which the similarity coefficient (S) and the simple matching coefficient (S) were calculated and the results of a single-linkage cluster-analysis technique and a minimum spanning tree technique were compared. The two cluster-analysis techniques gave almost identical results, but with the simple matching coefficient a few strains were grouped together on the basis of negative correlations.

Fixed levels of agreement were chosen to define genera and species. With the matching coefficient five major clusters were recognized at a similarity level of 87·5%. These clusters correspond to the genera and The genera and each contained a number of species. At an agreement level of 90% the clusters of and were identical to those at 87·5% which means that these genera each contain one species. The reference strains labelled and agreed on the 92·5% level which means that they form a single species. A ‘calculated median organism’ for each of these taxo-species is described. A serological grouping by immunofluorescence of all the organisms studied, by means of type-specific antisera to and , corresponded to the numerical grouping.

Characters, both constant and discriminative, were selected to form a set of tests useful for clinical laboratory identification of this group of bacteria.


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