International Journal of Systematic and Evolutionary Microbiology (2001), 51, 2221–2225 Printed in Great Britain TAXONOMIC NOTE Is characterization of a single isolate sufficient for valid publication of a new genus or species? Proposal to modify Recommendation 30b of the Bacteriological Code (1990 Revision) 1 Department of Veterinary Microbiology, Stigbøjlen 4, The Royal Veterinary and Agricultural University, DK-1870 Frederiksberg, Denmark 2 Rødding Tværvej 3, 6630 Rødding, Denmark 3 Institute of Medical Microbiology and Hospital Hygiene, Philipps University, Pilgrimstein 2, D-35037 Marburg, Germany 4 Institute of Veterinary Bacteriology, University of Bern, Laenggass-Strasse 122, CH-3012 Bern, Switzerland Henrik Christensen, 1 Magne Bisgaard, 1 Wilhelm Frederiksen, 2 Reinier Mutters, 3 Peter Kuhnert 4 and John Elmerdahl Olsen 1 Author for correspondence : Henrik Christensen. Tel : ›45 3528 2783. Fax: ›45 3528 2757. e-mail : hech!kvl.dk From 1990 to 2000, the number of published named taxa based upon new isolates at species and genus levels in International Journal of Systematic and Evolutionary Microbiology, formerly International Journal of Systematic Bacteriology, have increased by approximately four- and sevenfold, respectively. New taxa based upon characterization of only a single isolate remained at around 40 % for both categories. The Bacteriological Code (1990 Revision) has no recommendations on the number of strains required for definition of new taxa. For a few groups, a minimum number of 5–10 strains has been suggested in minimal standards. Since an exponential increase in new taxa can be expected in the future, the authors discuss problems related to naming new species and genera based upon descriptions of a single isolate and suggest that this practice is re-evaluated. It is proposed that the following should be added to Recommendation 30b of the Bacteriological Code : ‘ Descriptions should be based on as many strains as possible (minimum five), representing different sources with respect to geography and ecology in order to be well characterized both phenotypically and genotypically, to establish the centre (from which the type strain could be chosen) and the extent of the cluster to be named. In addition, comparative studies should be performed, including reference strains that represent neighbouring species and/or genera, in order to give descriptions that are sufficiently detailed to allow differentiation from these neighbours. ’ Keywords : single-isolate description, minimal standards, bacterial nomenclature, Bacteriological Code Present recommendations on the selection of isolates The rules of bacterial (domains Archaea and Bacteria) nomenclature are stated in the 1990 Revision of the International Code of Nomenclature of Bacteria (in short Bacteriological Code) (Lapage et al., 1992); however, the Bacteriological Code does not deal with the classification of bacteria, including the criteria for selection of isolates. Recommendation 30b of the Bacteriological Code (1990 Revision) states that ‘ Before publication of the name and description of a new species, the examination and description should conform at least to the minimal standards (if available) required for the relevant taxon of bacteria ’. Minimal standards were included in the Bacteriological Code to ensure the optimal circumscription of new taxa to be named within specific groups of bacteria (Lessel, 1971). Fewer than ten minimal standards have been published by the Subcommittees on Taxonomy under the In- ternational Committee on Systematic Bacteriology (ICSB). In these cases, recommendations as to the selection and the number of isolates have been given. It has been recommended to base descriptions of new species on the characterization of not fewer than five, or ten, and preferably as many isolates as possible. These isolates should be independently isolated and obtained from different locations (Dewhirst et al., 2000 ; Graham et al., 1991 ; Oren et al., 1997 ; Ursing et 01807 # 2001 IUMS 2221