Candidatus in scientific classification is a formal word that is placed before the genus and species name of bacteria that cannot be maintained in a Bacteriology Culture Collection. An example would be "Candidatus Phytoplasma allocasuarinae". Candidatus status may be used when a species or genus is well characterised but unculturable.
For valid publication, bacteria must be isolated, cultured, described, and deposited in a Bacteriology Culture Collection. However, some bacteria require special culture conditions and cannot be maintained in Bacteriology Culture Collections. These include obligately intracellular pathogens and endosymbionts, insect symbionts, and populations from oceans or sludge. In 1994, Murray and Schleifer published a taxonomic note in which they recommended that the new category of indefinite rank (Candidatus) be established for certain putative taxa that could not be described in sufficient detail to warrant establishment of a new taxon. It was also recommended that a Candidatus list should be established in the International Journal of Systematic Bacteriology (now International Journal of Systematic and Evolutionary Microbiology).
The taxonomic note proposing the establishment of the provisional status Candidatus for incompletely described procaryotes was published in the January 1995 issue of the IJSB. According to this note, the category Candidatus should be used for describing procaryotic entities for which more than a DNA sequence is available but for which characteristics required for description according to the Bacteriological Code are lacking. In addition to genomic information such as sequences, all information, including structural, metabolic, and reproductive features should be included in the description.
Newly proposed culturable bacteria must be deposited in at least two publicly accessible culture collections in different countries (Euzéby and Tindall, 2004).
According to the "Ad Hoc Committee for the re-evaluation of the species definition in bacteriology", microbiologists are encouraged to use the "Candidatus" concept for well characterised but as yet uncultured organisms.
The names included in the category Candidatus are usually written as follows: Candidatus (in italics), the subsequent name(s) in Roman type (with an initial cap for the genus name) and the entire name in quotation marks. For example, "Candidatus Phytoplasma", "Candidatus Phytoplasma allocasuarinae".
According to the "IRPCM Phytoplasma/Spiroplasma Working Team - Phytoplasma taxonomy group" the abbreviation for Candidatus should be Ca..
- MURRAY (R.G.E.) and SCHLEIFER (K.H.): Taxonomic notes: a proposal for recording the properties of putative taxa of procaryotes. Int. J. Syst. Bacteriol., 1994, 44, 174-176.
- JUDICIAL COMMISSION OF THE INTERNATIONAL COMMITTEE ON SYSTEMATIC BACTERIOLOGY: Minutes of the meetings, 2 and 6 July 1994, Prague, Czech Republic. Int. J. Syst. Bacteriol., 1995, 45, 195-196.
- MURRAY (R.G.E.) and STACKEBRANDT (E.): Taxonomic Note: implementation of the provisional status Candidatus for incompletely described procaryotes. Int. J. Syst. Bacteriol., 1995, 45, 186-187.
- JUDICIAL COMMISSION OF THE INTERNATIONAL COMMITTEE ON SYSTEMATIC BACTERIOLOGY: VIIth International Congress of Microbiology and Applied Bacteriology. Minutes of the meetings, 17 and 22 August 1996, Jerusalem, Israel. Int. J. Syst. Bacteriol., 1997, 47, 240-241.
- INTERNATIONAL COMMITTEE ON SYSTEMATIC BACTERIOLOGY: VIIth International Congress of Microbiology and Applied Bacteriology. Minutes of the meetings, 17, 18, and 22 August 1996, Jerusalem, Israel. Int. J. Syst. Bacteriol., 1997, 47, 597-600.
- DE VOS (P.), TRÜPER (H.G.) and TINDALL (B.J.): Judicial Commission of the International Committee on Systematics of Prokaryotes Xth International (IUMS) Congress of Bacteriology and Applied Microbiology. Minutes of the meetings, 28, 29 and 31 July and 1 August 2002, Paris, France. Int. J. Syst. Evol. Microbiol., 2005, 55, 525-532.
- STACKEBRANDT (E.), FREDERIKSEN (W.), GARRITY (G.M.), GRIMONT (P.A.D.), KÄMPFER (P.), MAIDEN (M.C.J.), NESME (X.), ROSSELLO-MORA (R.), SWINGS (J.), TRÜPER (H.G.), VAUTERIN (L.), WARD (A.C.) and WHITMAN (W.B.): Report of the ad hoc committee for the re-evaluation of the species definition in bacteriology. Int. J. Syst. Evol. Microbiol., 2002, 52, 1043-1047.
- The IRPCM Phytoplasma/Spiroplasma Working Team - Phytoplasma taxonomy group: 'Candidatus Phytoplasma', a taxon for the wall-less, non-helical prokaryotes that colonize plant phloem and insects. Int. J. Syst. Evol. Microbiol., 2004, 54, 1243-1255.
- INTERNATIONAL COMMITTEE ON SYSTEMATICS OF PROKARYOTES: REQUEST FOR AN OPINION:
EUZÉBY (J.P.) and B. J. TINDALL (B.J.): Status of strains that contravene Rules 27(3) and 30 of the Bacteriological Code. Request for an Opinion. Int. J. Syst. Evol. Microbiol., 2004, 54, 293-301.