Candidatus

In bacterial nomenclature, Candidatus (Latin for candidate of Roman office, named after the white gown worn by Roman senators) is a component of the taxonomic name for a bacterium that cannot be maintained in a bacteriology culture collection. It is an interim taxonomic status for yet-to-be-cultured organisms. An example would be "Candidatus Phytoplasma allocasuarinae". This can be abbreviated to "Ca. Phytoplasma allocasuarinae". Candidatus status may be used when a species or genus is well characterized but yet-uncultured. With today's technology much information is obtained by 16S ribosomal RNA or even near-complete genomes with modern metagenomics techniques.

For valid publication of a species, 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 such 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 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 - IJSB (now International Journal of Systematic and Evolutionary Microbiology).

The taxonomic note proposing the establishment of the provisional status Candidatus for incompletely described prokaryotes was published in the January 1995 issue of the IJSB. According to this note, the category Candidatus should be used for describing prokaryotic 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.

...
Wikipedia