"Candidatus Karelsulcia muelleri" is an aerobic, gram-negative, bacillus bacterium that is a part of the phylum Bacteroidota. "Ca. K. muelleri" is an obligate and mutualistic symbiotic microbe commonly found occupying specialized cell compartments of sap-feeding insects called bacteriocytes. A majority of the research done on "Ca. K. muelleri" has detailed its relationship with the host Homalodisca vitripennis. Other studies have documented the nature of its residency in other insects like the maize leafhopper (Cicadulina) or the spittlebug (Cercopoidea). "Ca. K. muelleri" is noted for its exceptionally minimal genome and it is currently identified as having the smallest known sequenced Bacteroidota genome at only 245 kilobases.
Discovery
"Ca. K. muelleri" was classified under microscope in 2005 by the evolutionary biologist Nancy A. Moran. The endosymbiont was found in the dissected bacteriocyte of the spittlebug (Calstopter arizonana). The genus "Candidatus Sulcia" is named after Vytváření Karel Šulc, a Moravian embryologist who was one of the first scientists to recognize that the insect bacteriome is an organ where bacteria reside. The name was amended to "Ca. Karelsulcia" in 2017 to avoid breach of the nomenclatural code, as Sulcia is already a genus of spiders. The species, muelleri, has been named in the honor of H. J. Müller, (not to be confused with Hermann Joseph Muller) who speculated in 1960 that there was a parallel evolutionary history between endosymbionts and a select clade of insect hosts known as Auchenorrhyncha. "Ca. K. muelleri" is a member of the order Flavobacteriales. It is currently not classified as a member of any taxonomic family.
Morphology
Little has been documented about the morphology of "Ca. K. muelleri".
"Ca. K. muelleri" is a rod-shaped bacterium measuring 5–7 μm in length, .7 μm in diameter and 2–5 μm in width. Because "Ca. K. muelleri" lacks most of the genes responsible for cell division and membrane synthesis, it is sometimes observed to extend to unusual lengths of up to 100 μm during part of its life cycle.
Like all other Flavobacteriales, "Ca. K. muelleri" is gram-negative.
Phylogeny
The phylogeny of "Ca. K. muelleri" has been discovered to follow the phylogeny of the Hemiptera clade, Auchenorrhyncha. The first association between "Ca. K. muelleri" and Auchenorrhyncha is estimated to have occurred sometime between 260 and 280 million years ago. Further evidence supports the idea that "Ca. K. muelleri" has coevolved with another symbiotic lineage from the taxonomic class Betaproteobacteria. The result of this coevolution can be noticed through the fact that both "Ca. K. muelleri" and its host leave cofactor and vitamin production to another member of the symbiotic relationship. Although the co-residents of "Ca. K. muelleri" are not always of the class Betaproteobacteria, contemporary analyses have shown that they often are. The Betaproteobacteria ancestor is suggested to have diversified into the genera Zinderia, Nasuia and Vidania.
There are currently 9 unique strains of "Ca. K. muelleri" that have been identified through a complete genome sequence. They can all be found here.
"Candidatus Karelsulcia" and the Flavobacteria
The tree below demonstrates the position of "Ca. K. muelleri" with respect to some other members of the class Flavobacteriia. The tree was constructed by comparing the peptide sequences of ten different types of proteins. The proteins used were the DNA polymerase III beta-subunit, initiation factor IF-2, leucyl-tRNA synthetase, the phenylalanine—tRNA ligase beta-subunit, VARS, elongation factor Tu, the RNA polymerase beta-subunit, and the ribosomal proteins L2, S5, and S11. Where "Ca. K. muelleri" is found occupying the body of Auchenorrhyncha hosts, the other members of Flavobacteriia are found residing in freshwater bodies and soils. The inference for the long, isolated stretch of the "Ca. K. muelleri" branch is that there has been a high frequency of base-pair substitution which has led to noticeable genetic differences between "Ca. K. muelleri" and most other Flavobacteriia.
Phylogeny using 120 bacterial markers from known genomes (see GTDB) place the bacterium under family Blattabacteriaceae. Using 11 full genomes of acceptable quality, the database is able to define two species-level groups.