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Box 1 Discovery and phylogenetic classification of the Christensenellaceae

From: The human gut bacteria Christensenellaceae are widespread, heritable, and associated with health

The family Christensenellaceae belongs to the bacterial phylum Firmicutes, the phylogenetically diverse and predominant phylum of the human gut microbiome. The name Christensenellaceae is derived from the isolate named Christensenella minuta (pictured in Fig. 1), which was first cultivated from the feces of a healthy Japanese male by Morotomi and colleagues and published in 2012 [7]. This isolate was named to honor the Danish microbiologist Henrik Christensen, and the species designated “minuta”, due to the small size of the cell (0.8–1.9 μM) and the colonies it forms on agar plates (only 0.1 mm in diameter). In their species description, Morotomi et al. compared C. minuta’s full length 16S rRNA against publicly available databases and identified Caldicoprobacter oshimai, a bacterium in the family Caldicoprobactereaceae (Clostridiales), as the closest relative, with 86.9% pairwise ID. Other related taxa included Tindallia californiensis (86.3% ID) and Clostridium ganghwense (86.1% ID), both of which are in the family Clostridiaceae in the phylum Firmicutes. They did note that other sequences were identified with matches greater than 98% ID; however, these were unclassified taxa from other 16S rRNA gene diversity surveys. C. minuta was designated to represent a novel family, Christensenellaceae, in the order Clostridiales in the phylum Firmicutes [7].
A closely related bacterium, Catabacter hongkongensis, was described in 2007 [8]. The 16S rRNA genes of C. minuta and Catabacter hongkongensis share 96.5% sequence identity, suggesting the two should be in the same family, and possibly the same genus [9] (Fig. 2). As a result, some databases use the family name Catabacteriaceae, some use Christensenellaceae, and some studies include both as two distinct families. The family name Christensenellaceae, however, is now considered with standing in nomenclature [10]. The Genome Taxonomy Database, a recent taxonomy developed by Phil Hugenholtz and colleagues that is based on whole genome comparisons rather than 16S rRNA gene sequences for reconstructing phylogeny, supports that Christensenella and Catabacter are separate genera in the family Christensenellaceae, within a new order Christensenellales [11].