Box 2 Cultured isolates of the family Christensenellaceae (2019)

The first isolate, Christensenella minuta (DSM 22607), was isolated from the feces of a healthy Japanese male. It is strictly anaerobic, non-sporulating, non-motile, and described as Gram-negative [7]. Intriguingly, others have described it as Gram-positive [12], which is also consistent with our unpublished observations. A Gram-positive cell wall is consistent with its classification as belonging to the phylum Firmicutes, which includes predominantly Gram-positive bacteria. However, C. minuta is able to produce small amounts of lipopolysaccharide, an attribute that is more typical of, but not exclusive to, Gram-negative bacteria [13]. Morotomi and colleagues demonstrated that C. minuta produces the short chain fatty acids acetate and butyrate, and is saccharolytic, with the ability to utilize arabinose, glucose, mannose, rhamnose, salicin, and xylose. C. minuta was negative for many of the standard biochemical assays used for characterization, which included catalase, oxidase, esculin and gelatin hydrolysis, indole production, and nitrate reduction [7]. The genome was published in 2017 [14], and is estimated as 2.94 Mb with 51.5% G + C content.

Catabacter hongkongensis (DSM 18959), first described in 2007, was isolated from the blood of patients who developed bacteremia in Canada and Hong Kong. Catabacter hongkongensis is described as strictly anaerobic, non-sporulating, and Gram-positive [8]. In contrast to the other Christensenella isolates, Catabacter hongkongensis is in fact motile. Catabacter has been associated with bacteremia in at least 12 additional instances, and there may be more due to the difficulty in many chemical-based methods of accurately identifying Catabacter hongkongensis [15,16,17]. Catabacter hongkongensis has a similar saccharolytic profile to C. minuta, with the exception of glycerol and rhamnose utilization depending on the isolate, and it was not able to utilize salicin. Catabacter hongkongensis differs from C. minuta in that it is catalase positive. Like C. minuta, it was negative for oxidase, esculin and gelatin hydrolysis, indole production, and nitrate reduction [8]. No short chain fatty acid production has been reported for Catabacter. The genome for this bacterium was published in 2015, and is 3.2 Mb with 48.5% G + C content. Annotation of the genome supported that Catabacter hongkongensis is motile, and the authors identified a number of antibiotic resistance genes, which may contribute to its pathogenicity [18].

Christensenella massiliensis (DSM 102344) and Christensenella timonensis (DSM 102800), both isolated from the feces of a diabetic patient in Marseilles, France, are described as strictly anaerobic, non-motile, non-sporulating, and Gram-negative, similar to C. minuta [19, 20]. Although 16S rRNA gene sequence comparisons place C. timonensis within the Christensenella genus (> 97% identity to C. minuta), whole genome taxonomy indicates it belongs to a genus distinct from both Christensenella and Catabacter [11]. No characterization of these isolates has been reported.