Resumen
- Spirochaetes is one of a few bacterial phyla that are characterized by a unifying diagnostic feature, namely, the helical morphology and motility conferred by axial periplasmic flagella. Their unique morphology and mode of propulsion also represent major pathogenicity factors of clinical spirochetes. Here we describe the genome sequences of two coccoid isolates of the recently described genus Sphaerochaeta which are members of the phylum Spirochaetes based on 16S rRNA gene and whole-genome phylogenies. Interestingly, the Sphaerochaeta genomes completely lack the motility and associated signal transduction genes present in all sequenced spirochete genomes. Additional analyses revealed that the lack of flagella is associated with a unique, nonrigid cell wall structure hallmarked by a lack of transpeptidase and transglycosylase genes, which is also unprecedented in spirochetes. The Sphaerochaeta genomes are highly enriched in fermentation and carbohydrate metabolism genes relative to other spirochetes, indicating a fermentative lifestyle. Remarkably, most of the enriched genes appear to have been acquired from nonspirochetes, particularly clostridia, in several massive horizontal gene transfer events (>40% of the total number of genes in each genome). Such a high level of direct interphylum genetic exchange is extremely rare among mesophilic organisms and has important implications for the assembly of the prokaryotic tree of life.