Brachyspira burroughii sp. nov., a novel intestinal spirochete isolated from pigs during swine dysentery surveillance: genomic and phenotypic characteristics

Brachyspira hyodysenteriae is the classical etiological agent of swine dysentery (SD); however, strongly β-hemolytic species such as B. hampsonii and B. suanatina also cause SD-like disease and complicate surveillance and diagnosis. Fecal samples from two separate swine farms were submitted to the Iowa State University Veterinary Diagnostic Laboratory for SD surveillance. The resulting isolates were weakly β-hemolytic, and MALDI-TOF identified them as B. hyodysenteriae or other Brachyspira spp. but with low confidence scores (1.80–1.90). PCR assays for known species were also inconclusive. Therefore, we combined classical phenotypic characterization with genome-based analyses to establish the distinctiveness of this novel Brachyspira species. The two isolates shared an identical biochemical and enzymatic profile that differed clearly from B. hyodysenteriae and B. pilosicoli . Morphologically, they fell within the expected size and structural range of the genus, with 8–11 periplasmic flagella observed per cell. Average nucleotide identity analysis demonstrated only 76.5–91% similarity to recognized species, supporting their designation as a novel species. The pairwise ANI between the two isolates was 94.5%, slightly below the traditional species cutoff. Core-genome SNP phylogeny placed the isolates in a distinct, well-supported clade, a finding also supported by the nox -based phylogeny, while 16S and 23S rRNA gene analyses showed lower discriminatory resolution among some Brachyspira species. These findings reveal a previously unrecognized Brachyspira species identified independently during routine SD surveillance. The recognition of Brachyspira burroughii highlights the importance of combining genomic analyses with classical phenotyping to improve detection and accurate identification of novel Brachyspira species.