Abstract
BackgroundBrachyspira spp. colonize the intestines of some mammalian and avian species and show different degrees of enteropathogenicity. Brachyspira intermedia can cause production losses in chickens and strain PWS/AT now becomes the fourth genome to be completed in the genus Brachyspira.Results15 classes of unique and shared genes were analyzed in B. intermedia, B. murdochii, B. hyodysenteriae and B. pilosicoli. The largest number of unique genes was found in B. intermedia and B. murdochii. This indicates the presence of larger pan-genomes. In general, hypothetical protein annotations are overrepresented among the unique genes. A 3.2 kb plasmid was found in B. intermedia strain PWS/AT. The plasmid was also present in the B. murdochii strain but not in nine other Brachyspira isolates. Within the Brachyspira genomes, genes had been translocated and also frequently switched between leading and lagging strands, a process that can be followed by different AT-skews in the third positions of synonymous codons. We also found evidence that bacteriophages were being remodeled and genes incorporated into them.ConclusionsThe accessory gene pool shapes species-specific traits. It is also influenced by reductive genome evolution and horizontal gene transfer. Gene-transfer events can cross both species and genus boundaries and bacteriophages appear to play an important role in this process. A mechanism for horizontal gene transfer appears to be gene translocations leading to remodeling of bacteriophages in combination with broad tropism.
Highlights
Brachyspira spp. colonize the intestines of some mammalian and avian species and show different degrees of enteropathogenicity
The phylogenetic distances to the other available genomes within the Brachyspira genus and to the spirochete Leptospira interrogans (AE016823-24), calculated from average nucleotide similarity of the whole core genome [24], showed that B. intermedia strain PWS/AT was most closely related to B. hyodysenteriae followed by B. murdochii; it was somewhat more distantly related to the B. pilosicoli genome (Figure 1)
Average similarity distances indicated that the B. pilosicoli genome was closest to B. murdochii followed by B. intermedia and furthest from B. hyodysenteriae
Summary
Brachyspira spp. colonize the intestines of some mammalian and avian species and show different degrees of enteropathogenicity. The genus Brachyspira currently comprises seven validly published species: B. aalborgi, B. alvinipulli, B. hyodysenteriae, B. innocens, B. intermedia, B. murdochii and B. pilosicoli. These form a distinct evolutionary line within the phylum Spirochaetes [1]. Brachyspira spp. are oxygen-tolerant anaerobes that colonize the intestines of some mammalian and avian species but they differ in enteropathogenicity from important pathogens of livestock to presumed commensals. B. hyodysenteriae and B. pilosicoli are important porcine pathogens, B. intermedia commonly colonize the large intestine of commercially farmed pigs and chickens and has been isolated from rodents [3] and ducks (Jansson, D. unpublished data). Data from field studies have suggested that B. intermedia may be a mild enteropathogen of pigs [6,7,8], but experimental challenge in porcine isolates has failed to produce clinical disease [9,10,11]
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