Intestinal Spirochaetes and Brachyspiral colitis David J Hampson School of Veterinary and Life Sciences Murdoch University Western Australia Tel: +61 8 9360 2287 Fax: +61 8 9319 4144 Email: d.hampson@murdoch.edu.au The “intestinal spirochaetes” are a group of anaerobic bac- teria assigned to various species in the genus Brachyspira. They inhabit the large intestines of birds and animals – but also may be found in human beings. These bacteria first came to prominence in the early 1970s when a spirochaete named Treponema hyodysenteriae (now Brachyspira hyodysen- teriae) was shown to be the agent of swine dysentery, a colonic infection of pigs that is endemic in many countries and is of considerable economic significance. Since the initial description, related spirochaetes have been identified and characterised and various name changes have occurred – finally resulting in the genus Brachyspira and its seven officially recognised species. Many different hosts are colo- nised with the various Brachyspira species, but disease is mainly reported in pigs and in adult chickens. Humans are colonised with the zoonotic Brachyspira pilosicoli and Brachyspira aalborgi. Reduced susceptibility to various antimicrobials is now starting to represent a major problem for effective control of Brachyspiral colitis in pigs and other species, and consequently attention is focusing on the de- velopment of new vaccines. The Brachyspira species have specialised growth requirements, and different species can take from three days to three weeks to form a thin film of visible growth on selective isolation plates. Genetic manip- ulation of individual strains remains difficult, and this has limited understanding of gene function and disease patho- genesis. Recently whole genomic sequencing projects have started to reveal much that was previously unknown about these specialised bacteria. Taxonomy and disease association Currently the genus Brachyspira includes seven officially named species and a variety of unofficially proposed members. Some species have well-established pathogenic potential in certain hosts, causing forms of Brachyspira colitis, whilst others are considered to be largely commensal (see Table 1). The Brachyspira are genetically distinct from other spirochaetes, and the close similarities between some of the species in their 16S rRNA sequences suggest that speciation in the genus has occurred relatively recently and rapidly. Apart from the strength of haemolysis, there are few clear pheno- typic differences between the species, and indeed the boundaries between some of the named species are indistinct both genetically and phenotypically. Population structures, evolution and genetic variation For some species, such as B. hyodysenteriae, studies using multi- locus sequence typing have provided clear evidence of the popu- lation structure being clonal 1,2 , while for B. pilosicoli the population appears to be recombinant 3 . In the case of B. hyodysenteriae, the adaptation to a lifestyle in the large intestine appears to have included acquisition of various genes from Escherichia coli and Clostridium species, especially those encoding proteins associated with transport and metabolism 4 . These are likely to have been acquired in the densely populated, complex and specialised envi- ronment of the large intestine. The existence of extensive genetic rearrangements can be observed within and between Brachyspira species, with sequence drift also generating diversity. The variation and fluidity of the genomes can be seen in the case of B. pilosicoli, where three sequenced strains had genome sizes of ~2,765, 2.890 and 2.596 Mb respectively 5 , with genome rearrangements that largely correlated with the positions of mobile genetic elements. Novel bacteriophages were detected, as they were in a previous genomic study on B. intermedia 6 . These bacteriophages, that have themselves undergone extensive gene remodelling, are involved in intra- and inter-species horizontal gene transfer, and are likely to be a major force in the evolution of the Brachyspira species. In addition, novel genetic information may be acquired through the activity of prophage-like gene transfer agents that are present in the genome of different Brachyspira species 7,8 . Evidence for rapid genetic change can be seen at the farm level, where, for example, “microevolution” of B. hyodysenteriae strains resulting in changed DNA profiles has been recorded over relatively short time periods 9,10 . Under the Microscope 34 MICROBIOLOGY AUSTRALIA * MARCH 2013 10.1071/MA13011