E. coli-like genes in the spirochete Brachyspira hyodysenteriae, the agent of swine dysentery

The genus Brachyspira comprises at least seven species of anaerobic spirochetes that live in the large intestines of various birds and animals (including humans).  One species, Brachyspira hyodysenteriae, causes swine dysentery, a disease that causes economic loss among pig farmers worldwide.  Afflicted pigs produce loose stools covered with mucus and blood.  In severe cases, necrotic chunks of colon lining are expelled with the stool.

An Australian group sequenced the genome of B. hyodysenteriae strain WA1 to figure out how the spirochete thrives in the complex nutritional environment of the large intestine and induces swine dysentery.  They found 2,122 protein-coding genes distributed between a 3,000,694 bp chromosome and a 35,940 bp plasmid.  A number of genes encoded degradative enzymes such as proteases, phospholipases, and hemolysins that may or may not account for the damage to the colon observed in swine dysentery cases.  Otherwise, no obvious pathogenic mechanism for the disease process could be gleaned from the genome sequence.

Remarkably, half of the proteins encoded in the B. hyodysenteriae genome were most similar in sequence to proteins of Escherichia and Clostridium, genera that are not even on the same branch on the bacterial evolutionary tree as spirochetes.  A mere 6.4% of B. hyodysenteriae proteins matched best to proteins of other spirochetes.

From Table 3 of Bellgard 2009.

Among the Escherichia- and Clostridium-like genes, those encoding proteins involved with amino acid and sugar metabolism and transport were over-represented.  Since E. coli, Clostridium species, and B. hyodysenteriae all live in the large intestine, the similarity in the proteins may simply reflect convergent evolution that enable the bacteria to metabolize the nutrients available in the colon.  The more attractive possibility is that B. hyodysenteriae acquired the genes from the other enteric bacteria by horizontal gene transfer thereby allowing the spirochete to adapt to the complex nutritional environment of the large intestine.  At least for the E. coli-like genes, examining their GC content may help distinguish between the two possibilities since the GC content of B. hyodysenteriae is only 27% versus 50% for E. coli.

How can B. hyodysenteriae acquire genes from other enteric bacteria?  A commentary in the journal Gut Pathogens raised the possibility that bacteriophage-like elements found in the B. hyodysenteriae genome could be involved, although bacteriophages generally do not transfer DNA between different species of bacteria.  Another possibility is that genes could be acquired from other bacteria by conjugation, a form of microbial mating.  Although the capacity of B. hyodysenteriae for acquiring DNA from other bacteria by conjugation is unknown, scientists have demonstrated that another spirochete could acquire DNA from E. coli by conjugation in the laboratory setting.

Image source

Sow with piglet, from Wikipedia

References

Bellgard, M.I., Wanchanthuek, P., La, T., Ryan, K., Moolhuijzen, P., Albertyn, Z., Shaban, B., Motro, Y., Dunn, D.S., Schibeci, D., Hunter, A., Barrero, R., Phillips, N.D., and Hampson, D.J. (2009).  Genome sequence of the pathogenic intestinal spirochete Brachyspira hyodysenteriae reveals adaptations to its lifestyle in the porcine large intestine.  PLoS ONE 4(3):e4641. DOI: 10.1371/journal.pone.0004641

Hampson, D.J. and Ahmed, N. (2009).  Spirochaetes as intestinal pathogens:  Lessons from a Brachyspira genome.  Gut Pathogens 1(1):10.  DOI: 10.1186/1757-4749-1-10

Cholera and spirochetes: Introducing Brachyspira!

Cholera results in a severe form of diarrhea that can lead to dehydration, shock, and ultimately death without prompt treatment. The disease afflicts the poor in developing countries lacking clean water sources and sanitation infrastructure. Vibrio cholerae is the causative agent and can be viewed by microscopic examination of the so-called "rice-water" stool samples from cholera patients.

rice-water stool from a cholera patient (Figure 1 from Sack et al., 2004)

As reported in a recent issue of Emerging Infectious Diseases, Nelson and colleagues, while examining a cholera outbreak in Bangladesh back in 2006, found that stool samples in over a third of cholera patients contained spirochetes mingling with V. cholerae. Samples were fluorescently stained to aid identification of bacteria. One example is shown below. V. cholerae were visualized with a FITC-conjugated monoclonal antibody to its lipopolysaccharide (in red), and bacterial DNA was stained wtih DAP I (green). Only the merged image is shown below. V. cholerae are the rods with a slight bend and appear yellow (combination of red and green) with a red edge; the spirochetes are the W-shaped forms stained green.

bar = 10 µm

This wasn't the first time spirochetes were observed in rice-water stool. Over a century ago, Theodor Escherich (the discoverer of E. coli) was the first to witness spiral-shaped microbes in fecal samples from cholera victims.

What was the identity of these spirochetes? They were not any of the "Big 3" of Borrelia, Treponema, and Leptospira, which garner the most attention from spirochete researchers (and from the writer of this blog). Nelson and colleagues guessed that they were members of the genus Brachyspira as they are the only spirochetes known to live in the human intestine. They turned out to be correct. They successfully amplified the gene encoding the 16s rRNA with Brachyspira-specific PCR primers. The sequence of the PCR product revealed the spirochetes to be Brachyspira pilosicoli and Brachyspira aalborgi.

Brachyspira account for most cases of human intestinal spirochetosis, defined as the presence of spirochetes in the colon. Although colonization of the large intestine by spirochetes is uncommon in the Western world, up to half of those in developing nations may harbor intestinal spirochetes. A typical example is shown below (click on image for larger version).

Figure 3 from Esteve et al., 2006

The sectioned tissue, which was stained with H&E, was obtained by colonic biopsy. The left panel reveals a fuzzy layer covering the colonic epithelium. These are Brachyspira attached at one end to the lining of the colon. The density of spirochetes can reach up to 1,700 per square millimeter. The right panel shows a colonic biopsy from the same patient after successful treatment with the antimicrobial agent metronidazole. Note that the fuzzy layer has disappeared.

Whether intestinal spirochetes cause disease in humans is unclear. Many people with intestinal spirochetes do not suffer any ill effects, but others endure chronic diarrhea. The mode of transmission of Brachyspira is unknown, but scientists have surmised that ingestion of contaminated water is involved.

The role of Brachyspira in cholera, if any, is even more of a mystery. In the conclusion to their article, Nelson et al. present the hypothesis that intestinal spirochetes exacerbate the already devastating clinical course of cholera.

Featured paper

Nelson, E.J., Tanudra, A., Chowdhury, A., Kane, A.V., Qadri, F., Calderwood, S.B., Coburn, J., Camilli, A. (2009). High Prevalence of Spirochetosis in Cholera Patients, Bangladesh Emerging Infectious Diseases, 15 (4), 571-573 DOI: 10.3201/eid1504.081214

Other references

Esteve, M., Salas, A., Fernandez-Banares, F., Lloreta, J., Marine, M., Gonzalez, C.I., Forne, M., Casalots, J., Santaolalla, R., Espinos, J.C., Munshi, M.A., Hampson, D.J., and Viver, J.M. (2006). Intestinal spirochetosis and chronic watery diarrhea: Clinical and histological response to treatment and long-term follow up. Journal of Gastroenterology and Hepatology 21(8):1326-1333. DOI: 10.1111/j.1440-1746.2006.04150.x

Sack, D.A., Sack, R.B., Nair, G.B., and Siddique, A.K. (2004). Cholera. Lancet 363(9404):223-233. DOI: 10.1016/S0140-6736(03)15328-7