Do nonspiral spirochetes help clean our environment?

Members of the spirochete phylum Spirochaetes are recognized easily by their long spiral shape, which allows their periplasmic flagella to power them through viscous environments.  But scientists are discovering that not all spirochetes share this peculiar shape.  Two bacterial isolates recovered from freshwater sediments in Michigan were spherical and lacked flagella, yet phylogenetic analysis of their 16S rRNA and other genes placed them firmly within the Spirochaetes.  The genus Sphaerochaeta was created to accommodate the new isolates, which were designated Sphaerochaeta globosa and Sphaerochaeta pleomorpha.

Sphaerochaeta pleomorpha viewed by phase contrast microscopy.  Arrowheads point to protrusions.  Panel B shows the round spirochetes organized as "strings of pearls."  Figure 1a and 1b from Ritalahti et al., 2012.

Sphaerochaeta globosa viewed by phase contrast microscopy.  Figure 2a from Ritalahti et al., 2012.

The disease-causing spirochetes such as Borrelia burgdorferi and Leptospira species are shape changers.  Although they are often observed with the familiar spiral morphology, they sometimes morph into nonmotile round bodies when stressed, only to revert to the spiral form when conditions improve (see images below).  Could the Sphaerochaeta strains sprout flagella and morph into the spiral form under the right conditions?  It doesn't appear likely.  Sphaerochaeta retain their round shape under a variety of growth conditions, and their genomes lack motility and chemotaxis genes, including those encoding the components of the flagellum.

The Lyme disease spirochete B. burgdorferi viewed by electron microscopy.  Panel A:  B. burgdorferi in its standard growth medium BSKII, which contains serum.  Panel B:  Most of the spirochetes appear as round bodies after being starved for serum for 48 hours.  Bar, 2 µm.   Figure 1A and 1B from Alban et al., 2000.

Views of B. burgdorferi by phase contrast microscopy.  Panel A: B. burgdorferi starved for serum for 48 hours.  Panel B:  Less than one minute after the culture is replenished with serum, the round bodies convert back to the spiral form.  Bar, 5 µm.  Figure 2A and 2B from Alban et al., 2000.

Sphaerochaeta spirochetes have another unusual property.  Electron microscopy revealed what could be a peptidoglycan-layered cell wall (see image below), yet they grow fine even when high concentrations of ampicillin are dumped into the growth meduim.  The genome sequence revealed the reason for their resistance to the antibiotic.  Although the two Sphaerochaeta strains had the genes necessary to make peptidoglycan, they were missing the genes encoding the enzymes that strengthen the cell wall by cross-linking the peptidoglycan.  These missing enzymes are the targets of β-lactams, the penicillin class of antibiotics that includes ampicillin.  Without the cross-linking enzymes, one may expect the cell wall to be fragile, but it isn't.  The strains grow fine in hypotonic medium, which would have caused the bacteria to burst if they had a weak cell wall.  What strengthens the Sphaerochaeta cell wall to keep it intact under physical strain remains a mystery.

Cell wall architecture of Sphaerochaeta pleomorpha viewed by electron microscopy.  OM, outer membrane; PS, periplasmic space; CW, cell wall.  Figure 1d from Ritalahti et al., 2012.

Even though Sphaerochaeta reside in oxygen-poor environments, they don't live alone.  They are members of a close-knit microbial community that includes bacteria of the genus Dehalococcoides, which respire by reducing organic chlorides instead of oxygen.  Dehalococcoides have attracted attention because of their potential for cleaning up groundwater and other sensitive environments contaminated with chlorinated organic compounds, pollutants that originated mainly from past industrial and agricultural activities.  Although the production of these toxic compounds has ceased in many countries, the pollutants persist in the environment and must be detoxified.  This is where Dehalococcoides bacteria may be beneficial.  They obtain energy by anaerobic respiration of chlorinated organic molecules, which strips off the chloride atoms, rendering the compounds nontoxic.

Dehaloccoides bacteria do not grow well on their own unless other members of the microbial community are also present.  This indicates that the other microbes provide something that the Dehalococcoides need for optimal growth.  Sphaerochaeta bacteria extract energy from sugars by fermentation, generating a mixture of waste products that include acetate and H₂.  Dehalococcoides have a strict requirement for acetate as a carbon source, and they must use hydrogen as the electron donor for anaerobic respiration of organic chlorides.  Members of Sphaerochaeta may provide these critical substrates to Dehalococcoides.

S. globosa and S. pleomorpha are the best-characterized nonspiral spirochetes, but they were not the first round spirochetes to be found.  A report from 1992 described a round, cold-loving spirochete recovered from Ace Lake in Antarctica.  This spirochete is a member of the genus Spirochaeta, the closest relative of Sphaerochaeta.  More recently, another round nonmotile spirochete, Spirochaeta coccoides, was isolated from the hindgut of a termite.  Based on its genome sequence, reclassification of Spirochaeta coccoides into the genus Sphaerochaeta was proposed recently.  The residence of nonspiral spirochetes in such diverse environments could mean that they are more widespread than we think.


References

Caro-Quintero, A., Ritalahti, K.M., Cusick, K.D., Loffler, F.E., & Konstantinidis, K.T. (2012). The chimeric genome of Sphaerochaeta: Nonspiral spirochetes that break with the prevalent dogma in spirochete biology mBio, 3 (3) DOI: 10.1128/mBio.00025-12

Ritalahti, K.M., Justicia-Leon, S.D., Cusick, K.D., Ramos-Hernandez, N., Rubin, M., Dornbush, J., & Loffler, F.E. (2011). Sphaerochaeta globosa gen. nov., sp. nov. and Sphaerochaeta pleomorpha sp. nov., free-living, spherical spirochaetes INTERNATIONAL JOURNAL OF SYSTEMATIC AND EVOLUTIONARY MICROBIOLOGY, 62 (1), 210-216 DOI: 10.1099/ijs.0.023986-0

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Franzmann P.D., & Dobson S.J. (1992). Cell wall-less, free-living spirochetes in Antarctica. FEMS microbiology letters, 76 (3), 289-292 PMID: 1385265

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