|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.|
|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.
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