Significations et usages de Spiroplasma

Spiroplasma is a genus of Mollicutes, a group of small bacteria without cell walls. Spiroplasma shares the simple metabolism, parasitic lifestyle, fried-egg colony morphology and small genome of other Mollicutes, but has a distinctive helical morphology, unlike Mycoplasma. It has a spiral shape and moves in a corkscrew motion. Most spiroplasmas are found either in the gut or hemolymph of insects, or in the phloem of plants. Spiroplasmas are fastidious organisms, which require a rich culture medium. Typically they grow well at 30°C, but not at 37°C. A few species, notably Spiroplasma mirum, grow well at 37°C (human body temperature), and cause cataracts and neurological damage in suckling mice. The best studied species of spiroplasmas are Spiroplasma citri, the causative agent of Citrus Stuborn Disease, and Spiroplasma kunkelii, the causative agent of Corn Stunt Disease.

  Corn Stunt Spiroplasma in phloem cells. Thick section (0.4 micrometers) observed in a TEM. Magnified 75,000X.

There is some disputed evidence for the role of spiroplasmas in the etiology of Transmissible Spongiform Encephalopathies (TSEs), due primarily to the work of Dr. Bastian, summarized below. Other researchers, such as Leach et al. (1983) have failed to replicate this work, while the prion model for TSEs has gained very wide acceptance. The most recent work of Alexeeva et al. (2006) appears to refute the role of spiroplasmas in the best small animal scrapie model (hamsters). Bastian et al. (2007) have responded to this challenge with the isolation of a spiroplasma species from scrapie-infected tissue, grown it in cell-free culture, and demonstrated its infectivity in ruminants.

According to Frank O. Bastian, MD:

"spiroplasmas contain internal fibrillar proteins, that have morphological and immunological similarities to scrapie- and CJD-related fibrillar proteins. This comparison is noteworthy since mycoplasmologists consider these fibril proteins unique to this prokaryote.
In vivo and in vitro experimental Spiroplasma infections produce cytopathic effects similar to those of the scrapie agent. Experimental Spiroplasma brain infection in the suckling rat is characterized by vacuolar encephalopathy with localization of the microbe to gray matter.
[...] Spiralins are chemically bound to Spiroplasma-associated fibrils (SpFs) and are separated with difficulty.' SpFs are unique internal fibrils of spiroplasmas with a molecular weight of 55 kDa. Recently, SpFs have been shown to bear close morphological resemblance to scrapie-associated fibrils (SAFS), ' and show cross-reactivity using SAF antibody."^{[verification needed]}

In addition, a Spiroplasma species had been shown to kill males of the Plain Tiger butterfly on infection, leading to interesting consequences for population genetics and consequently speciation similar to the effects caused by some strains of Wolbachia (Jiggins et al. 2000).

  References

• Alexeeva, I.; Elliott, E. J.; Rollins, S.; Gasparich, G. E.; Lazar, J. & Rohwer, R. G. (2006): Absence of Spiroplasma or Other Bacterial 16S rRNA Genes in Brain Tissue of Hamsters with Scrapie. Journal of Clinical Microbiology 44(1): 91-97. PMID 16390954 doi:10.1128/JCM.44.1.91-97.2006 PDF fulltext

• Bastian, F. O.; Sanders DE, Forbes, W.A.; Hagius, S.D.; Walker, J.V.; Henk, W.G.; Enright, F.M.& Elzer, P.H. (2007): Spiroplasma spp. from transmissible spongiform encephalopathy brains or ticks induce spongiform encephalopathy in ruminants. Journal of Medical Microbiology 56(9):1235-1252. PMID 17761489 doi:10.1099/jmm.0.47159-0

• Jiggins, F. M.; Hurst, G. D. D.; Jiggins, C. D.; Schulenburg, J. H. G. v. D. & Majerus, M. E. N. (2000): The butterfly Danaus chrysippus is infected by a male-killing Spiroplasma bacterium. Parasitology 120(5): 439–446. doi:10.1017/S0031182099005867 (HTML abstract)

• Leach, R. H.; Mathews, W. B. & Will, R. (1983): Creutzfeldt-Jakob disease. Failure to detect spiroplasmas by cultivation and serological tests. Journal of Neurological Science 59(3): 349-353. PMID 6348215 (HTML abstract)

  External links

• Spiroplasma may cause Creutzfeldt-Jakob Disease. An interview with a leading expert in infectious diseases: Frank O. Bastsian, MD.
• Spiroplasma & Transmissible Spongiform Encephalopathies, Ed Gehrman
• Spiroplasma Genome Projects from Genomes OnLine Database