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Concept
Spiroplasma
Summary
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. Many Spiroplasma are found either in the gut or haemolymph of insects where they can act to manipulate host reproduction, or defend the host as endosymbionts. Spiroplasma are also disease-causing agents 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 poulsonii, a reproductive manipulator and defensive insect symbiont, Spiroplasma citri, the causative agent of citrus stubborn disease, and Spiroplasma kunkelii, the causative agent of corn stunt disease.
There is some disputed evidence for the role of spiroplasmas in the etiology of transmissible spongiform encephalopathies (TSEs), due primarily to the work of Frank Bastian, summarized below. Other researchers have failed to replicate this work, while the prion model for TSEs has gained very wide acceptance. A 2006 study 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.
Many Spiroplasma strains are vertically-transmitted endosymbionts of Drosophila species, with a variety of host-altering mechanisms similar to Wolbachia. These strains are from the Spiroplasma poulsonii clade, and can have important effects on host fitness. The S. poulsonii strain of Drosophila neotestacea protects its host against parasitic nematodes.
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