| Spiroplasma citri, Mollicutes, Acholeplasma, Phytoplasma, Wolbachia|
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 haemolymph 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 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 Dr. 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 endosymbionts of Drosophila species, with a variety of host-altering mechanisms similar to Wolbachia. Currently, a Spiroplasma species is receiving attention for its protective effects against parasitic nematodes in the fruit fly Drosophila neotestacea as a model for evolution through symbiosis. The Spiroplasma species restores fertility in flies infected with nematodes that otherwise sterilize females. This case study highlights a growing movement to consider heritable symbionts as important drivers in patterns of evolution. Spiroplasma are found in many insects and arthropods, including the plain tiger butterfly. When infected, male butterfly offspring are killed by the Spiroplasma, leading to interesting consequences for population genetics and consequently speciation.