Fine morphology and phylogeny of Spiroplasma sp. isolated from eyes of scrapie sheep

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Spiroplasma sp. were isolated from eyes of sheep with terminal transmissible spongiform encephalopathy (scrapie). These isolates were previously reported to produce intracellular pathology typical for neurodegenerative diseases in experimentally infected cell lines (Bastian et al., 2014), but have not been characterized morphologically or genetically. The microorganisms were grown cell-free following a period of cultivation in bovine corneal endothelium cells. This paper presents light and electron microscopic description of these microorganisms, as well as 16S rDNA sequence-based phylogenetic analysis. The microorganisms were elongated cells of transient helicity up to 10 µm long, and about 100 nm in diameter. Transmission and scanning electron microscopies revealed variable morphology, including presence of helical and non-helical forms. The 16S rDNA from the novel isolates revealed >99.9% similarity to the orthologues from S. chrysopicola and S. syrphidicola, common commensals of flies of Tabanidae and Syrphidae families (Diptera). On 16S rDNA-inferred phylogenetic trees the scrapie sheep isolates fell into the S chrysopicola-S. syrphidicola-TAAS group of Diptera-associated species suggesting likely route of infection through flies contacting eyes of sheep in pastures. The question, whether microorganisms occasionally contaminate eyes of sheep with terminal scrapie upon contacts with flies, or spiroplasmas are linked with the disease, and tabanids play a role of their obligate or facultative vectors and reservoirs, warrants further investigation.