Spiroplasma spp.: a plant, arthropod, animal and human pathogen

Mollicutes is a class of smallest and free-living bacteria. They have no cell wall and their plasma membrane contains cholesterol; nevertheless, cellular organization does not dif- fer from that of other prokaryotes. They are used as simple model systems for studying general biological problems, such as those concerning membrane structure and func- tions, symbiosis between arthropods and microrganisms, animal and plant pathogens. Mollicutes includes the family of Spiroplamataceae, which contains Spiroplasma genus, a group of species associated, in different manner, with arthropods (insects, mites, crusta- ceans). Spiroplasma species can be commensals or parasites and even be involved in more close symbiosis, such as synergism or mutualism. Out of 38 described Spiroplasma spe-cies, only three have been associated with plant diseases and three with arthropod dis-eases. Moreover, some species have been related to animal diseases, such as transmissible spongiform encephalopathy (TSE), and their role in human disease has been assessed. The chapter describes the taxonomic situation of the genus and reports the most impor-tant diseases due to the presence of Spiroplasma in different living organisms with special emphasis on citrus in which it causes one of the most economically damaging infectious diseases in a number of citrus growing areas worldwide.