SPIROPLASMAS - INFECTIONS AGENTS OF PLANTS, ARTHROPODS AND VERTEBRATES

The spiroplasmas are mollicutes characterized by motility and helical morphology. They were discovered through studies on corn stunt and cit rus stubborn diseases. The stubborn agent was the first mollicute of p lant origin to be obtained in culture and the first cultured mollicute to possess a helical morphology. The citrus pathogen has been known a s Spiroplasma citri since 1973. The corn stunt agent was cultured in 1 975 and fully characterized as Spiroplasma kunkelii by 1986. The third and only other phytopathogenic spiroplasma is Spiroplasma phoeniceum, cultured from naturally infected periwinkle plants in Syria and descr ibed in 1986. These three spiroplasmas are restricted to the phloem si eve-tubes of the infected plants and are transmitted from plant by var ious phloem feeding leafhopper vectors in which the spiroplasmas multi ply. Following the pioneering work on S. citri and S. kunkelii, close to fifty other spiroplasma species or proposed species have been disco vered. All spiroplasmas have been isolated from insects, ticks and pla nts. insects are particularly rich sources of spiroplasmas. Some insec t-derived spiroplasmas are entomopathogens. S. melliferum and S. apis are honey bee pathogens. They cross the insect-gut barrier and reach t he hemolymph, where they multiply abundantly and kill the bee. Spiropl asma floricola is the agent of lethargy disease of Melolontha melolont ha (cockchafer). Spiroplasma poulsonii infects the neotropical species of Drosophila, is transmitted transovarially and kills the male proge ny of an infected female fly, hence the name sex ratio spiroplasma. So me insect-derived spiroplasmas are also found on plant (flower) surfac es. For instance, S. apis was cultured from the surfaces of flowers gr owing in the vicinity of affected beehives. This suggests that the pla nt surface spiroplasmas are deposited on these surfaces by contaminate d insects. Many insect spiroplasmas are not pathogenic, are often rest ricted to the gut and may be regarded as mutualists or incidental comm ensals. Of the three known tick spiroplasmas, only Spiroplasma mirum o btained from rabbit ticks is pathogenic to the vertebrate animal (chic k embryo, new-born rodents, adult rabbit), but only upon experimental inoculation of the spiroplasma. Strain SMCA induces high incidence of cataracts in new born rodents. With strain GT-48 no cat aracts are obs erved, but fatal encephalitis occurs. Spiral membranous inclusions res embling spiroplasmas have been seen in brain biopsies taken from patie nts with Creutzfeldt-Jakob disease. However, failure to detect spiropl asmas by serology and culture points to the absence of spiroplasmal in volvement in spongiform encephalopathies. Transposon Tn 4001 mutagenes is has been applied for the first lime to Spiroplasma citri, and patho genicity can now be studied at the genetic level. One Tn 4001 mutant d oes not multiply in the leafhoppers and is, therefore, not transmitted to the plant. Another mutant multiplies well in the plant and is tran smitted to the plant, where it reaches high titers, but without induci ng symptoms in the plant. In this non-phytopathogenic mutant, Tn 4001 is inserted in the spiroplasmal fructose operon, and the mutant is una ble to use fructose. Finally, to study involvement of spiroplasma[ mot ility in pathogenicity, a non-motile mutant has been obtained. Motilit y was restored by complementation with the wild type genes. This is th e first time that successful complementation has been reported, not on ly in the spiroplasmas but in the mollicutes in general. Undoubtedly, studies an pathogenicity have entered a new era.