Spiroplasmas: Agents of Animal and Plant Diseases

Abstract

Mycoplasmas are placed in a separate class, Mollicutes, which removes them from bacteria. The fundamental characteristics of Mollicutes are the absence of a cellular wall and the inability to synthesize the peptidoglycan polymer or its precursors. The lack of a cell wall accounts for the pleomorphism of mycoplasmas, their osmotic sensitivity, susceptibility to lysis by detergents and alcohol, the ability to grow in fibrillar agar gel, and resistance to antibiotics (such as penicillin) that inhibit peptidoglycan polymerization and synthesis. The genome size of mycoplasmas is less than one-half the size reported for most bacteria. Taxonomically, the class Mollicutes comprises three families: Mycoplasmataceae, Acholeplasmataceae, and Spiroplasmataceae. Spiroplasmas constitute a new group of microorganisms, recognized less than a decade ago. Their discovery was preceded by the breakthrough made in Japan in 1967, when several plant diseases, earlier considered to be caused by viruses, were found associated with mycoplasma-resembling microorganisms (Doi et al. 1967, Ishiie et al. 1967, Nasu et al. 1967). Japanese plant pathologists and entomologists reported that plants with so-called yellows-type and witches' broom diseases contained wall-less prokaryotes in sieve elements, and that similar microorganisms were present in insect vectors carrying the infectious agents from plant to plant. Diseased plants treated with tetracycline antibiotics recovered temporarily, but suffered