A proposal for minimal standards for descriptions of new species of the class is presented here with the approval of the International Committee on Systematic Bacteriology (ICSB) and its Judicial Commission. The proposal is a revised and extended version of a similar proposal of minimal standards for descriptions of species of the order previously published by the Subcommittee. The major characteristics of the class are summarized. The classification of an organism as a member of the class is based primarily on the following criteria: Lack of a cell wall, typical colonial appearance, filterability through a membrane filter with a pore size 450 nm in diameter, and absence of reversion to a bacterium under conditions ordinarily favorable for reversion. Placement of these organisms into families is based on genome size, sterol requirements, and morphology; their assignment to genera is based on morphological, cultural, and biochemical properties. Demonstration that an organism differs from existing species requires determination of serological, cultural, and biochemical characteristics. Other tests may be useful or, under certain conditions, necessary.


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