Definition: An order of gram-negative bacteria containing cells that are bounded by a three-layered membrane but do not possess a true cell wall. The minimal reproductive units are 0.2–0.3 mcm in diameter. Pathogenic and saprophytic species occur. These organisms reproduce through the breaking up of branched filaments into coccoid, filterable elementary bodies. The order includes the so-called pleuropneumonialike organisms, under organism (PPLO).