Reversion of 3A type L forms of Proteus mirabilis.

Reversion of L forms to the normal bacterial state has been described in general terms in a variety of bacterial cell-L-form systems (Dienes and Weinberger, 1951). The relatively large 3B type L colony formed on penicillin-containing medium is composed entirely of spherical bodies of varying sizes (2 to 10 ,u in diameter) that appear to propagate by a budding process. In contrast, 3A type L forms, also induced by penicillin, produce a tiny, subsurface colony composed of very small, deeply staining granules. Estimated rates of L form reversion vary widely from rapid and efficient reversion of 3B type L bodies to rare and delayed reversion of 3A type L forms and finally to the apparently permanent (stable) 3A L forms encountered in many investigations (Kawatomari, 1958; Medill-Brown, Hutchinson, and Cocklin, 1960; Freimer, Krause, and McCarty, 1959; Sharp, Hijmans, and Dienes, 1957). There is general agreement that 3A type L forms represent a more extensive departure from normal bacterial cells than do 3B L forms (Taubeneck and Schuhmann, 1959) and consequently that reversion to the bacterial state by 3A L bodies is basically a more complex phenomenon than reversion of 3B type L bodies. The studies reported below present the initial attempts to define quantitatively some parameters of reversion of 3A type L forms of Proteus mirabilis. These data show that reversion of such L forms is indeed a rare event but with a definite probability of occurrence which can be altered by modifications of the growth environment. Furthermore, the available evidence indicates that reversion of these L forms is a physiological and not a genetic event. Finally, the quantitative characteristics of reversion of 3A L bodies suggest a general explanation for permanence or impermanence of L forms.