Ultrastructure of Brucella abortus L-forms induced by penicillin in a liquid and in a semisolid medium

Brucella abortus L-forms were induced by 5.0 or 10.0 mug of penicillin/ml in a broth medium containing 0.3 m sucrose, and in a semisolid medium containing 10% calf serum and 20.0, 40.0, or 60.0 mug of penicillin/ml. After 96 hr of incubation, L-forms of various sizes and shapes were observed. Basic structures of the L-forms were similar whether induced in liquid or semisolid medium. L-forms had two "unit" membranes, each consisting of two outer dense layers separated by a lucent layer. A few large, irregularly shaped organisms in penicillin-treated broth cultures had additional surface material and were referred to as "transitional" forms. In contrast with L-forms, the bacterial cells were fairly uniform in size and shape, were smaller, and had a more complex cell wall structure. Small bodies limited by a "unit" membrane were present within and around numerous L-forms from liquid and semisolid medium cultures. Other internal membranous structures were also seen in some L-forms. Most Brucella L-forms described in this paper reverted to bacteria in the absence of penicillin and were structurally characteristic of unstable L-forms.