Fine structure of Listeria monocytogenes in relation to protoplast formation.

Among the eight strains of Listeria monocytogenes tested for lysozyme sensitivity, two were resistant to lysozyme but became sensitive after lipase pretreatment. Among the other six, one was very sensitive to lipase and another one was extremely susceptible to lysozyme. Stable protoplasts were formed from the lysozyme-resistant strain (42) by lipase and lysozyme treatment, which completely digested the cell wall. The cell wall (uranyl acetate-lead stained) was of a thick triple-layered profile, with the intermediate layer of low density. Lipase treatment for a short time (60 min) did not cause any alteration in structure, but prolonged treatment (180 min) caused extensive digestion of the plasma membrane and the cell wall, liberating cytoplasmic material. When the cells were treated with either lipase or lysozyme, a small number of protoplasts were extruded through the partly digested or weakened transverse cell wall, leaving an almost intact cell wall ghost. There were small vesicular structures in the interspace between cell wall and plasma membrane. Mesosomes of varied organization were prominent in electron micrographs, both in sections and in negatively stained preparations. These were largely everted during protoplasting in the form of tubules and as small peripheral buds; a few small vesicles also remained as intrusive structures, some of which were very unusual because they appeared to be enclosed by the inner layer of plasma membrane alone. Lysis of the protoplasts by dilution of the sucrose, while maintaining a constant ionic environment, liberated many small vesicular structures and fibrillar nuclear material.