Electron microscopy of ultra-thin sections of bacteria. II. Sporulation of Bacillus megaterium and Bacillus cereus.

The lack of clarity of the cytological representations of non-nuclear structures appearing in figures which accompany various papers describing the sporulation process has always left the author somewhat dissatisfied. Nuclear elements typically are distinctly stained while the cytoplasm and cell walls usually are either indistinguishable or very faintly discernible. The techniques which have been developed for these studies have admittedly been designed to demonstrate the nucleus. That this aim has been achieved, with some degree of success, can be seen by observing the figures provided by Knaysi (1955), Fitz-James (1954), Robinow (1953a), and DeLamater and Hunter (1952). Since it is frequently necessary in cytology to sacrifice one structure in order to demonstrate another, the lack of appearance of non-nuclear structures in the above preparations in no way detracts from the contributions made. A desire to determine what structures could be observed in the cytoplasm of sporulating cells led the author to apply the techniques of ultrathin sectioning to the problem. This procedure was described in the preceding paper in this series (Chapman and Hillier, 1953). Following the publication of this article, there have appeared several papers which have presented the results obtained when ultra-thin sectioning techniques were used. All of the papers owe much of their merit to the added resolution provided by the electron microscope, used in conjunction with ultra-thin sections. Thus, ultra-thin sections have provided striking information concerning cellular division (Chapman and Hillier, 1953), nuclear division (Bradfield, 1954), effect of culture age on cell appearance (Birch-Andersen et al., 1953), spore structure (Robinow, 1953b), phage-host interaction ('Maal0e et al., 1954), and the cytology of sporulation (this paper). The only attempt to add to the techniques in this new area of bacterial study was made by Bradfield, who developed a modification of the Feulgen stain. Although the results presented in all of the papers have been highly informative and often dramatically clarifying and have helped to establish the basis upon which further analytical work may proceed, it is only by the development of auxiliary techniques that we may expect to obtain a substantial part of the information which ultra-thin sectioning promises to reveal.