The ultrastructure of the cytoplasmic membranes, cell walls, and mesosomes of Escherichia coli and Bacillus megaterium were found to be significantly affected by different fixation procedures.Logarithmic cells were divided into equal portions and different fixation procedures employed. A 6% glutaraldehyde solution followed by 1% 0s04 fixa-tion (1). The Ryter-Kellenberger (R-K) fixation (2) and Luft's ruthenium red (R-R) (3). The glutaraldehyde-OsO4 and R-K were post fixed with 2% and 0.5% uranyl acetate respectively. Thin sections were post stained with uranylacetate and lead citrate. One sub-portion of Bacillus megaterium was criti-cally-point dried. Glutaraldehyde-OsO4 critically-point dried whole mounts (Fig. 1) showed little background material and smooth walls. The R-K cells (Fig. 2) had irregular outer walls. The R-R cells (Fig. 3) were irregular and revealed external polysaccharide. There is a distinct difference in the internal morphology of the wall layers using the E. coli (Figs. 4,5,6). A heavier staining of the CM was observed in B. megaterium (Fig. 7).
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