The osmolality of the fixative was adjusted with NaCl to equal that of the growth medium.
After four washes with the same buffer, cells were embedded in 2% (w/v) agar. The agar block was divided into
small cubes which were fixed with buffered 1 % (w/v) OsO, for 90 min at 5 "C. The cubes were dehydrated by
passing them through an ascending series of acetone and were then embedded in Epon. Electron micrographs
were recorded with an Opton electron microscope.