Electron microscopy. Cells were fixed with 3% (v/v) glutaraldehyde in cacodylate buffer pH 7.5 for 90 min at
room temperature. 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.