Samples were prepared by diluting the cell culture broth
with the appropriate amount of isotonic diluent (Fisher Scientific,
Pittsburgh, PA). The cell concentration was determined
by a microprocessor-controlled electronic particle
counter, Coulter Counter model ZM with a Channelizer
model C256 (both from Coulter Electronics, Inc., Hialeah,
FL) as well as by occasional confirming counts using a
hemacytometer. Since the average cell size was highly variable
(about 100 fL/cell or pm3/cell while the cells are actively
growing and about 30 fL/cell at stationary phase), the
total cell volume [ = cell concentration (cells/mL) X average
cell volume (fL/cell)] was used to calculate the volumetric
specific growth rate and thus the doubling time.