The baby-machine system, which produces newborn Escherichia coli cells from
cultures immobilized on a membrane, was developed many years ago in an att
empt to attain optimal synchrony with minimal disturbance of steady-state g
rowth. In the present article, we describe in some detail a model designed
to analyse such cells with a view to characterizing the nature and quality
of the synchrony in a quantitative manner; it can also serve to evaluate th
e methodology itself, its potential and its limitations.
The model consists of five elements, giving rise to five adjustable paramet
ers (and a proportionality constant): a major, essentially synchronous grou
p of cells with ages distributed normally about zero; a minor, random compo
nent from a steady-state population on the membrane that had undergone only
very little age selection during the elution process; a fixed background c
ount, to account for the signals recorded by the electronic particle counte
r produced by debris and electronic noise; a time-shift, to allow for diffe
rences between collection time and sampling time; and the coefficient of va
riation of the interdivision-time distribution, taken to be a Pearson type
III.
The model is fitted by nonlinear least-squares to data from cells grown in
glucose minimal medium. The standard errors of the parameters are quite sma
ll, making their estimates all highly significant; the quality of the fit i
s striking.
We also provide a simple yet rigorous procedure for correcting cell counts
obtained in an electronic particle counter for the effect of coincidence. A
n example using real data produces an excellent fit. (C) 2001 Academic Pres
s.