The influence of environmental stress on the distributions of times to first division in populations of Escherichia coli as determined by digital image analysis of individual cells

School of Food Biosciences, The University of Reading, PO Box 226, Whiteknights, Reading RG6 6AP United Kingdom; Dstl Porton Down, Salisbury, Wiltshire, SP4 0JQ, United Kingdom

^* To whom correspondence should be addressed. Email: gwniven{at}dstl.gov.uk.

	Abstract

The distributions of times to first cell division were determined for populations of stationary-phase cells of Escherichia coli inoculated onto agar media. This was accomplished by using automated analysis of digital images of individual cells growing on agar, and calculation of the "box area ratio". Using approximately 300 cells per experiment, the mean time to first division and standard deviation for cells grown in liquid medium at 37 °C and inoculated on agar and incubated at 20 °C were determined as 3.0 h and 0.7 h, respectively. Distributions were observed to tail towards the higher values, but no definitive model distribution was identified. Both pre-inoculation stress by heating cultures at 50 °C and post-inoculation stress by growth in the presence of higher concentrations of NaCl increased mean times to first division. Both stresses also resulted in an increase in the spread of the distributions that was proportional to the mean division time, the coefficient of variation being constant at approximately 0.2 in all cases. The "relative division time", which is the time to first division for individual cells expressed in terms of the cell size doubling time, was used as measure of the "work to be done" to prepare for cell division. Relative division times were greater for heat-stressed cells than for those growing under osmotic stress.