Variations in Growth Rate of Closely Related Escherichia coli Strains

Growth rate of closely related bacterial strains depends sensitively not only on their growth conditions but also on their genetic make-up. To investigate the effect of small genetic variations on different E. coli K12 sub strains, the growth rate of two strains, MG1655 and BW25113, was measured and compared in different growth conditions. The genetic differences between these two sub strains are well documented. The growth rate was obtained through measurements of the optical density (OD) of a cell culture using a spectrophotometer. The OD was assumed to be proportional to the number of cells in the culture and the growth rate was determined from the linear portion of the cells log(OD) vs time curve. Cells were grown in 12 ml plastic tubes with 250 rpm shaking, while the temperature and composition of growth medium was varied. The data collected indicated that an increased temperature and/or a glucose medium sped up growth, while a decreased temperature and/or a glycerin medium slowed down growth. MG1655 had an average doubling time of 148 ± 8 and 102± 8 mins at 28°C and 37°C in glucose, and 280 ± 35 and 106 ± 13 mins at 28°C and 37°C in glycerol. BW25113 had an average doubling time of 151 ± 8 and 76 mins at 28°C and 37°C in glucose, and 223 ± 23 and 103 mins at 28°C and 37°C in glycerol. In conclusion, our measurements show that BW25113 despite harboring additional genetic deletions grow faster than its parental strain MG1655 in minimal medium. One needs to be aware of these differences when comparing published results from these two sub strains