To grow Escherichia coli in liquid medium, a small amount of a growth medium (5ml or less) is inoculated with few cells (a colony grown on LB-agar plate) of E. coli. Inoculated medium is incubated 37°C with shaking. As E. coli cells multiply the culture becomes a bit hazy. The culture growth with time can be monitored by measuring the optical density at 600 nm (OD600). Increasing OD600 values suggest that E. coli is multiplying and the cell density is increasing.
E. coli growth in liquid culture follow standard growth curve that has different growth phases:
Lag phase: When the liquid culture is freshly inoculated with a colony of bacteria, E. coli cells take some time to adapt in this new atmosphere. It is a preparative phase and is characterized by no growth (Bertrand 2019).
Log phase. The slow growth phase is followed by the exponential growth phase, the log phase. In this growth phase bacterial cells divide rapidly. This phase is characterized by availability of enriched medium and oxygen, and low concentration of toxic metabolic waste. How fast E. coli cells will divide depends on the type of growth medium. Usually in rich growth medium, most laboratory strains of E. coli have doubling time between 20 – 30 min.
Log phase can also be divided into three subphases: early-log phase, mid-log phase, and late-log phase.
Early log phase is the transition period from lag to mid-log phase and is characterized by E. coli cells gaining momentum to divide fast while late log phase is a transition from mid-log phase to stationary phase and is characterized by losing the momentum to divide fast. It happens due to the accumulation of toxic wastes and non-availability of nutrients and oxygen in the solution.
Stationary phase: Stationary phase is characterized by slow cell division and no significant increase of cell density due to an equilibrium attain between cell division and cell death. The stationary phase represents a plateau in the growth curve. This happens due to accumulation of substantial amounts of waste and scarcity of nutrients in the growth medium (Pletnev et al., 2015).
- Bertrand 2019. Lag Phase Is a Dynamic, Organized, Adaptive, and Evolvable Period That Prepares Bacteria for Cell Division. J Bacteriol. 201(7), e00697-18. PMID-30642990; Full Text Links: asm, PMC6416914
- Pletnev et al., 2015. Survival guide: Escherichia coli in the stationary phase. Acta Naturae. 7(4), 22-33. PMID-26798489; Full Text Link: PMC4717247