Escherichia coli (E. coli) grows in liquid medium through a process of binary fission, where one cell divides into two. Under optimal conditions, this bacterial growth is rapid and can be quantified.
Optimal Growth Conditions and Doubling Time
When E. coli is provided with optimal growth conditions in a rich liquid broth medium such as Luria-Bertani (LB) broth, it exhibits a characteristic growth pattern:
- Temperature: A temperature of 37°C is ideal for optimal growth.
- Aeration: Sufficient aeration is necessary as E. coli is a facultative anaerobe that prefers aerobic conditions.
- pH: The medium should have a neutral pH around 7.0 for the best results.
- Nutrients: A rich medium like LB broth provides essential nutrients for rapid growth.
Under these conditions, E. coli has:
- Doubling Time: A doubling time of approximately 20 minutes, meaning the population doubles in number every 20 minutes.
- Cell Density: It will reach an overnight cell density of approximately 109 colony-forming units per milliliter (cfu/mL) of culture.
Phases of Growth
While the reference focuses on optimal growth, here's a more complete look at growth phases:
- Lag Phase: When initially inoculated into fresh medium, there's a lag phase where E. coli adjusts to its new environment. There's little to no cell division at this point.
- Exponential (Log) Phase: Once adapted, E. coli enters the exponential phase with rapid, consistent doubling, as described in the reference. The population increases exponentially and the doubling time is usually constant during this phase.
- Stationary Phase: Eventually, due to nutrient depletion or the accumulation of waste products, growth slows and enters a stationary phase. New cells are produced at the same rate as old cells die.
- Death Phase: If conditions remain unfavorable, the death phase begins and the population decreases.
Practical Insights
- Maintaining Aeration: When growing E. coli in liquid culture in a flask, the culture must be agitated or shaken to ensure sufficient oxygenation.
- Monitoring Growth: Turbidity (cloudiness) of the culture can be measured using a spectrophotometer to estimate cell density. A higher turbidity reading usually means a higher concentration of cells.
- Subculturing: To maintain cells in the exponential phase, they should be subcultured into fresh media before reaching the stationary phase.
Conclusion
E. coli growth in liquid media is a well-defined process with rapid doubling times when provided with ideal conditions such as rich nutrients, proper temperature, aeration, and pH. The cells go through different phases of growth, each having distinct characteristics, from lag to death phase, but exponential growth is characterized by doubling roughly every 20 minutes under ideal conditions like those found in Luria-Bertani broth.
