When bacteria are inoculated into a suitable culture media into an enclosed vessel, such as a tube or a flask, and incubated, their growth follows a definite course. A bacterial growth curve is obtained when a bacterial count of such culture is determined at different time intervals and plotted.
A typical bacterial growth curve consists of four phases: lag, log, stationary, and death. This bacterial growth curve reflects the events in the bacterial populations when grown in a closed system of microbial culture of fixed volume (i.e., batch culture). This classical growth curve for batch cultures of bacteria was proposed by Buchanan (1918).
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When a microbial population is inoculated into a fresh medium, growth usually does not begin immediately but only after a period called the lag phase. The duration of the lag phase may be brief or extended depending on the history of culture and growth conditions.
- No lag phase: If exponentially growing culture is inoculated into the same medium under the same condition of growth:
- Old or stationary culture inoculated into the same medium: lag phase (time required for bacteria to synthesize essential cell constituent and to be able to go for binary fission).
- Damaged bacterial culture (heat, radiation, or toxic chemicals): long lag phase (time required to repair damaged cells and time required to synthesize cell constituents)
Log phase (exponential phase)
In this phase, bacterial cell numbers double during each unit (i.e., generation time). For example, a culture containing 1,000 organisms per ml with a generation time of 20 minutes would contain 2,000 organisms per ml after 20 minutes, 4,000 after 40 minutes, 8,000 after 60 minutes, and 16,000 after 120 minutes. The generation time of most bacteria is between 20 minutes to 20 hours. Mycobacterium tuberculosis and Mycobacterium leprae have longer generation times.
When the cell number from such an experiment is graphed on arithmetic coordinates as a function of elapsed time, one obtains a curve with a constantly increasing slope. The increase in cell number is slow initially but in the later stage, the cell numbers increase explosively. The rate of exponential growth varies between bacterial genera (i.e. genetic characteristics of bacteria) and is also influenced by cultural conditions.
Logarithmic growth can be maintained using a device, a chemostat. The chemostat has a reservoir and growth chamber. Fresh medium is continuously added to the growth chamber, and the old medium is removed.
Interesting and unbelievable fact about bacterial growth: If a single bacterium (having weight 10-12 gram) with a generation time of 20 min continued to grow exponentially for 48 hours, produce a population that weighed about 4000 times the weight of the earth.
The number of new cells produced balances the number of cells that die, resulting in a steady state. In batch culture, exponential growth cannot occur indefinitely because the culture medium’s essential nutrients are used and organisms’ waste products accumulate in the environment. There is no net increase or decrease in cell number in the stationary phase. The cells function, such as energy metabolism and some biosynthetic processes. (Note: cells grown in a chemostat do not enter the stationary phase)
If incubation continues after a bacterial population reaches the stationary phase, the cells may start dying. Cell death may be due to cell lysis, and this is also an exponential process but much slower than exponential growth. During the decline phase, many cells undergo involution, i.e., assume various unusual shapes. Spore-forming organisms form spores as a part of survival mechanisms.
References and further readings
- Madigan, M., Martinko, J., Stahl, D., & Clark, D. (2012). Brock Biology of Microorganisms (13th ed). Pearson Education
- Pelczar Jr., M., Chan, E., & Krieg, N. (2007). Microbiology (5th edition). Tata McGraw-Hill.