Typical growth curve of bacterial population in enclosed vessel (Batch Culture)
In the Phases of Bacterial growth curve, we discuss reflection of events in bacterial population. In this post we are discussing about bacterial population growth in a closed system of microbial culture of fixed volume (i.e. Batch Culture). The typical growth curve for population of cells can be divided into several distinct phases called the lag phase, Exponential phase (log phase), stationary phase and death phase.
Lag phase: When a microbial population is inoculated into a fresh medium, growth usually does not begin immediately but only after a period of time called the lag phase, which may be brief or extended depending on the history of culture and growth conditions.
Condition of bacterial culture used:
- Exponentially growing culture inoculated into same medium: No lag phase.
- Old or stationary culture inoculated into 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 damage plus time required to synthesize cell constituents)
Exponential Phase: This is the phase of bacterial growth curve in which the bacterial cell numbers doubles during each unit time period. When the cell number from such as experiment is graphed on arithmetic coordinates as function of elapsed time, one obtains a curve with a constantly increasing slope. The rate of increase in cell number is slow initially but in later stage the cell numbers increases explosively. The rate of exponential growth varies between bacterial genera (i.e. Genetic characteristics of bacteria) and is also influenced by environmental conditions.
Interesting and unbelievable fact about bacterial growth: If a single bacterium (10-12 (Note: i.e. ten power minus 12, word press is not supporting superscript) gram with a generation time of 20 min would, if it continued to grow exponentially for 48 hour, produce a population that weighed about 4000 times the weight of the earth.
In batch culture, exponential growth cannot occur indefinitely because the essential nutrients of the culture medium are used and waste products of organisms build in the environment. In stationary phase there is no net increase or decrease in cell number. The cells functions such as energy metabolism and some biosynthetic processes go on.
Death 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 that of exponential growth.