Bacterial population growth studies require cultivation of viable cells in a fresh sterile broth medium and incubation in a closed culture vessel with a single batch of medium under optimum temperature, pH, and gaseous conditions. Under these conditions, the cells will reproduce rapidly and the dynamics of the microbial growth can be charted by means of a population growth curve, which is constructed by plotting the increase in cell numbers versus time of incubation and can be used to delineate stages of the growth cycle. Growth involves an increase in cell mass and number of ribosomes, duplication of the bacterial chromosome, synthesis of new cell wall and plasma membrane, partitioning of the two chromosomes, septum formation, and cell division.
Generation time is the time required for bacteria to grow and divide i.e. one complete cell division. Some microbes are able to divide as rapidly as once every 12 to 15 minutes, others require up to several hours, and a few very slow growing bacteria may require more than 24 hours per cell division.
Because no fresh medium is provided during incubation, nutrient concentrations decline and concentrations of wastes increase. The growth of microorganisms reproducing by binary fission can be plotted as the logarithm of the number of viable cells versus the incubation time. The resulting curve has four distinct phases
The growth of microorganism can be measured by: –
1- increase in size but this a poor criterion of growth.
2- increase in the number of bacterial cell by either counting the number of living cells (viable count) or all cells (total count).
3- measurement of some component of cell structures such as protein or DNA as an indication of microbial increase (growth) or decrease (death).
Bacterial Growth Curve
When microorganisms are grown in a suitable liquid medium (batch culture or closed system) and incubated its growth follows a definite process. If bacterial counts are carried out at intervals after inoculation and plotted in relation to time, a growth curve is obtained.
The typical growth curve is divided into the following phase:
1. Lag phase
2. Log phase or exponential phase
3. Stationary phase
4. Death or decline phase
1. Lag phase
When a bacterial population is inoculated into new fresh media the cells do not reproduce immediately in a new medium. During the lag phase, bacteria take some time adapt themselves to the new growth conditions.
The lag phase is characterized by
• No cell division
• No increase in the number of cells.
• Increase in size of bacteria
• Synthesis of RNA, enzymes, and co-enzymes for physiological activities.
• Duration of the lag phase varies according to conditions and species of bacteria. For example, if the culture microorganism is taken from old culture, the duration will be longer but if the culture is fresh, duration is short. Likewise, if the culture media is different from the previous culture then duration is long because bacteria takes some more time to adapt to the fresh media.
2. Log or exponential phase
During exponential phase,
• Microorganisms start dividing at a constant rate
• Bacterial cell numbers double with time
• Rate of growth remains constant
• Bacteria have smallest size
• Generation time is shortest during this phase
• The rate of exponential growth varies between bacterial genera and is also influenced by cultural conditions.
A stationary phase is attained at a bacterial population level of around 109 cells per ml. During stationary phase,
there is no net increase in the number of bacterial cells
• Cell division stops due to nutrient exhaustion and accumulation of toxic products.
• The viable count remains stationary as equilibrium exists between the dying cells and the newly formed cells.
• Production of antibiotics such as Penicillin, streptomycin etc and enzymes by certain bacteria occur during this phase
• In endospore forming bacteria, sporulation occur as the bacteria enter stationary phase.
4.Phase of decline
This is the phase when the population decreased due to cell death. Since it is a closed system, there is no way to add nutrients or remove the waste products. Eventually, this leads to unfavourable conditions and a decrease in the number of living cells in the population.