Growth of Micro organisms refers to increase in population size, I.e the number of individuals rather than the increase in cell size. Increase in population size is due to reproduction. Under favourable conditions, micro organisms reproduce asexually by binary fusion. A bacterium grows to a certain maximum size, then it undergoes binary fission to form two daughter cells. A species capable of rapid growth can divide every 20 minutes. This time interval is known as doubling or generation time. Generation time varies from species to species and is greatly dependent on environmental factors such as availability of food, space, temperature, etc.
Ideal Growth
Under idea laboratory conditions, the generation time is about
• 30 minutes for Escherichia coli, and
• 12 hours for mycobacterium tuberculosis.
Theoretically, one E. Coli would divide in two two after 30 minutes, the two to four in the next 30 minutes, and so on. The E.coli population is doubling in size at intervals of 30 minutes (or at every generation). This type of population increase is exponential or logarithmic. We get
•, an exponential curve by plotting the number the number of bacteria against time, and
• a straight line graph by plotting the logarithm of the number of bacteria against time.
Actual Growth
The growth of e. Coli can be observed by inoculating the bacteria into a liquid culture medium and measuring the growth at regular intervals of time. The resulting growth curve shows
• an initial slow growth phase (lag phase)
• an exponential phase,
• a stationary phase,
• a phase of decline.
When E. Coli us introduced into the culture medium, it takes some time to adjust to its new environment. This accounts for the initial slow growth phase. After that growth proceeds at maximum rate as shown by exponential phase. However, as population size increases, there is competition for food and space. This slows the growth rate until the population ceases to grow. This is the stationery phase. As toxic waste increase and food decreases, the cells stop dividing. The dormant cells begin to die causing a decline in growth rate.
