Fundamentals of Microbiology

2.5.3 The growth Curve

When microorganisms are cultured in a liquid medium, they are grown in a batch culture or closed system. In this closed system or batch culture, they are incubated in a closed culture vessel with a single batch of medium. The growth of microorganisms reproducing by binary fission can be plotted as the logarithm of cell number versus the incubation time. The graph or curve has four distinct phases.

1. Lag Phase:

When microorganisms are inoculated into fresh culture medium, usually no immediate increase in cell number occurs. Therefore this period is called the lag phase. Lag phase is also called tooling up phase, as the cells prepare themselves for growth in the new medium. The cells synthesize new compounds prior to start of cell division. The lag phase varies considerably in length depending upon factors like: (1) Nature of the medium (2) condition of microorganisms.

Exponential phase:

During exponential or log phase, microorganisms grow and divide at rapid rate depending upon their genetic potential, the nature of medium and growth conditions. The rate of growth is constant during the exponential phase. The population is uniform in terms of chemical and physiological properties during this phase and therefore exponential phase cultures are usually used in biochemical and physiological studies.

Stationary phase

Population growth ceases and growth curve becomes horizontal. This phase usually is attained by bacteria at a population density of around 10⁹ cells per ml. Protozoans and algal cultures have maximum densities of about 10⁶ per ml. This occurs as the number of cells dividing would equal cell death and the number of viable microorganisms remains constant. Microbial populations enter the stationary phase due to

i. nutrient limitation

ii. Depletion of oxygen ( in case of aerobes)

iii. Production of toxic metabolites

Declining phase (or) Death Phase:

Changes like nutrient depletion and build up of toxic waste lead to decline in the number of viable cells. The death of the population is usually logarithmic similar to its growth.