Micro-organisms are able to grow rapidly under suitable conditions. Exponential growth occurs, where the number/mass of bacteria double every growth cycle.
The time taken for the number of bacterial cells to double is known as the doubling rate.
The growth of microbes can be graphed and has four main stages, as shown below.
There is little/no change in number of microbes but their metabolic activity is high. The microbes adjust to the specific conditions of the growth media and enzymes are induced to metabolise the substrates that are available.
The microbes multiply at a constant rate. This is the most rapid growth of micro-organisms, as long as there is a constant supply of nutrients and appropriate environmental conditions (temperature, pH, O2 levels).
The growth medium is depleted of nutrients and toxic metabolites are produced (pH usually increases as a result). Secondary metabolites are also produced. These are pathways and small molecules that are not essential for survival. Antibiotics are an example of secondary metabolites.
In nature, these can confer an ecological advantage to the micro-organisms which produce them. This can allow them to outcompete other micro-organisms.
During this phase, the number of bacteria being produced is the same as the number dying so there is no overall growth.
The microbes die due to the lack of appropriate nutrients and the build up of toxic metabolites.
When producing or interpreting growth curves of micro-organisms a semi-logarithmic scale is used because the data range on the y-axis is extremely large.
When carrying out a viable cell count only living micro-organisms will be included whereas when carrying out a total cell count both viable and dead cells are included. Only viable cell counts will show a death phase where the cell numbers are decreasing.