Microbiology experiments require special techniques and precautions, for two good reasons: Health and Safety - to prevent the escape of any experimental microbes to the surrounding environment, where they may cause disease; Validity - to prevent contamination of experiments by microbes from the environment
Microbial cultures can have a very large concentration of cells in them, so you often start by making a serial dilution of your culture to give you a large range of concentrations
Counts viable cells by taking a small sample from a broth culture, diluting it, and spreading it on an agar plate to grow into colonies that can be counted
For most cultures there will be too many colonies to count, so a serial dilution is used to get a range of cell concentrations, and a sample from each dilution is spread on an agar plate
But in one of the dilutions there will be a good number (20-200) of individual colonies, and from this we can calculate the concentration of viable cells in the original culture
This method is very accurate, but tedious. It is the only good way to count viable cells, because only those cells that grow into living colonies are counted
1. Moulds growing on a solid agar plate tend to form fairly flat circular mycelia, so the area of the mycelium is an easy measure of fungal mass
2. If the mycelium is not circular it can be treated as an ellipse, and the area calculated using: area = π x r1 x r2, where r1 and r2 are the longest and shortest radii
Lag phase - cell division and cell death rates are both low, so the population does not change much
Exponential phase - cells divide without any external limiting factors, population doubles in a set time known as the generation time
Stationary phase - cell division slows down and cell death increases until the two rates are approximately equal, so the population of viable cells is roughly constant
Death phase - cell death rate is greater than cell division rate, so number of viable cells decreases