I am doing an experiment in which I am growing S. mutans in agar dishes, and I am not sure how I would measure the growth of the S. mutans. I am also not sure if I would do this by measuring cell mass or by cell count. Any ideas?
The most simple way is seeding the plates with a suspension of bacteria ensuring that you spread the solution properly. Then you can count the number of colonies, wich would be equal to the number of single cells.
If you want to mesure the growth speed, usually it's simpler to just measure the diameter of the colonies, always ensuring you inoculate the plates with the same amount of inoculum.
Lastly, it's even easier to estimate the growth if your culture is un liquid media and you measure the optic density with an spectophotometer.
you will almost certainly want to perform serial dilutions of your suspension, and plate the serial dilutions using the spread plate method (or pour plate method). The dilutions will make it much easier to count the number of bacteria.
Generally, it is not recommended to count more than 250 colonies on a plate for reasons of accuracy (but I never count more than like 50, because that is the whole reason for doing serial dilutions). Here is a nice picture of what I am talking about. It includes a description of how to calculate the number of organisms in the original solution. They have chosen to count the number of bacteria on the fourth plate (1:10,000 dilution) in order to estimate the number of bacteria in the original inoculum. Hope this helps]1