Direct Methods

1. Microscopic counts – A set volume of a liquid culture is placed onto a microscope slide and one actually counts the number of cells.  The advantage of this method is that no incubation time is required to obtain results.  However, it can take a great deal of time to count that many cells and there is no way to distinguish between living and dead cells.
2. MPN (Most Probable Number) – This is a statistical estimation that will be discussed in greater detail later in the semester.
3. Standard Plate Count (SPC) – In this method, a liquid culture undergoes a series of dilutions.  Set aliquots of the final, dilute solution are spread onto an agar plate and the plates are incubated overnight.  Every cell that was spread onto the plate develops into a visible colony.  These colonies can be counted and since they originated from a single cell they represent the number of cells in the final aliquot that was spread onto the plate.  The dilutions made from the original culture can then be taken into account and one can work backwards to determine the concentration (titer) of the original culture.

Today in lab, we will use the Standard Plate Count.  In order to do a standard plate count, one must understand how to figure dilutions.  Before beginning the dilution section, review "exponents" in the Appendix.

Rachel Watson, M.S.
AG 5010
766-3524
Cell: 307-760-2942
rwatson@uwyo.edu