Microbiology, 2021

9.1 • How Microbes Grow 339
Figure 9.12 In the pour plate method of cell counting, the sample is mixed in liquid warm agar (45–50 °C) poured into a sterile Petri dish
and further mixed by swirling. This process is repeated for each serial dilution prepared. The resulting colonies are counted and provide an
estimate of the number of cells in the original volume sampled.
Figure 9.13
In the spread plate method of cell counting, the sample is poured onto solid agar and then spread using a sterile spreader.
This process is repeated for each serial dilution prepared. The resulting colonies are counted and provide an estimate of the number of cells
in the original volume samples.
A very dilute sample—drinking water, for example—may not contain enough organisms to use either of the
plate count methods described. In such cases, the original sample must be concentrated rather than diluted
before plating. This can be accomplished using a modification of the plate count technique called the
membrane filtration technique. Known volumes are vacuum-filtered aseptically through a membrane with a
pore size small enough to trap microorganisms. The membrane is transferred to a Petri plate containing an
appropriate growth medium. Colonies are counted after incubation. Calculation of the cell density is made by
dividing the cell count by the volume of filtered liquid.
LINK TO LEARNING
Watch this video (https://openstax.org/l/22serdilpltcvid) for demonstrations of serial dilutions and spread plate
techniques.
The Most Probable Number
The number of microorganisms in dilute samples is usually too low to be detected by the plate count methods
described thus far. For these specimens, microbiologists routinely use the most probable number (MPN)
method, a statistical procedure for estimating of the number of viable microorganisms in a sample. Often used
for water and food samples, the MPN method evaluates detectable growth by observing changes in turbidity or
color due to metabolic activity.
A typical application of MPN method is the estimation of the number of coliforms in a sample of pond water.