This cannot be done with a fluid solution since 1) one cannot identify purity of the specimen, and 2) there is no way to enumerate the cells in a liquid. The agar plate allows accurate counting of the microorganisms, resulting from the equal distribution across the agar plate. The procedure described above produces a set of pour plates from many dilutions, but spread plates (sample spread on top of solidified agar) can be used also. Colonies grow within the agar, as well as on top of the agar and below the agar (between the agar and the lower dish). The agar solidifies, with the bacterial cells locked inside of the agar. A set of serial dilutions is made, a sample of each is placed into a liquefied agar medium, and the medium poured into a petri dish. This protocol is specific for bacterial counts (colony-forming units, CFUs), but can be modified for fungi (CFUs) and viruses (plaque-forming units, PFUs for viral counts). The purpose can be determination of bacterial, fungal, or viral counts (indirectly). The following is a step-by-step procedure to working dilution problems, and includes some practice problems at the end. Most specimens have high enough numbers of microorganisms that the specimen has to be serially diluted to quantitate effectively. A serial dilution is simply a series of simple dilutions which amplifies the dilution factor quickly beginning with a small initial quantity of. coli in nutrient broth all the way to soil samples and hamburger meat. And so forth for 3 more serial dilution steps (giving 1/8,000, 1/16,000, and 1/32,000). It is a common practice to determine microbial counts for both liquid and solid specimens-suspensions of E. In widely accepted single day absorbance QC. Understand how to quantify bacterial cells. Measurement of the dilution accuracy is not easily accomplished using traditional quality control methods.

Serial dilution vs simple dilution