|MadSci Network: Microbiology|
Hi Taylor -
You can find some additional information to help you with a search for CFU agar on our search engine.
The best way to answer your question involves analyzing the number of "colony forming units" or CFU in a defined volume of the water, commonly 0.1 milliliters (mL). To do this spread 0.1mL of your sample on a plate, incubate it, and count the number of colonies the next day, and after 2 days of incubation. Each colony grows from a single cell, so the number of colonies is related to the number of cells you put on the plate.
This procedure will not pick up *all* bacteria in a sample. Your culture conditions will likely be performed in ambient atmosphere, and at ~25°C (room temperature) or 37°C (body temperature). Some bacteria will not grow well, or at all, in the concentration of oxygen gas present in the air, or at the given temperature(s) you select. However, these conditions should be adequate for most species commonly encountered in environmental water sources. Furthermore, if you use *selective* media, you may only be culturing for particular kinds of bacteria. We commonly use an agar media called MacConkey agar to grow coliform bacteria such as E. coli and related species. If you use a nutrient media such as Brain-Heart Infusion, Sheep's Blood Agar, or Luria-Bretani (LB) agar, you may pick up additional species.
If you have a lot of bacteria, you may get a "lawn" of microbial growth on the plate, something of little help, other than it tells you that you probably have >5,000 CFU in your sample, or a swarming species of bacterium that took over the plate. If this happens, perform a dilution series to determine how many bacteria are in your sample.
Starting tube 10X dilution 100X dilution 1000X dilution 0.1mL --> +0.9mL | =1.0mL V take 0.1mL --> +0.9mL plate 0.1mL to | =1.0mL agar plate V take 0.1mL --> +0.9mL plate 0.1mL to | =1.0mL agar plate V | plate 0.1mL to V agar plate plate 0.1mL to agar plate `0.1 or 10-1 10-2 10-3 10-4 FINAL DILUTIONS PER MILLILITERYou then do the following:
# colonies X 10+dilution
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