Starting with 10mL of original culture, you created three serial dilutions as fo

Question

Starting with 10mL of original culture, you created three serial dilutions as follows. You removed 0.1 ml from the original culture diluted it into 0.9 ml of broth to make dilution "1". You removed 0.1 ml from dilution "1" and added it to 0.9 ml of broth to make dilution "2". You removed 0.1 ml from dilution "2" and added it to 0.9 ml of broth to make dilution "3". You then plated 10 microliters from the each dilution onto agar plates. The next day, you counted that 10 microliter of dilution "1" produced TNTC colonies, dilution "2" produced 31 colonies, and dilutions "3" and "4" produced no colonies at all. What was the total number of bacteria in the original culture?

Explanation / Answer

0.1 ml = 100x dilution [Plate 1] = Too Many to Count 0.1 ml = 100x * 100x [Plate 2] = 31 0.1 ml = 100x * 100x * 100x [Plate 3] 0 Where is the data for plate 4?! Not needed, but whatever. 10 uL plated is = 0.001 milimeters = 1000x dilution factor Only plate 2 produced anything so: 100 * 100 = 10000 [Dilution Factor] 10000 * 1000 = 1 X 10^7 1 X 10^7 x 31 = 3.1 X 10^8 CFU/mL

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