You dilute an unknown sample of bacteria to 1 x 10-6 and plate out 10 onto a Petri dish. The next day you count 64 colonies. The number of viable bacteria in the original sample was:. a. 6.4x105 cells/ul b. 6.4x 106 cells/ul c. 6.4 x 107 cells/ul d. 6.4 x 108 cells/ul

b

Explanation:

The correct answer would be 6.4 x cells/ul.

For 1 x 10-6 dilution, the plating factor is . To find the number of bacteria in the original sample, the plating factor is multiplied by the colony count per ul.

Number of viable bacteria = plating factor x colony count/ul

Since 10 ul gives 64 colonies.  1 ul will give:

         64/10 = 6.4 colonies.

Therefore, the viable bacteria in the original sample = 6.4 x cells/ul

The correct option is b.