Question

14. Analyze the following serial dilution experiment and answer the questions, each (a and b) worth 10 pts. Given: One ml of the original bacterial culture (tube 0) was added to experiment tube 1, containing 9 ml of media. 7 more experiment tubes were serially diluted in order, the same way, for a total of 8. Each originally contained 9 ml of media, and each received 1 ml from the tube number preceeding it. Two drops from tube #8 were spread across a culture plate and incubated. After one week, a total of 30 small spherical colonies were counted on the culture plate. We know each drop contains 50 microliters (50 x 10-2 ml). Question a.--draw each tube 0 thru 8 below, and show the dilution factor for each numbered tube, here is a start hint "tube 0 has dilution factor equal to 10° X, equals no dilution, tube 1 has dilution factor = 10X equals diluted 10 times......." and so on. Question b. Show your calculation below to figure out how many bacteria per ml were in the original culture tube 0, here is a hint...it may be easier for you to write out how many bacteria are in one ml from each tube, knowing that for the last tube #8, "if each drop = 50 microliters, then 2 drops = 100 microliters = (100 x 10-3 ml = (1 X 10-1) ml. If 30 colonies were counted from this, then one ml from tube #8 had 10 times (10) this many = 30 X 10 = 300 bacteria per ml in tube 8 ......

Answer 1

Q a)

Ans:

Dilution factor in tubes 1-8 1 2 3 un E 2 10 10 10 10 10 10 10 10 dilution factor

Q b)

2 drops from tube 8 = 30 colonies

1drop = 50 x 10^-3 ml

Therefore 2 drops = 100x 10^-3 ml

or 0.1 ml

Hence 0.1ml = 30 colonies of bacteria

1ml = 30 colonies of bacteria x 1/0.1 = 300 colonies of bacteria

or 300 colonies of bacteria/ ml

Colonies of bacteria/ml in tube 0 =  300 colonies of bacteria /ml in tube 8 x dilution factor

Therefore

Colonies of bacteria/ml in tube 0 =  300 colonies of bacteria /ml in tube8 x 10⁷

Colonies of bacteria/ml in tube 0 =  3 x 10⁹ colonies of bacteria /ml