Question

Problem#1 ctice en the following bacterial dilution scheme Name: 1 ml 1 ml 1 ml 1 ml Bacterial culture 4 ml 9 ml 9ml 2ml 0.1 m 1 ml1 m0.1 m 400 CFU 400 CFU 140 CFU 16 CFU Individual tube dilutions Dilution Factor (-Total tube dilutions) Plated dilution factor 1. Explain why plates E and F have the same colony count. 2. Which plate is the countable plate? (30-300 countable range) Calculate the titer of the original culture in cfu/ml.

Answer 1

	A	B	C	D	D
Individual tube dilutions	5	10	10	3

	A'	B'	C'	D'	D'
Dilution factor	1:5	1:50	1:500	1:1500

	-	E	F	G	H
Plate dilution factor		1:10	1:1	1:1	1:10

1)

Although tube C is 10 times more dilute than tube B, 1 mL from it was plated onto plate F while 0.1 mL from tube B was plated onto plate E.

So 1 mL from a 1:500 dilution tube and 0.1 mL from a 1:50 dilution have the same dilution and hence the same number of colonies.

2) Plate G

3)

CFU/mL =

No. of colonies on countable plate × final dilution factor

= 140 × plate dilution factor of Plate G × total dilution factor of Tube D

= 140 × 1 × 1500

= 210,000 CFU/mL of original culture.