Question

Can anyone explain the process behind this please?

Starting with a culture of E. coli, you make a 1/100 dilution, followed by a 1/30 dilution, followed by a 1/5 dilution and then plate 0.5 ml and get 250 colonies on a plate, what was the cfu/ml in the initial culture.

		1.88 X 106
		3.75 X 106
		7.50 X 106
		3.75 X 107
		2.50 X 107

Answer 1

because you have not specified the conc. of stock, so we will consider 5 ml of cultural bacteria is added to 45 ml of sterile diluent, as in most cases.

First thing we need to calculate is the Dilution Factor, or how much the original sample is diluted:

here Initially 5mL in 45mL = Final Volume / Sample volume = 50/5 = 10.

Then three serial dilutions of 1/100, 1/30 and 1/5

Total Dilution Factor = 10X100X30X5 = 150000

CFU/mL = cfu/ml = (no. of colonies x dilution factor) / volume of culture plate

= (250 * 150000)/0.5

= 75000000 or 7.50X10⁶

So Total colony forming units in initial culture= 7.5X10⁶ CFU/mL