Question

C - For the following 4 different experiments, you find the number of colony forming units (CFU) seen on a plate using the volume of the dilutions given. In each case calculate the number of CFU/100ml of the original solution.

                                                                                                                                          

Number of colonies	Dilution	Volume Plated	CFU/100ml
100	1/200	0.001 ml
50	10-4	100 µl
75	1/5	60 µl

D - What is the total count of S.aureus, when 43gm of meat is homogenized in 350 ml of buffer then 1ml transferred from the dilution 1:90 in 60ml. standard amount of the last dilution is plated on agar media and 143 colonies of S.aureus were subsequently counted on this plate?

Answer 1

CfU/100ml=no of colonies /(total dilution x100)

Total dilution=dilutionx volume plated

Number of colonies	Dilution	Volume Plated	Total dilution =D=dilutionx volume plated	CFU/100ml=no of colonies/(total dilutionx100ml)
100	1/200	0.001 ml	(1/200)x0.001 ml=5x10-6ml	cfu/100ml=100/(5x10-6x100ml)= 2x105 colonies per 100ml
50	10-4	100 µl	10-4x100x10-3ml= 10-5ml	Cfu/100ml=50/(10-5x100ml)=5x104
75	1/5	60 µl	(1/5)x60 x10-3ml=12x10-3ml	Cfu/100ml=75/(12x10-3x100ml)= 6.25x101

Ansd)

Total dilution=(43g/350ml)x(1/90)x(1/60)ml=2.3x10^-5gm

CFU/gm=143/2.3x10^-5=6.2x10⁶ colonies /gm.