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O.D. and cell number

About microscopic forms of life, including Bacteria, Archea, protozoans, algae and fungi. Topics relating to viruses, viroids and prions also belong here.

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O.D. and cell number

Postby 2810712 » Fri Nov 02, 2007 11:11 am

What is the relation between O.D. of a bacterial culture [ in log phase atleast] and the number of cells? O.D. when plotted against time , a straight line is obtained for a log phase culture...this resembles the graph of logN [N is number of cells] Vs time. what does it indicate about mathematical relationship between O.D. and cell number or concentration of cells..?
It will be useful if some one has a standard curve & expresssion relating O.D. & concentration of cells that can help to calculate N.

I have read that O.D. increases directly with cell mass of bacteria for some range. But can we relate this with number of cells..?
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Postby MrMistery » Fri Nov 02, 2007 11:52 am

I am afraid you cannot do that. It depends on the type of bacteria. From what i know, when you start with a culture, you first need to do paralel determinations with the spectophotometer and serial dillutions, and only then can you use OD
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Postby MrMistery » Fri Nov 02, 2007 11:52 am

PS: welcome back numbers. Haven't seen you around for a while
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Postby canalon » Sat Nov 03, 2007 5:20 pm

For E. coli, it is usually calculated than 1 OD unit is 10e9 bacteria per ml. This works pretty well, but in some conditions it is better to reasses the number for specific strains
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Postby 2810712 » Tue Nov 06, 2007 4:41 pm

thanks for the reply,
We have used E. coli, so this relation may be checked for in this case.
Which factors influence the O.D. of a bacterial suspension..?
I think, mass concentration [mass/vol], number concentration[number/vol], or percentage volume are related to each other and thus to O.D. also.
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Postby BDeis » Tue Nov 06, 2007 6:29 pm

I have found that motility can also play a role in absorption measurements. Motile bacteria can move to the walls of your wells. This will give you a higher density reading than what you really have.
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Postby canalon » Wed Nov 07, 2007 2:01 am

If you work well (i.e. fast and with agitation before measurement) motility should not be an important factor. But there are strains variations in cell size and other slight difference of membrane composition that can influence light diffusion, that is why, although you can usually go with a rough relation as I said above, if you really need good measurement it is better to plate dilution to improve the conversion factor.
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Postby 2810712 » Thu Nov 08, 2007 2:14 pm

Sodium ion concentration also influences the O.D.
I think the factors influencing the multiplication of bacteria can effect the O.D.
%viability also effects the O.D.

I think these will total to too many factors influencing the O.D. ,
But i think many of them will produce minor variations...
ain't it?
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Postby GeneticsLover » Wed Nov 21, 2007 8:40 pm

I'd have to agree that a plate dilution is very helpful. You can get a good idea of how many becteria per m/l you have and then relate that to a 1:1 dilution for your OD, makes for a great standard curve.
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Postby microworld » Sat Nov 24, 2007 9:11 am

you can make a standard curve to show the relationship between O.D. and CFU according to youre culture during the log phase.Not all the methods are proper for you even you use the same strain cause O.D. is affected by the culture media.
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