Question on a MacFarland Standards

Biology mpiraee at hotmail.com
Sun Mar 7 18:22:24 EST 1999

I believe that bacteria in a solution are actually in suspension state.
Therefore we need to have a standard which is also a suspension, which at the
same time has almost the same particle size (mean) of the most of bacteria.
Moreoever that the standard solution, if was a biological one, will replicate
and change the concentration by time.
I believe also that BaCl2 is an  insoluble salt, thus making a good suspension
in H2SO4. We know that even with very low solubility some of the salt will
disolve that is very similar to case when some soluble compounds are excreted
from bacteria to the broth, or bacteria suspension.
nice to hear your comments


William J. Mason wrote:

> I am currently teaching an honors microbiology lab and was posed with a
> question that I could not fully answer.
> The MacFarland standards are solutions of BaCl2 and H2SO4 that are added
> to a spec cuvette to create a turbid solution of measurable OD.  This tube
> is then correlated to the OD of a growing bacterial sample.  Once the ODs
> are the same an alloquat of the bacterial culture is removed, diluted, and
> CFU/ml are determined.  By increasing the concentration of the BaCl2, one
> can establish a set of ODs that correspond to a specific number of
> bacteria/ml (obviously for a specific bacteria).  The question was Why is
> it necessry to use the chemicals to create the standard.  I tried to
> explain that it is better to have a non-biological solution to create the
> standards, but this did not satisfy the student.  She left with an
> understanding of how to do it, but not a good and detailed reason as to
> WHY?  Does anyone have any practical but good advice on how to explain
> this?  Your help would be greatly appreciated.
> Jeff Mason
> University of Arkansas, Biological Sciences/Microbiology
> wmason at comp.uark.edu
> http://comp.uark.edu/~wmason

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