Question on a MacFarland Standards

Regine Fischeder fischeder at lw-online.de
Wed Mar 3 05:08:43 EST 1999


"William J. Mason" schrieb in Nachricht ...
>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
>
Using a chemical standard means much more stability than using a "living"
standard. You can keep the standards for a long time without a change, while
bacteria would grow and die off or (if you use dead bacteria) there would be
a degradation anyway.





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