Question on a MacFarland Standards

William J. Mason wmason at COMP.UARK.EDU
Tue Mar 2 23:45:20 EST 1999

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

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