Question on a MacFarland Standards

Bob Friedel rfriedel at perrittlab.com
Wed Mar 3 16:45:29 EST 1999

As mentioned in the previous post, MacFarland Standards are utilized due to
their chemical stability and ease of use.  They should be used in
conjunction with a "Wickerham card".  This is business size card containing
a white background with black horizontal lines.

I used MacFarland Standards in my last job to prepare bacterial and yeast
culture solutions for performing Preservative Challenge Testing of
Over-The-Counter Pharmaceuticals.  In my current position, our laboratory
uses a spectrophotometer.  Obviously this entails a larger capital
investment.  Both methods arrive at the same point, only one is much
cheaper.  The standards are available in a series from 1 to 10.

Standard #1    equals 3.0 x 10E8
Standard #5    equals 1.5 x 10E9
Standard #10  equals 3.0 x 10E9

The Difco Manual is a good reference source (see 2nd ed., p. 564, 1984).

By the way, the standards last much longer than the one year expiration date
that the manufacturer suggests.  Unfortunately, GMP requirements forbid use
of reagents after their expiration date.

Robert R. Friedel
Quality Assurance Manager
Laboratory Research & Analysis Groups
Perritt Laboratories, Inc.
rfriedel at perrittlab.com

"William J. Mason" wrote in message ...
>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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